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diff --git a/gcc-4.4.3/libjava/classpath/doc/cp-vmintegration.texinfo b/gcc-4.4.3/libjava/classpath/doc/cp-vmintegration.texinfo new file mode 100644 index 000000000..0b2d78c84 --- /dev/null +++ b/gcc-4.4.3/libjava/classpath/doc/cp-vmintegration.texinfo @@ -0,0 +1,1999 @@ +\input texinfo @c -*-texinfo-*- + +@c %**start of header +@setfilename cp-vmintegration.info +@settitle GNU Classpath VM Integration Guide +@c %**end of header + +@setchapternewpage off + +@ifinfo +This file contains important information you will need to know if you +are going to write an interface between GNU Classpath and a Virtual +Machine. + +Copyright (C) 1998-2002, 2004, 2005, 2006, 2007 Free Software Foundation, Inc. + +@ifnotplaintext +@dircategory GNU Libraries +@direntry +* VM Integration: (cp-vmintegration). GNU Classpath VM Integration Guide +@end direntry +@end ifnotplaintext +@end ifinfo + +@titlepage +@title GNU Classpath VM Integration Guide +@author John Keiser +@author C. Brian Jones +@author Mark Wielaard + +@page +@vskip 0pt plus 1filll +Copyright @copyright{} 1998-2002 Free Software Foundation, Inc. +@sp 2 +Permission is granted to make and distribute verbatim copies of +this document provided the copyright notice and this permission notice +are preserved on all copies. + +Permission is granted to copy and distribute modified versions of this +document under the conditions for verbatim copying, provided that the +entire resulting derived work is distributed under the terms of a +permission notice identical to this one. + +Permission is granted to copy and distribute translations of this manual +into another language, under the above conditions for modified versions, +except that this permission notice may be stated in a translation +approved by the Free Software Foundation. + +@end titlepage + +@ifinfo +@node Top, Introduction, (dir), (dir) +@top GNU Classpath Hacker's Guide + +This file contains important information you will need to know if you +are going to write an interface between GNU Classpath and a Virtual +Machine. + +This document is incomplete, as we are still in alpha with the interface. + +@end ifinfo + +@menu +* Introduction:: An introduction to the Classpath project +* Initialization:: Initializing the classes +* Classpath Hooks:: Hooks from Classpath to the VM +* VM Hooks:: Hooks from the underlying VM to Classpath +* JNI Implementation:: Hooking the VM to jni.h +* JVMTI Implementation:: Hooking the VM to jvmti.h +* Miscellaneous VM Requirements:: +@end menu + +@node Introduction, Initialization, Top, Top +@comment node-name, next, previous, up +@chapter Introduction + +The Classpath Project's ambition to be a 100% clean room implementation +of the standard Java class libraries cannot be fulfilled without some +level of integration with the Virtual Machine, the underlying machinery +that actually runs Java. + +There are several VMs out there, here is a small list. + +@itemize @bullet +@item @uref{http://www.hungry.com/old-hungry/products/japhar/,Japhar} +Japhar was the first VM to use GNU Classpath. Today you can see that +sort of relationship in the source tree which denotes several Japhar +specific files as a reference implementation of those pieces. This VM +has been primarily tested against Linux and lacks garbage collections, a +JIT, and suffers recently from slow development. + +@item @uref{http://www.intel.com/research/mrl/orp/,Intel's Open Runtime Platform} +Intel surprised us not long ago with the release of this rather advanced +VM that uses GNU Classpath for a set of class libraries and works on +Linux and Windows 2000. As of June, 2004, it does not appear that ORP +is under active development. + +@item @uref{http://www.sablevm.org/,SableVM} +SableVM is a robust, extremely portable, efficient, and +specifications-compliant Java Virtual Machine that aims to be easy to +maintain and to extend. It features a state-of-the-art, efficient +interpreter engine. Its source code is very accessible and easy to +understand, and has many robustness features that have been the object +of careful design. + +@item @uref{http://www.kaffe.org,Kaffe} +Kaffe is an advanced VM and together with its own class libraries +provides a Java 1.1 compatible environment. + +@item @uref{http://www.mozilla.org/projects/ef,Electrical Fire} +The Electrical File VM continues to be listed as a Mozilla project +though development has been somewhat quiet. A number of concepts from +EF were expected at one point to be rolled into Japhar, but that +development has not occurred as of yet. + +@item @uref{http://latte.snu.ac.kr/,LaTTe} +This VM project so far supports only Sun UltraSparc processors using the +proprietary Solaris 2.5.1 or higher operating system. LaTTe was derived +from Kaffe but claims a number of improvements. + +@item @uref{http://gcc.gnu.org/java/,GNU Compiler for Java (GCJ)} +This is a portable, optimizing, ahead-of-time compiler for the Java +Programming Language. It can compile Java source code directly to native +machine code, Java source code to Java bytecode (class files), and Java +bytecode to native machine code. Compiled applications are linked with the +GCJ runtime, libgcj which is based on the GNU Classpath code, which provides +the core class libraries, a garbage collector, and a bytecode interpreter. +libgcj can dynamically load and interpret class files, resulting in mixed +compiled/interpreted applications. +GCJ is part of the GNU Compiler Collection (@uref{http://gcc.gnu.org/,GCC}). +On March 6 2000 the libgcj and GNU Classpath projects were officially merged +and there is active work on merging all the classes between the projects. +Licensed under GPL+exception, just as GNU Classpath is. + +@item @uref{http://kissme.sourceforge.net/,Kissme} +This is a free Java Virtual Machine that is being developed on GNU/Linux +and can run console Java applications. Kissme also provides support for +orthogonally persistent Java. +@c I don't know what ``orthogonally persistent Java'' is, and I bet +@c there are other people don't know either. -- Steve Augart, 4 June 2004 + +@item @uref{http://jamvm.sourceforge.net/,JamVM} +A simple, small bytecode interpreter that works out-of-the-box with +pure GNU Classpath; it is emerging as the preferred platform for +quickly testing a new build of GNU Classpath. Licensed under the GPL. + +@item @uref{http://jikesrvm.sourceforge.net/,Jikes RVM} +A free runtime environment for Java, written in Java. Works +out-of-the-box with pure GNU Classpath. Features an optimizing JIT. +Runs on the x86 and PowerPC architectures, on the AIX, Linux, and Mac +OS/X operating systems. Licensed under the CPL (Common Public +License). Extensively documented. Actively developed as of June, +2004. + +@end itemize + +In the past integration efforts were focused mainly on Japhar with an eye +towards getting Electrical Fire to work. Most information contained in +this document is gleaned from these efforts. Recently more work has been +done on getting gcj, orp and kissme to work out of the box with GNU Classpath +but there is much to do before that becomes a reality. + + +@node Initialization, Classpath Hooks, Introduction, Top +@comment node-name, next, previous, up +@chapter Initialization + +The order of initialization, as far as I can tell, doesn't matter just +yet. However, when we move to 1.2 support, it probably will matter, so +we'll have a note in here at that time. + +The initialization order is currently documented in the +@file{Runtime.java} source file. + +@node Classpath Hooks, VM Hooks, Initialization, Top +@comment node-name, next, previous, up +@chapter Classpath Hooks + +The primary method of interaction between Classpath and the VM is via +the helper classes, which are named after the relevant core library +class, but include an additional `VM' prefix. The library classes from +Classpath call out to these to get certain VM-specific dirty work done. +A reference copy of each VM class exists. The majority consist of a +series of static methods, some of which are simply declared +@code{native}, and some which provide a default implementation. VMs may +either use these as is, or create their own local variations. When +using the default implementations, the VM is responsible for +implementing any of the code marked as @code{native} which corresponds +to functionality they wish their VM to provide. When using their own +versions of the classes, VM implementors may choose to change the mix of +native and non-native methods from that below, so as to best suit their +implementation. + +@menu +* java.lang:: +* gnu.classpath:: +* java.util:: +* java.io:: +* java.security:: +* java.net:: +* java.nio:: +* java.nio.channels:: +* gnu.java.nio:: +* java.lang.reflect:: +* gnu.java.lang:: +* gnu.java.lang.management:: +* java.lang.management:: +* Classpath Callbacks:: +@end menu + +@node java.lang, gnu.classpath, Classpath Hooks, Classpath Hooks +@comment node-name, next, previous, up + +@section @code{java.lang} + +@code{java.lang} is the core Java package, being imported automatically by all +classes. It includes basic classes as @code{Object} and @code{String}. +A VM must implement at least some parts of this package in order to +become operable. + +@menu +* java.lang.VMClass:: +* java.lang.VMObject:: +* java.lang.VMClassLoader:: +* java.lang.VMSystem:: +* java.lang.VMThrowable:: +* java.lang.VMCompiler:: +* java.lang.VMDouble:: +* java.lang.VMFloat:: +* java.lang.VMProcess:: +* java.lang.VMRuntime:: +* java.lang.VMString:: +* java.lang.VMThread:: +* java.lang.VMMath:: +@end menu + +@node java.lang.VMClass, java.lang.VMObject ,java.lang,java.lang +@subsection @code{java.lang.VMClass} + +The core class, @code{java.lang.Class}, and the corresponding VM class, +@code{java.lang.VMClass}, provide two main functions within GNU Classpath. + +@enumerate +@item For basic VM operation, @code{java.lang.Class} provides the link between +the Java-based representation of a class it embodies and the VM's own +internal structure for a class. @xref{VM Hooks}. + +@item As far as the user is concerned, the main function of +@code{java.lang.Class} is as an entry point to the reflection +facilities, and so it also provides this functionality, backed by the +VM class. +@end enumerate + +This VM class lists the following methods, organized by the version of the +Java specification in which they occur. All are @code{native}, unless +otherwise specified, and pertain to reflection. As a result, the VM only +needs to implement these methods in order to provide reflection support, +and then only to the degree required. + +@itemize @bullet +@item 1.0 +@itemize @bullet +@item @code{isInterface(Class)} -- This is simply a property test, and matches +the presence of an appropriate flag within the class file. +@item @code{getName(Class)} -- Returns the fully-qualified name of the class. +@item @code{getSuperclass(Class)} -- Returns a @code{Class} instance which +represents the superclass. Again, the class file contains an element directly +relating to this. @code{null} is returned for primitives, interfaces and +@code{Object}. +@item @code{getInterfaces(Class)} -- Same as the above, but the implemented +or extended interfaces rather than the superclass. An empty array should +be returned, rather than @code{null}. +@item @code{getDeclaredClasses(Class,boolean)} -- Returns the internal classes +this instance declares directly. The flag determines whether or not the +VM should filter out non-public classes. +@item @code{getDeclaredFields(Class,boolean)} -- The same for fields. +@item @code{getDeclaredMethods(Class,boolean)} -- And for methods. +@item @code{getDeclaredConstructors(Class,boolean)} -- And constructors. +@item @code{getClassLoader(Class)} -- Returns the @code{ClassLoader} instance +which is responsible for the specified class. +@item @code{forName(String, boolean, ClassLoader)} -- The VM should create a +@code{Class} instance corresponding to the named class. As noted in +@ref{VM Hooks}, the internal content of the instance is the +responsibility of the VM@. The supplied class loader is recorded as that +which loaded the class, and the boolean specifies whether or not to +run the class initializer. +@item @code{isArray(Class)} -- Another property test, corresponding to a +class file flag. +@item @code{initialize(Class)} -- The VM should initialize the class fully, +if it has not already done so. +@item @code{loadArrayClass(String,ClassLoader)} -- This is called if +@code{forName} returns @code{null} and the string specifies an array class. +The specified array class should be loaded with the supplied class loader. +@item @code{throwException(Throwable)} -- The VM should throw the supplied +checked exception, without declaring it. +@end itemize +@item 1.1 +@itemize @bullet +@item @code{isInstance(Class,Object)} -- This is a reflection-based equivalent +of the @code{instanceof} operator. +@item @code{isAssignableFrom(Class,Class)} -- Mainly a shorthand for the above, +removing the need to create an instance to test assignability. +@item @code{isPrimitive(Class)} -- Returns true if this class is simply +a representation of one of the primitive types: @code{boolean}, @code{byte}, +@code{char}, @code{short}, @code{int}, @code{long}, @code{float}, +@code{double} and @code{void}. +@item @code{getComponentType(Class)} -- Produces a @code{Class} instance which +represents the type of the members of the array the class instance represents. +Classes which don't represent an array type return @code{null}. +@item @code{getModifiers(Class,boolean)} -- Returns an integer which encodes +the class' modifiers, such as @code{public}. Again, this relates to +information stored in the class file. +@item @code{getDeclaringClass(Class)} -- Returns the class that declared +an inner or member class, or @code{null} if the instance refers to a top-level +class. +@end itemize +@item 1.5 +@itemize @bullet +@item @code{isSynthetic(Class)} -- Returns true if the flags for this class +mark it as synthetic. +@item @code{isAnnotation(Class)} -- Returns true if the flags for this class +mark it as an annotation. +@item @code{isEnum(Class)} -- Returns true if the flags for this class +mark it as an enumeration. +@item @code{getSimpleName(Class)} -- Returns the simple name of the class. +A default implementation is provided, but a more efficient version may instead +be provided by the VM. +@item @code{getCanonicalName(Class)} -- Returns the canonical name of the +class. A default implementation is provided, but a more efficient +version may instead be provided by the VM. +@item @code{getEnclosingClass(Class)} -- Returns the immediately enclosing +class (null for a top-level class). +@item @code{getEnclosingConstructor(Class)} -- Returns the constructor +which immediately encloses the supplied class. +@item @code{getEnclosingMethod(Class)} -- Returns the method +which immediately encloses the supplied class. +@item @code{getClassSignature(Class)} -- Returns the generic signature of +the class or null if there isn't one. +@item @code{isAnonymousClass(Class)} -- Returns true if the class is an +anonymous class. +@item @code{isLocalClass(Class)} -- Returns true if the class is an +local class. +@item @code{isMemberClass(Class)} -- Returns true if the class is an +member class. +@end itemize +@end itemize + +@node java.lang.VMObject, java.lang.VMClassLoader, java.lang.VMClass, java.lang +@subsection @code{java.lang.VMObject} + +@code{VMObject} is the bridge between the low level @code{Object} +facilities such as making a clone, getting the class of the object and +the wait/notify semantics. This is accomplished using the following +@code{native} methods. + +@itemize @bullet +@item @code{getClass(Object)} -- Returns the @code{Class} instance for the +object. @code{Class} objects are produced by the VM, as described in +@ref{VM Hooks}. +@item @code{clone(Cloneable)} -- The VM should produce a low-level clone of the +specified object, creating a field-by-field shallow copy of the original. +The only difference between the two is that the new object should still be +@code{finalizable}, even if the original is not. +@item @code{notify(Object)} -- The VM should choose one of the threads waiting +for a lock on the specified object arbitrarily, and wake it. If the current +thread does not currently hold the lock on the object, then an +@code{IllegalMonitorStateException} should be thrown. +@item @code{notifyAll(Object)} -- Same as the above, but all threads are +awakened. +@item @code{wait(Object,long,int)} -- The VM should set the current thread +into a waiting state, which persists until it receives a notify signal or the +specified time (in milliseconds and nanoseconds) is exceeded. The nanoseconds +restriction may be ignored if such granularity is not available, and a +@code{IllegalMonitorStateException} should be thrown if the current thread +doesn't own the object. +@end itemize + +@node java.lang.VMClassLoader, java.lang.VMSystem, java.lang.VMObject, java.lang +@subsection @code{java.lang.VMClassLoader} +@code{VMClassLoader} provides methods for defining and resolving core and +primitive classes, as well as handling resources, packages and assertions. +The class is a mixture of @code{native} methods and Java-based +implementations, with some of the latter being @emph{stubs}. + +@itemize @bullet +@item Native Methods +@itemize @bullet +@item @code{defineClass(ClassLoader,String,byte[],int,int,ProtectionDomain)} +-- The VM should create a @code{Class} instance from the supplied byte array. +@item @code{resolveClass(Class)} -- Resolve references to other classes in the +supplied class. +@item @code{loadClass(name,boolean)} -- Load a class using the bootstrap +loader. +@item @code{getPrimitiveClass(char)} -- The VM should provide a @code{Class} +implementation for one of the primitive classes. The supplied character +matches the JNI code for the primitive class e.g.@: `B' for byte and +`Z' for boolean. +@end itemize +@item Java Methods +@itemize @bullet +@item @code{getResource(String)} -- The default implementation calls +@code{getResources} and returns the first element in the returned enumeration, +or @code{null} if there are no elements. +@item @code{getResources(String)} -- By default, this compiles a list of +URLs via the boot class path. Any matching files within a zip file are added, +and directories on the boot class path are automatically converted to file +URLs that refer to join the directory with the resource name (whether or not +it actually exists). +@item @code{getPackage(String)} -- Always returns null, which may be suitable +if the VM does not wish to return a @code{Package} implementation. Otherwise, +it may be necessary to make this a @code{native} method. +@item @code{getPackages()} -- As with the last, a default stub implementation +exists (returning an empty array) which may be replaced if support is +required. +@item @code{defaultAssertionStatus()} -- A stub which can be implemented +by VMs providing assertion support. At present, it always returns @code{true}. +@item @code{packageAssertionStatus()} -- Much the same status as the above. +The method should return a map converting package names to boolean status +values. The stub implementation provides an empty map. +@item @code{classAssertionStatus()} -- Same as the last, but for classes. +@item @code{getSystemClassLoader()} -- The default calls @code{ClassLoader} +to create a new auxiliary class loader with a system and extension class +loader. The VM may wish to replace it if it wishes to supply its own custom +system class loader. +@end itemize +@end itemize +@node java.lang.VMSystem, java.lang.VMThrowable, java.lang.VMClassLoader, java.lang +@subsection @code{java.lang.VMSystem} +@code{VMSystem} handles the default I/O streams, provides access to the +system clock and environment variables and provides methods for +@code{System.arraycopy} and the @code{identityHashCode} of an +@code{Object}. It consists of @code{native} methods, but the default +implementation also provides some helper methods to simplify stream +creation. + +@itemize @bullet +@item Native Methods +@itemize @bullet +@item @code{arraycopy(Object,int,Object,int,int)} -- The VM should copy +a specified number of array objects from one array to another, with +appropriate checks for compatible typing, available elements and space. +The VM should be able to perform this more efficiently using native code +and direct memory manipulation than would have been achieved by using Java. +@item @code{identityHashCode(Object)} -- This is the hashcode for +@code{Object}, which relates to the actual location of the object in memory. +@item @code{setIn(InputStream)} -- Set the system input stream. +@item @code{setOut(PrintStream)} -- Set the system output stream. +@item @code{setErr(PrintStream)} -- Set the system error stream. +@item @code{currentTimeMillis()} -- Gets the system time in milliseconds. +@item @code{getenv(String)} -- Returns the value of the specified environment +variable. +@item @code{getenv()} -- Returns a list of `name=value' pairs which correspond +to the environment variables. +@end itemize +@item Java Methods +@itemize @bullet +@item @code{makeStandardInputStream()} -- Helps provide the functionality of +@code{System.in} by wrapping the appropriate file descriptor in a +buffered file input stream. VMs may choose to create the stream from +the descriptor differently rather than using this method. +@item @code{makeStandardOutputStream()} -- Helps provide the functionality of +@code{System.out} by wrapping the appropriate file descriptor in a buffered +file output stream. VMs may choose to create the stream from the descriptor +differently rather than using this method. +@item @code{makeStandardErrorStream()} -- Helps provide the functionality of +@code{System.err} by wrapping the appropriate file descriptor in a buffered +file output stream. VMs may choose to create the stream from the descriptor +differently rather than using this method. +@end itemize +@end itemize + +Classpath also provides native implementations of + +@itemize @bullet +@item @code{setIn(InputStream)} +@item @code{setOut(PrintStream)} +@item @code{setErr(PrintStream)} +@item @code{currentTimeMillis()} +@item @code{getenv(String)} +@end itemize + +making a VM implementation optional. + +@node java.lang.VMThrowable, java.lang.VMCompiler, java.lang.VMSystem, java.lang +@subsection @code{java.lang.VMThrowable} +@code{VMThrowable} is used to hold the VM state of a throwable, created either +when a @code{Throwable} is created or the @code{fillInStackTrace()} method is +called (i.e., when the actual stack trace is needed, as a lot of exceptions are +never actually used). The actual class has two @code{native} methods, +one (@code{fillInStackTrace()}) being a method of the class used to obtain +instances, and the other an instance method, @code{getStackTrace()}. +@itemize @bullet +@item @code{fillInStackTrace(Throwable)} -- The VM should return the current +execution state of the @code{Throwable} in the form of a @code{VMThrowable} +instance. The VM may also return @code{null} if it does not support this +functionality. +@item @code{getStackTrace()} -- This is used to create a real +@code{StackTraceElement} array for the exception, using the state data +stored during creation of the instance. +@end itemize + +@node java.lang.VMCompiler, java.lang.VMDouble, java.lang.VMThrowable, java.lang +@subsection @code{java.lang.VMCompiler} + +@code{VMCompiler} provides an interface for VMs which wish to provide +JIT compilation support. The default implementation is simply a series +of stubs. The property, @code{java.compiler}, should point to a library +containing the function @code{java_lang_Compiler_start()} if such support +is to be provided. + +@itemize @bullet +@item @code{compileClass(Class)} -- Invoke the compiler to compile the specific +class, returning @code{true} if successful. +@item @code{compileClasses(String)} -- The compiler should compile the classes +matching the specified string, again returning @code{true} on success. +@item @code{command(Object)} -- The object represents a command given to the +compiler, and is specific to the compiler implementation. +@item @code{enable} -- Enable the operation of the compiler. +@item @code{disable} -- Disable compiler operation. +@end itemize + +@node java.lang.VMDouble, java.lang.VMFloat, java.lang.VMCompiler, java.lang +@subsection @code{java.lang.VMDouble} + +@code{VMDouble} provides native support for the conversion and parsing +of doubles. + +@itemize @bullet +@item @code{doubleToRawLongBits(double)} -- Converts the double to the IEEE 754 +bit layout, preserving NaNs. +@item @code{longBitsToDouble(long)} -- This is the inverse of the last method, +preserving NaNs so that the output of one can be fed into the other without +data loss. +@item @code{toString(double,boolean)} -- Converts the double to a string, +giving a shorter value if the flag @code{isFloat} is @code{true}, indicating +that the conversion was requested by @code{java.lang.Float} rather than +@code{java.lang.Double}. +@item @code{initIDs} -- Used by JNI-based solutions to initialize the cache +of the static field IDs. The default @code{VMDouble} implementation has a +static initializer which loads the JNI library and calls this method. +@item @code{parseDouble} -- Turn the string into a usable double value. +@end itemize + +Classpath provides native implementations of all these, making VM +implementation optional. + +@node java.lang.VMFloat, java.lang.VMProcess, java.lang.VMDouble, java.lang +@subsection @code{java.lang.VMFloat} + +@code{VMFloat} provides native support for the conversion of floats. + +@itemize @bullet +@item @code{floatToRawIntBits(float)} -- Converts the float to the IEEE 754 +bit layout, preserving NaNs. +@item @code{intBitsToFloat(int)} -- This is the inverse of the last method, +preserving NaNs so that the output of one can be fed into the other without +data loss. +@end itemize + +Classpath provides native implementations of all these, making VM +implementation optional. + +@node java.lang.VMProcess, java.lang.VMRuntime, java.lang.VMFloat, java.lang +@subsection @code{java.lang.VMProcess} + +@code{VMProcess} handles the execution of external processes. In the +default implementation, threads are spawned and reaped by @code{ProcessThread}. +A constructor creates a new @code{VMProcess}, which extends rather than +complements @code{Process}, using an array of arguments, an array of +environment variables and a working directory. The instance maintains +system input, output and error streams linked to the external process. +Three @code{native} methods are used, and implementations are provided +for all three by Classpath, making VM implementation optional. These use +the POSIX functions, @code{fork()}, @code{waitpid()} and @code{kill()}. + +@itemize @bullet +@item @code{nativeSpawn(String[],String[],File,boolean)} -- The VM should +create a new process which uses the specified command-line arguments, +environment variables and working directory. Unlike the other two +methods, this method is linked to an instance, and must call +@code{setProcessInfo()} with the results before returning. The +boolean argument maps to the @code{redirectErrorStream} property of +@code{java.lang.ProcessBuilder}. When true, the output and error streams +are merged. +@item @code{nativeReap()} -- This is called to perform a reap of any +zombie processes, and should not block, instead returning a boolean as to +whether reaping actually took place. +@item @code{nativeKill(long)} -- The VM should terminate the specified PID. +@end itemize + +@node java.lang.VMRuntime, java.lang.VMString, java.lang.VMProcess, java.lang +@subsection @code{java.lang.VMRuntime} + +The @code{VMRuntime} class provides a series of native methods +which divulge information about the runtime or invoke certain +operations. This includes retrieving the amount of available memory, +and scheduling the garbage collector. There are two exceptions: the +@code{enableShutdownHooks} method, which allows the VM to put in its own +shutdown hooks when @code{Runtime.addShutdownHook()} is first invoked, +and @code{exec(String[],String[],File)} which spawns an external process. +These are Java-based static methods instead. The first is simply a stub by +default, while the second simply links to the functionality of +@code{VMProcess} (and should be changed if a different @code{Process} +implementation is used). + +@itemize @bullet +@item @code{availableProcessors()} -- Returns the number of processors +available to the VM. +@item @code{freeMemory()} -- Returns the amount of memory the VM has available +on the heap for allocating. +@item @code{totalMemory()} -- Returns the size of the heap. +@item @code{maxMemory()} -- Returns the maximum memory block the VM will +attempt to allocate. May be simply @code{Long.MAX_VALUE} (8 exabytes!) +@item @code{gc()} -- Allows users to explicitly invoke the garbage collector. +This is a suggestion to the VM, rather than a command, and the garbage +collector should run anyway @emph{without} it being invoked. +@item @code{runFinalization()} -- Like the above, but related to the +finalilzation of objects rather than the garbage collector. +@item @code{runFinalizationForExit()} -- Called immediately prior to VM +shutdown in order to finalize all objects (including `live' ones) +@item @code{traceInstructions(boolean)} -- This turns on and off the optional +VM functionality of printing a trace of executed bytecode instructions. +@item @code{traceMethodCalls(boolean)} -- This turns on and off the optional +VM functionality of printing a trace of methods called. +@item @code{runFinalizersOnExit(boolean)} -- A toggleable setting for +running the finalization process at exit. +@item @code{exit(int)} -- The VM should shutdown with the specified exit code. +@item @code{nativeLoad(String,ClassLoader)} -- Attempts to load a file, +returning an integer which is non-zero for success. Nothing happens if the +file has already been loaded. +@item @code{mapLibraryName(String)} -- The VM should map the system-independent +library name supplied to the platform-dependent equivalent (e.g.@: a @code{.so} +or @code{.dll} file) +@end itemize + +@node java.lang.VMString, java.lang.VMThread, java.lang.VMRuntime, java.lang +@subsection @code{java.lang.VMString} +@code{VMString} is responsible for handling interned strings. If two strings +are equal (using the @code{equals()} method), then the results of calling +the @code{intern()} method on each of them makes them equal +(using @code{==}). Thus, the same string object is always returned by +@code{intern} if the two strings are equal. The default implementation +is Java-based and implements @code{intern(String)} by maintaining a +@code{WeakHashMap} which links the strings to their @code{WeakReference}. +A new mapping is created for each new string being @code{intern}ed. +A VM may implement this differently by implementing this method, +which is @code{static} and the only one in @code{VMString}. + +@node java.lang.VMThread, java.lang.VMMath, java.lang.VMString, java.lang +@subsection @code{java.lang.VMThread} + +@code{VMThread} provides the link between Java's threads and the platform +threading support. A @code{VMThread} is created via a private constructor +and linked to a @code{Thread} instance. This occurs when the @code{Thread} +instance is started by the static @code{create(Thread,long)} method (the second +argument requests a certain stack size, usually zero). The thread itself is +executed via the @code{run()} method, which handles any problems with the +running of the thread and its eventual death. + +@code{VMThread} provides the following accessors and mutators for accessing +the thread state via @code{VMThread}, + +@itemize @bullet +@item @code{getName()} +@item @code{setName(String)} +@item @code{getPriority()} +@item @code{setPriotity(int)} +@item @code{isDaemon()} +@end itemize + +all of which refer to the @code{Thread} instance. @code{setPriority(int)} also +calls the appropriate native method. @code{stop(Throwable)} similarly wraps +a native method, merely adding in a check for the state of the thread. + +The default implementation also provides Java-based implementations of +@code{join(long,int)}, @code{sleep(long,int)} and +@code{holdsLock(Object)}. @code{join} and @code{sleep} simply wait for +the appropriate amount of time, with @code{join} additionally waiting +for the thread instance to become @code{null}. @code{holdsLock} simply +checks if an object is locked by the current thread by trying to invoke +the @code{notify} method, and catching the failing exception if this is +not the case. + +The remainder of the class is a series of @code{native} methods, some of +which are mandatory for VM implementation and others which provide optional +or deprecated functionality. + +@itemize @bullet +@item Mandatory Instance Methods +@itemize @bullet +@item @code{start(long)} -- The VM should create the native thread and start +it running using the @code{run} method of the @code{VMThread} instance on +which this method is called. +@item @code{interrupt()} -- The VM should interrupt the running thread and +throw an appropriate exception. +@item @code{isInterrupted()} -- Checks the interrupted state of the thread. +@item @code{suspend()} -- The thread should be suspended until resumed. +@item @code{resume()} -- The thread should be resumed from its suspended state. +This pair of methods are deprecated, due to the possibility of a deadlock +occurring when a thread with locks is suspended. +@item @code{nativeSetPriority(int)} -- Called by @code{setPriority} +to allow the setting to flow down to the native thread. +@item @code{nativeStop(Throwable)} -- The VM should stop the thread abnormally +and throw the specified exception. This is clearly deprecated, due to the +ambiguous state an abruptly-stopped thread may leave. +@item @code{getState()} -- Returns the VM's impression of the current state +of the thread. The applicable states are supplied by the @code{State} +enumeration in @code{java.lang.Thread}. +@end itemize +@item Mandatory Class Methods +@itemize @bullet +@item @code{currentThread()} -- Return a reference to the thread currently +being executed. +@item @code{yield()} -- The VM should allow some other thread to run. +The current thread maintains its locks even though it stops executing for +the time being. +@item @code{interrupted()} -- A shortcut to obtaining the interrupted state +of the current thread. +@end itemize +@item Other Methods +@itemize @bullet +@item @code{countStackFrames()} -- Returns a count of the number of stack +frames in the thread. This depends on the deprecated method @code{suspend()} +having returned true, and is thus deprecated as a result. +@end itemize +@end itemize + +@node java.lang.VMMath,, java.lang.VMThread, java.lang +@subsection @code{java.lang.VMMath} + +The @code{VMMath} class provides a series of native methods +for some of the mathematical functions present in @code{java.lang.Math}. +Classpath provides a default implementation of these which maps the +functions to those provided by @code{fdlibm}. VM implementors are welcome +to replace this with more efficient implementations, as long as the accuracy +contract of these methods, specified in @code{java.lang.Math}, is maintained. + +@itemize @bullet +@item 1.0 +@itemize @bullet +@item @code{sin(double)} -- Returns the sine value for the given angle. +@item @code{cos(double)} -- Returns the cosine value for the given angle. +@item @code{tan(double)} -- Returns the tangent value for the given angle. +@item @code{asin(double)} -- Returns the arc sine value for the given angle. +@item @code{acos(double)} -- Returns the arc cosine value for the given angle. +@item @code{atan(double)} -- Returns the arc tangent value for the given angle. +@item @code{atan2(double,double)} -- Returns the arc tangent of the ratio of +the two arguments. +@item @code{exp(double)} -- Returns the exponent raised to the given power. +@item @code{log(double)} -- Returns the natural logarithm for the given value. +@item @code{sqrt(double)} -- Returns the square root of the value. +@item @code{pow(double,double)} -- Returns x to the power of y. +@item @code{IEEEremainder(double,double)} -- Returns the IEEE 754 remainder +for the two values. +@item @code{ceil(double)} -- Returns the nearest integer >= the value. +@item @code{floor(double)} -- Returns the nearest integer <= the value. +@item @code{rint(double)} -- Returns the nearest integer or the even one +if the distance between the two is equal. +@end itemize +@item 1.5 +@itemize @bullet +@item @code{cbrt(double)} -- Returns the cube root of the value. +@item @code{cosh(double)} -- Returns the hyperbolic cosine value for the given +angle. +@item @code{expm1(double)} -- Returns the exponent of the value minus one. +@item @code{hypot(double,double)} -- Returns the hypotenuse corresponding to +x and y. +@item @code{log10(double)} -- Returns the base 10 logarithm of the given value. +@item @code{log1p(double)} -- Returns the natural logarithm of the value plus +one. +@item @code{sinh(double)} -- Returns the hyperbolic sine value for the given +angle. +@item @code{tanh(double)} -- Returns the hyperbolic tangent value for the given angle. +@end itemize +@end itemize + +@node gnu.classpath, java.util, java.lang, Classpath Hooks +@section @code{gnu.classpath} + +The @code{gnu.classpath} package provides Classpath-specific functionality, +primarily relating to the features in @code{java.lang}. At present, this +includes the context of a class (the stack) and the system properties. + +@menu +* gnu.classpath.VMStackWalker:: +* gnu.classpath.VMSystemProperties:: +* gnu.classpath.Unsafe:: +@end menu + +@node gnu.classpath.VMStackWalker,gnu.classpath.VMSystemProperties,gnu.classpath,gnu.classpath +@subsection @code{gnu.classpath.VMStackWalker} + +@code{VMStackWalker} provides access to the class context or stack. The +default implementation consists of a @code{native} @code{static} method, +@code{getClassContext()}, which obtains the class context, and two helper +methods which obtain the calling class (the 3rd element in the context array) +and its class loader, respectively. + +@itemize @bullet +@item @code{getClassContext()} -- The VM should return an array of +@code{Class} objects, each of which relates to the method currently being +executed at that point on the stack. Thus, the first item (index 0) is the +class that contains this method. +@item @code{getCallingClass()} -- A Java-based helper method which returns +the @code{Class} object which contains the method that called the method +accessing @code{getCallingClass()}. +@item @code{getCallingClassLoader()} -- Like the last, but returning the class +loader of the class. +@end itemize + +@node gnu.classpath.VMSystemProperties,gnu.classpath.Unsafe,gnu.classpath.VMStackWalker,gnu.classpath +@subsection @code{gnu.classpath.VMSystemProperties} + +@code{VMSystemProperties} allows the VM to hook into the property creation +process, both before and after the system properties are added by GNU +Classpath. The default implementation assumes that the VM will add its +properties first, by making the pre-initialisation method @code{native}, +and that the Classpath properties may then be altered by a Java-based +post-initialisation method. + +As these methods are called as part of the bootstrap process, caution should +be used as to what classes are used, and properties should only be set +using @code{Properties.setProperty()}. Specifically, I/O classes should be +avoided at this early stage. + +@itemize @bullet +@item @code{preInit(Properties)} -- Allows the VM to add properties +@emph{before} the Classpath properties are added. The default implementation +includes a full list of properties that @emph{must} be added by the VM, but +additional VM-specific ones may also be added. +@item @code{postInit(Properties)} -- Same as the last, but called after the +Classpath properties have been added. The main purpose of this is to allow +the VM to alter the properties added by GNU Classpath to suit it. +@end itemize + +@node gnu.classpath.Unsafe,,gnu.classpath.VMSystemProperties,gnu.classpath +@subsection @code{gnu.classpath.Unsafe} + +The @code{Unsafe} class provides access to some low-level unsafe operations +as required by the addition of the java.util.concurrent classes. These +focus on direct memory access to the fields within the VM and providing +atomic update methods. + +@itemize @bullet +@item @code{objectFieldOffset(Field)} -- Provides the caller with the memory +offset of a particular field. +@item @code{compareAndSwap*(Object,long,*,*)} -- One of these methods is +provided for each of int, long and Object (hence the *s). The value of +a field pointed to by the given Object and offset is compared with the +first value and replaced with the second if they are the same. The reason +for this method is to make this change operation atomic. +@item @code{put/get*(Object,long,*)} -- These are like the last set of +methods, handling integers, longs and Objects, but the field is always +changed on a put. Different methods are provided for different semantics. +Ordered variants perform a lazy put, in that the change does not +immediately propogate to other threads, while the others provide +volatile or 'normal' semantics. +@item @code{arrayBaseOffset(Class)} and @code{arrayIndexScale(Class)} -- +These two methods allow an array class to be traversed by pointer +arithmetic, by gaining the address of the first element and then +scaling appropriately for the later ones. +@item @code{park(boolean,long)} and @code{unpark(Thread)} -- These methods +block and unblock threads respectively, with an optional timeout being +provided for the blocking. @code{unpark} is unsafe as the thread may have +been destroyed by native code. +@end itemize + +@node java.util, java.io, gnu.classpath, Classpath Hooks +@section java.util + +The @code{java.util} VM hooks provide links between the mix of functionality +present in that package, which includes collections, date and time handling +and parsing. At present, there is only one hook, which connects GNU Classpath +to the timezone information provided by the underlying platform. + +@menu +* java.util.VMTimeZone:: +@end menu + +@node java.util.VMTimeZone,,java.util,java.util +@subsection @code{java.util.VMTimeZone} + +@code{VMTimeZone} joins @code{TimeZone} to the platform timezone information +via the static method, @code{getDefaultTimeZoneId()}. The VM hook is +expected to return a @code{TimeZone} instance that represents the current +timezone in use by the platform. The default implementation provides +this functionality for POSIX or GNU-like systems, and VMs that want this +functionality can keep this implementation and implement the native +method, @code{getSystemTimeZoneId()}. This method is only called when +obtaining the timezone name from the @code{TZ} environment variable, +@code{/etc/timezone} and @code{/etc/localtime} all fail. This fallback +mechanism also means that a system which doesn't provide the above three +methods, but does provide a timezone in string form, can still use this +implementation. + +@node java.io, java.security, java.util, Classpath Hooks +@section java.io + +The @code{java.io} package is heavily reliant on access to the I/O facilities +of the underlying platform. As far as its VM hooks go, they provide two +areas of functionality to GNU Classpath, these being + +@itemize @bullet +@item File and directory queries and manipulation +@item Serialization of objects +@end itemize + +The first corresponds directly to most of the @code{File} class, while +the latter underlies the functionality provided by the +@code{ObjectInputStream} and @code{ObjectOutputStream}. More low-level I/O +is provided by @ref{java.nio}. + +@menu +* java.io.VMFile:: +* java.io.VMObjectInputStream:: +* java.io.VMObjectStreamClass:: +@end menu + +@node java.io.VMFile,java.io.VMObjectInputStream,java.io,java.io +@subsection @code{java.io.VMFile} + +@code{VMFile} allows GNU Classpath's @code{File} representations to +probe and modify the file system using the native functions of the +platform. The default implementation (which consists of both a +@code{VMFile} class and the native methods) is primarily UNIX-centric, +working with POSIX functions and assuming case-sensitive filenames, +without the restriction of the 8.3 format. It consists mainly of +@code{static} @code{native} methods, with a few Java helper methods. +The native methods represent the file as a string containing its path, +rather than using the object itself. + +@itemize @bullet +@item Native Methods +@itemize @bullet +@item @code{lastModified(String)} -- The native method should return a +@code{long} value that represents the last modified date of the file. +@item @code{setReadOnly(String)} -- Sets the file's permissions to read only, +in whichever way this is realised by the platform. +@item @code{create(String)} -- Create the named file. +@item @code{list(String)} -- The native method opens the named directory, +reads the contents and returns them as a Java @code{String} array. +@item @code{renameTo(String,String)} -- Renames the first file to the second. +@item @code{length(String)} -- Returns a @code{long} value representing +the file size. +@item @code{exists(String)} -- Tests for the existence of the named file +or directory. +@item @code{delete(String)} -- Deletes the file or directory. +@item @code{setLastModified(String,long)} -- Change the last modified time. +@item @code{mkdir(String)} -- Creates the named directory. +@item @code{isFile(String)} -- Tests that the named path references a file. +@item @code{canWrite(String)} -- Tests that the file can be written to. +This method is @code{synchronized}, so the object is locked during the check. +@item @code{canRead(String)} -- Complement of the last method. +@item @code{isDirectory(String)} -- Tests that the named path references +a directory. +@end itemize +@item Java Helper Methods +@itemize @bullet +@item @code{canWriteDirectory(File)} -- Checks that the directory can be +written to, by trying to create a temporary file in it. +@item @code{listRoots()} -- Returns the root of a GNU filesystem, i.e.@: `/' +in an array. +@item @code{isHidden(String)} -- Checks whether the file starts with `.', +which is how files are hidden on UNIX-style systems. +@item @code{getName(String)} -- Pulls the actual filename from the end of +the path, by breaking off the characters after the last occurrence of the +platform's file separator. +@item @code{getCanonicalForm(String)} -- This converts a UNIX path to +its canonical form by removing the `.' and `..' sections that occur within. +@end itemize +@end itemize + +@node java.io.VMObjectInputStream,java.io.VMObjectStreamClass,java.io.VMFile,java.io +@subsection @code{java.io.VMObjectInputStream} + +This class consists of two methods which provide functionality used in +deserializing an object. @code{currentClassLoader()} provides the first +user-defined class loader from the class context +(@xref{gnu.classpath.VMStackWalker},) via a @code{PrivilegedAction}. +@code{allocateObject(Class,Class,Constructor)} is a @code{native} method +(a reference implementation is provided) which creates an object but +calls the constructor of another class, which is a superclass of the +object's class. + +@node java.io.VMObjectStreamClass,,java.io.VMObjectInputStream,java.io +@subsection @code{java.io.VMObjectStreamClass} + +@code{VMObjectStreamClass} is a series of @code{static} @code{native} +methods that provide some of the groundwork for @code{ObjectStreamClass} +and @code{ObjectStreamField}. @code{hasClassInitializer(Class)} works +with the former, and checks for the presence of a static initializer. +The remaining methods are of the form @code{setXXXNative(Field,Object,XXX)} +and support @code{ObjectStreamField}. One exists for each of the main types +(boolean, float, double, long, int, short, char, byte and object) and is used +to set the specified field in the supplied instance to the given value. + +A default implementation is provided for all of them, so a VM implementation +is optional. + +@node java.security, java.net, java.io, Classpath Hooks +@section java.security + +The @code{java.security} package provides support for Java's security +architecture. + +@menu +* java.security.VMAccessController:: +* java.security.VMSecureRandom:: +@end menu + +@node java.security.VMAccessController,java.security.VMSecureRandom,java.security,java.security +@subsection @code{java.security.VMAccessController} + +The @code{AccessController} is used to perform privileged actions. Its +hook class, @code{VMAccessController}, maintains the +@code{AccessControlContext} and the default implementation is purely +Java-based. The VM may choose to replace this with their own. +The methods in the reference version are as follows: + +@itemize @bullet +@item @code{pushContext(AccessControlContext)} -- Adds a new context to the +stack for the current thread. This is called before a privileged action +takes place. +@item @code{popContext()} -- Removes the top context from the stack. This +is performed after the privileged action takes place. +@item @code{getContext()} -- Either derives a context based on the +@code{ProtectionDomain}s of the call stack (see the next method) or returns +the top of the context stack. +@item @code{getStack()} -- Provides access to the call stack as a pair of +arrays of classes and method names. The actual implementation returns +an empty array, indicating that there are no permissions. +@end itemize + +@node java.security.VMSecureRandom,,java.security.VMAccessController,java.security +@subsection @code{java.security.VMSecureRandom} + +The @code{VMSecureRandom} class is used to provide access to +cryptographically secure random numbers. The default implementation +of the class runs eight threads that increment counters in a tight +loop, and XORs each counter to produce one byte of seed data. This is +not very efficient, and is not guaranteed to be random (the thread +scheduler is probably deterministic, after all). VM implementors +should provide a version of this class, which implements the method +@code{generateSeed(byte[],int,int)}, so that it fills the buffer using +a random seed from a system facility, such as a system entropy +gathering device or hardware random number generator. The parameters +are the usual set of buffer, offset and length and the method returns +the number of bytes actually generated, which may be less than that +requested. + +@node java.net, java.nio, java.security, Classpath Hooks +@section java.net + +The @code{java.net} package is heavily reliant on access to the networking +facilities of the underlying platform. The VM hooks provide information +about the available network interfaces, and access to lookup facilities +for network addresses. + +@menu +* java.net.VMInetAddress:: +* java.net.VMNetworkInterface:: +@end menu + +@node java.net.VMInetAddress,java.net.VMNetworkInterface,java.net,java.net +@subsection @code{java.net.VMInetAddress} + +@code{VMInetAddress} is a series of @code{static} @code{native} methods +which provide access to the platform's lookup facilities. All the methods +are implemented by GNU Classpath, making VM implementation optional, and +are as follows: + +@itemize @bullet +@item @code{getLocalHostname()} -- Wraps the @code{gethostname} function, and +falls back on `localhost'. +@item @code{lookupInaddrAny()} -- Returns the value of @code{INADDR_ANY}. +@item @code{getHostByAddr(byte[])} -- Looks up the hostname based on an IP +address. +@item @code{getHostByName(String)} -- The reverse of the last method, it +returns the IP addresses which the given host name resolves to. +@end itemize + +@node java.net.VMNetworkInterface,,java.net.VMInetAddress,java.net +@subsection @code{java.net.VMNetworkInterface} + +@code{VMNetworkInterface} currently consists of a single @code{static} +@code{native} method, @code{getInterfaces()}, which retrieves the +network interfaces available on the underlying platform as a @code{Vector}. +The current GNU Classpath implementation is a native stub. + +@node java.nio, java.nio.channels, java.net, Classpath Hooks +@section java.nio + +The @code{java.nio} package is part of the New I/O framework added in +Java 1.4. This splits I/O into the concepts of @emph{buffers}, +@emph{charsets}, @emph{channels} and @emph{selectors}, and +@code{java.nio} defines the buffer classes. As far as native and VM +code is concerned, the new package needs support for low-level efficient +buffer operations. + +@menu +* java.nio.VMDirectByteBuffer:: +@end menu + +@node java.nio.VMDirectByteBuffer,,java.nio,java.nio +@subsection @code{java.nio.VMDirectByteBuffer} + +A @code{ByteBuffer} maintains a buffer of bytes, and allows it to be +manipulated using primitive operations such as @code{get}, @code{put}, +@code{allocate} and @code{free}. A direct buffer avoids intermediate +copying, and uses native data which shouldn't be manipulated by a +garbage collector. The VM class consists of @code{static} @code{native} +methods, all of which are given default implementations by GNU +Classpath. + +@itemize @bullet +@item @code{init()} -- Creates an instance of an appropriate +@code{gnu.classpath.RawData} class. This class is not garbage +collected, is created natively and is used in the other methods to reference +the buffered data. +@item @code{allocate(int)} -- Allocates the memory for the buffer using +@code{malloc} and returns a reference to the @code{RawData} class. +@item @code{free(RawData)} -- Frees the memory used by the buffer. +@item @code{get(RawData,int)} -- Returns the data at the specified index. +@item @code{get(RawData,int,byte[],int,int)} -- Copies a section of the +data into a byte array using @code{memcpy}. +@item @code{put(RawData,int,byte)} -- Puts the given data in the buffer +at the specified index. +@item @code{adjustAddress(RawData,int)} -- Adjusts the pointer into the buffer. +@item @code{shiftDown(RawData,int,int,int)} -- Moves the content of the buffer +at an offset down to a new offset using @code{memmove}. +@end itemize + +@node java.nio.channels, gnu.java.nio, java.nio, Classpath Hooks +@section java.nio.channels + +Channels provide the data for the buffers with the New I/O packages. +For example, a channel may wrap a file or a socket. The VM hooks, +at the moment, simply allow the channels to be accessed by @code{java.io} +streams. + +@menu +* java.nio.channels.VMChannels:: +@end menu + +@node java.nio.channels.VMChannels,,java.nio.channels,java.nio.channels +@subsection @code{java.nio.channels.VMChannels} + +@code{VMChannels} provides the methods that create the channels or +streams. The default implementation is in pure Java and simply wraps +the channels in standard I/O classes from @code{java.io}. + +@itemize @bullet +@item @code{createStream(Class,Channel)} -- Creates a @code{FileChannel} +which wraps an instance of the specified stream class, created by reflection. +This method is private, and is used by the other two. +@item @code{newInputStream(ReadableByteChannel)} -- Wraps the channel +in a @code{FileInputStream}. +@item @code{newOutputStream(WritableByteChannel)} -- Wraps the channel +in a @code{FileOutputStream}. +@end itemize + +@node gnu.java.nio, java.lang.reflect, java.nio.channels, Classpath Hooks +@section gnu.java.nio + +The @code{gnu.java.nio} class provides Classpath implementations of the +interfaces provided by @code{java.nio}. The VM classes provide the native +support necessary to implement @emph{pipes} and @emph{selectors}. + +@menu +* gnu.java.nio.VMPipe:: +* gnu.java.nio.VMSelector:: +@end menu + +@node gnu.java.nio.VMPipe,gnu.java.nio.VMSelector,gnu.java.nio,gnu.java.nio +@subsection @code{gnu.java.nio.VMPipe} + +@code{VMPipe} provides the native functionality for a uni-directional pipe +between a source and a destination (sink) channel. It consists of one +@code{static} @code{native} method, @code{init(PipeImpl,SelectorProvider)}, +the reference implementation of which is currently a native stub. Ideally, +this should initialise the pipe at the native level. + +@node gnu.java.nio.VMSelector,,gnu.java.nio.VMPipe,gnu.java.nio +@subsection @code{gnu.java.nio.VMSelector} + +A @code{Selector} selects between multiple @code{SelectableChannel}s based +on their readiness and a key set. The VM hook for the Classpath implementation +of this is @code{VMSelector}, and this allows the actual @code{select()} +operation to be performed. This is represented by the @code{static} +@code{native} method, @code{select(int[],int[],int[],long)}, and a default +implementation of this is provided. + +@node java.lang.reflect, gnu.java.lang, gnu.java.nio, Classpath Hooks +@section @code{java.lang.reflect} +@code{java.lang.reflect} provides the interface to Java's reflection +facilities. Via reflection, programmers can obtain type information about +a particular instance at runtime or dynamically create new instances. + +@menu +* java.lang.reflect.VMArray:: +@end menu + +@node java.lang.reflect.VMArray,,,java.lang.reflect +@subsection @code{java.lang.reflect.VMArray} + +The @code{VMArray} class provides a hook, @code{createObjectArray}, +which the VM uses to generate a new non-primitive array of a +particular class and size. The default implementation simply passes +the job down to the standard JNI function, @code{NewObjectArray}. + +@node gnu.java.lang, gnu.java.lang.management, java.lang.reflect, Classpath Hooks +@section @code{gnu.java.lang} + +@code{gnu.java.lang} provides VM interfaces for the GNU +implementations of features in java.lang. Currently, this includes the +implementation of instrumentation. + +@menu +* gnu.java.lang.VMInstrumentationImpl:: +@end menu + +@node gnu.java.lang.VMInstrumentationImpl,,,gnu.java.lang +@subsection @code{gnu.java.lang.VMInstrumentationImpl} + +The @code{gnu.java.lang.VMInstrumentationImpl} and +@code{gnu.java.lang.InstrumentationImpl} classes provide an implementation of the +@code{java.lang.instrument.Instrument} interface. +A @code{InstrumentationImpl} object should be created by the VM when agents +are given in the command line (see the @code{java.lang.instrument} package +documentation). The VM has to set the static field +@code{VMClassLoader.instrumenter} to this object. The VM should implement the +static native methods of the @code{VMInstrumentationImpl} class. + +@itemize @bullet +@item @code{isRedefineClassesSupported()} -- Returns true if the JVM supports +class redefinition. +@item @code{redefineClasses()} -- Gives a set of classes with new bytecodes. +The VM must redefine the classes by reading the new bytecodes. +@item @code{getAllLoadedClass()} -- Returns an array of all loaded classes. +@item @code{getInitiatedClass()} -- Returns an array of all classes loaded +by a specific class loader. +@item @code{getObjectSize()} -- Gives the size of an object. +@end itemize + +Instrumentation allows to modify the bytecode of a class before it gets read +by the VM@. In GNU Classpath, the @code{ClassLoader.defineClass} method calls +the @code{VMClassLoader.defineClassWithTransformers} method which first checks +if @code{VMClassLoader.instrumenter} is @code{null}. If it's the case, it +directly calls @code{VMClassLoader.defineClass}. If it's not the case, the +method calls at first the @code{InstrumentationImpl.callTransformers} method, +which calls each transformer registered to the @code{InstrumentationImpl} +object and returns a new bytecode array. Then, it calls the +@code{VMClassLoader.defineClass} method with this new bytecode array. + +The second use of instrumentation is to redefine a class after it has been +loaded by the VM@. This is done in the Java application by calling the +@code{Instrumentation.redefineClasses} method of the standard interface on +a @code{Instrumentation} object. The @code{InstrumentationImpl.redefineClasses} +method calls the @code{VMInstrumentationImpl.redefineClasses} native method +which must be implemented by the VM@. The implementation should call the +@code{InstrumentationImpl.callTransformers} method. + +@node gnu.java.lang.management, java.lang.management, gnu.java.lang, Classpath Hooks +@section @code{gnu.java.lang.management} + +@code{gnu.java.lang.management} provides the VM interfaces for the GNU +implementations of the management beans. + +@menu +* gnu.java.lang.management.VMRuntimeMXBeanImpl:: +* gnu.java.lang.management.VMClassLoadingMXBeanImpl:: +* gnu.java.lang.management.VMThreadMXBeanImpl:: +* gnu.java.lang.management.VMMemoryMXBeanImpl:: +* gnu.java.lang.management.VMCompilationMXBeanImpl:: +* gnu.java.lang.management.VMMemoryPoolMXBeanImpl:: +* gnu.java.lang.management.VMMemoryManagerMXBeanImpl:: +* gnu.java.lang.management.VMGarbageCollectorMXBeanImpl:: +@end menu + +@node gnu.java.lang.management.VMRuntimeMXBeanImpl,gnu.java.lang.management.VMClassLoadingMXBeanImpl,,gnu.java.lang.management +@subsection @code{gnu.java.lang.management.VMRuntimeMXBeanImpl} + +The @code{gnu.java.lang.management.RuntimeMXBeanImpl} provides an +implementation of the @code{java.lang.management.RuntimeMXBean} interface, +and is supported by VM functionality in the form of +@code{gnu.java.lang.management.VMRuntimeMXBeanImpl}. This provides a +series of methods, which should be implemented by the virtual machine +in order to provide the required information for the bean. The VM +methods are generally representative of information that is only +available from the virtual machine, such as the command-line arguments +it was given at startup. + +The methods are as follows: + +@itemize @bullet +@item @code{(getInputArguments())} -- The VM should supply +a @code{String} array containing each of the command-line +arguments, excluding those that are directed at the +@code{main()} method. The reference implementation expects +this to be a native method. +@item @code{(getName())} -- The VM developer should choose +an appropriate name for the virtual machine. This name can +be instance-specific e.g.@: it can include things like the +process identifier or host name of the machine, which only +apply to the current running instance. Thus, the intention is +that this name refers to the entity that the other information +refers to, rather than the VM in general. The reference +implementation supplies a default concatenation of the VM +name and version. +@item @code{(getStartTime())} -- This should return the number +of milliseconds at which the virtual machine was started. +The uptime property of the bean is provided relative to this +value. Again, the reference implementation also expects +this method to be native. +@end itemize + +The virtual machine also needs to provide either the +@code{sun.boot.class.path} or @code{java.boot.class.path} +property in order to support the optional boot class path +retrieval functionality. + +@node gnu.java.lang.management.VMClassLoadingMXBeanImpl,gnu.java.lang.management.VMThreadMXBeanImpl,gnu.java.lang.management.VMRuntimeMXBeanImpl,gnu.java.lang.management +@subsection @code{gnu.java.lang.management.VMClassLoadingMXBeanImpl} + +The @code{gnu.java.lang.management.ClassLoadingMXBeanImpl} provides an +implementation of the @code{java.lang.management.ClassLoadingMXBean} interface, +and is supported by VM functionality in the form of +@code{gnu.java.lang.management.VMClassLoadingMXBeanImpl}. This provides a +series of methods, which should be implemented by the virtual machine +in order to provide the required information for the bean. Implementing +this bean requires the VM to monitor when classes are loaded and unloaded, +and provide the option of verbose class loading output. + +The methods are as follows: + +@itemize @bullet +@item @code{(getLoadedClassCount())} -- This should return +the number of classes that are currently loaded by the VM. +@item @code{(getUnloadedClassCount())} -- This should return +the number of classes that have been loaded by the VM, but +have since been unloaded. +@item @code{(isVerbose())} -- This should return @code{true} +or @code{false}, depending on whether verbose class loading +output is turned or not, respectively. +@item @code{(setVerbose(boolean))} -- This should allow the +verbose class loading output to be turned on and off. +@end itemize + +@node gnu.java.lang.management.VMThreadMXBeanImpl,gnu.java.lang.management.VMMemoryMXBeanImpl,gnu.java.lang.management.VMClassLoadingMXBeanImpl,gnu.java.lang.management +@subsection @code{gnu.java.lang.management.VMThreadMXBeanImpl} + +The @code{gnu.java.lang.management.ThreadMXBeanImpl} provides an +implementation of the @code{java.lang.management.ThreadMXBean} interface, +and is supported by VM functionality in the form of +@code{gnu.java.lang.management.VMThreadMXBeanImpl}. This provides a +series of methods, which should be implemented by the virtual machine +in order to provide the required information for the bean. Implementing +this bean requires the VM to monitor thread-related statistics such as +how often the blocked and waiting states have been entered, as well as +additional optional support for time and contention monitoring. + +Optional support is determined by the following properties: + +@itemize @bullet +@item @code{gnu.java.lang.management.CurrentThreadTimeSupport} -- +This property should be present if the VM supports monitoring the +time used by the current thread. If time monitoring for all threads +is supported, this need not be provided. +@item @code{gnu.java.lang.management.ThreadTimeSupport} -- +This property should be present if the VM supports monitoring the +time used by all threads. +@item @code{gnu.java.lang.management.ThreadContentionSupport} -- +This property should be present if the VM supports thread contention +monitoring. +@item @code{gnu.java.lang.management.MonitorUsageMonitoringSupport} -- +This property should be present if the VM supports the monitoring +of object monitor usage. +@item @code{gnu.java.lang.management.OwnableSynchronizerUsageMonitoringSupport} -- +This property should be present if the VM supports the monitoring +of ownable synchronizer usage. +@end itemize + +In addition, the property +@code{gnu.java.lang.management.ThreadTimeInitallyEnabled} may be +set to the @code{String} value, @code{"true"}, if time monitoring +is enabled at startup. + +The methods are as follows: + +@itemize @bullet +@item @code{(findDeadlockedThreads())} -- This should return +an array of thread identifiers which match threads involved in +deadlock cycles (where each thread is waiting to obtain a lock +held by one of the others) on object monitors or ownable +synchronizers. This is specified as a native method in the +reference implementation, and is optional. It is only called +when the VM supports ownable synchronizer monitoring. +@item @code{(findMonitorDeadlockedThreads())} -- This should return +an array of thread identifiers which match threads involved in +deadlock cycles (where each thread is waiting to obtain a lock +held by one of the others) on object monitors. This is specified +as a native method in the reference implementation. +@item @code{(getAllThreads())} -- This should return an array of +all live threads and set the @code{filled} variable to the number +found. A default implementation is provided. +@item @code{(getAllThreadIds())} -- This should return an array of +all live thread identifiers. An implementation is provided against +@code{getAllThreads()} by default. +@item @code{(getCurrentThreadCpuTime())} -- This should return the +approximate number of nanoseconds of CPU time the current thread +has used. This is an optional native method, which is used by VMs +supporting time monitoring. +@item @code{(getCurrentThreadUserTime())} -- This should return the +approximate number of nanoseconds of user time the current thread +has used. This is an optional native method, which is used by VMs +supporting time monitoring. +@item @code{(getDaemonThreadCount())} -- This should return the number +of live daemon threads. A default implementation is provided, based +on @code{getAllThreads()}. +@item @code{(getLockInfo(ThreadInfo))} -- This is an optional native +method called when the VM supports ownable synchronizer usage monitoring +and the user has requested information for a particular thread. The +supplied @code{ThreadInfo} object should be filled out with an +array of @code{LockInfo} objects, providing details on each lock. +@item @code{(getMonitorInfo(ThreadInfo))} -- This is an optional native +method called when the VM supports object monitor usage monitoring +and the user has requested information for a particular thread. The +supplied @code{ThreadInfo} object should be filled out with an +array of @code{MonitorInfo} objects, providing details on each lock. +@item @code{(getPeakThreadCount())} -- The VM should maintain a record +of the peak number of live threads, and return it when this method is +called. This is specified as a native method in the reference +implementation. +@item @code{(resetPeakThreadCount())} -- This should reset the record +of the peak number of live threads to the current number of live +threads. This is specified as a native method in the reference +implementation. +@item @code{(getThreadCount())} -- This should return the number of +live threads. A default implementation is provided, based on +@code{getAllThreads()}. +@item @code{(getThreadCpuTime(long))} -- This should return the +approximate number of nanoseconds of CPU time the specified thread +has used. This is an optional native method, which is used by VMs +supporting time monitoring. +@item @code{(getThreadUserTime(long))} -- This should return the +approximate number of nanoseconds of CPU time the specified thread +has used. This is an optional native method, which is used by VMs +supporting time monitoring. +@item @code{(getThreadInfoForId(long, int))} -- This return an instance +of @code{java.lang.management.ThreadInfo} for the specified thread. +The class includes a private constructor which VMs should use to initialise +it with the appropriate values for the thread. The second argument +given here specifies the depth of the stack trace supplied on construction +of the instance. Special values are 0 (return an empty array) and +@code{Integer.MAX_VALUE} (return the maximum depth possible). This +is specified as a native method in the reference implementation. +@item @code{(getTotalStartedThreadCount())} -- This should return the +total number of threads that have been started by the VM, including ones +that have died. This is specified as a native method in the reference +implementation. +@end itemize + +@node gnu.java.lang.management.VMMemoryMXBeanImpl,gnu.java.lang.management.VMCompilationMXBeanImpl,gnu.java.lang.management.VMThreadMXBeanImpl,gnu.java.lang.management +@subsection @code{gnu.java.lang.management.VMMemoryMXBeanImpl} + +The @code{gnu.java.lang.management.MemoryMXBeanImpl} provides an +implementation of the @code{java.lang.management.MemoryMXBean} interface, +and is supported by VM functionality in the form of +@code{gnu.java.lang.management.VMMemoryMXBeanImpl}. This provides a +series of methods, which should be implemented by the virtual machine +in order to provide the required information for the bean. Implementing +this bean requires the VM to monitor the levels of heap and non-heap +memory, and provide the number of objects which are eligible for garbage +collection. + +The methods are as follows: + +@itemize @bullet +@item @code{(getHeapMemoryUsage())} -- This should return +an instance of @code{java.lang.management.MemoryUsage} with +values pertaining to the heap. A default implementation is +provided, based on @code{java.lang.Runtime}'s methods. +@item @code{(getNonHeapMemoryUsage())} -- This should return +an instance of @code{java.lang.management.MemoryUsage} with +values pertaining to non-heap memory. +@item @code{(getObjectPendingFinalizationCount())} -- Returns +the number of objects which are no longer referenced, and which +will thus be garbage collected on the next run of the garbage +collector. +@item @code{(isVerbose())} -- This should return @code{true} +or @code{false}, depending on whether verbose memory management +output is turned or not, respectively. +@item @code{(setVerbose(boolean))} -- This should allow the +verbose memory management output to be turned on and off. +@end itemize + +@node gnu.java.lang.management.VMCompilationMXBeanImpl,gnu.java.lang.management.VMMemoryPoolMXBeanImpl,gnu.java.lang.management.VMMemoryMXBeanImpl,gnu.java.lang.management +@subsection @code{gnu.java.lang.management.VMCompilationMXBeanImpl} + +The @code{gnu.java.lang.management.CompilationMXBeanImpl} provides an +implementation of the optional @code{java.lang.management.CompilationMXBean} +interface, and is supported by VM functionality in the form of +@code{gnu.java.lang.management.VMCompilationMXBeanImpl}. This provides a +single method for returning the number of milliseconds the virtual +machine's Just-In-Time (JIT) compiler has spent compiling. Even if +a JIT compiler is available and an instance of the bean supplied, this +method is still optional. + +Optional support is determined by the following properties: + +@itemize @bullet +@item @code{gnu.java.lang.compiler.name} -- This property should +specify the name of the JIT compiler. Classpath also uses this, +within @code{java.lang.management.ManagementFactory}, to determine +whether a bean should be created. If this property is set to a +non-null value, a bean will be created and its @code{getName()} +method will return this value. +@item @code{gnu.java.lang.management.CompilationTimeSupport} -- +This property should be present if the VM supports monitoring the +time spent compiling. +@end itemize + +Time support is implemented by the following method: + +@itemize @bullet +@item @code{(getTotalCompilationTime())} -- This should return the +number of milliseconds the JIT compiler has spent compiling. +@end itemize + +@node gnu.java.lang.management.VMMemoryPoolMXBeanImpl,gnu.java.lang.management.VMMemoryManagerMXBeanImpl,gnu.java.lang.management.VMCompilationMXBeanImpl,gnu.java.lang.management +@subsection @code{gnu.java.lang.management.VMMemoryPoolMXBeanImpl} + +The @code{gnu.java.lang.management.MemoryPoolMXBeanImpl} provides an +implementation of the optional @code{java.lang.management.MemoryPoolMXBean} +interface, and is supported by VM functionality in the form of +@code{gnu.java.lang.management.VMMemoryPoolMXBeanImpl}. Providing +this interface requires implementing a number of methods for each supported +pool. These return statistics on memory usage, and, optionally, allows +monitoring of when memory usage exceeds a preset threshold. + +Optional support is determined by the following properties: + +@itemize @bullet +@item @code{gnu.java.lang.management.CollectionUsageThresholdSupport} -- +This property should be present if the VM supports setting a collection +usage threshold and monitoring when it is matched or exceeded. Collection +usage thresholds are related to the remaining memory usage following a +garbage collection cycle. +@item @code{gnu.java.lang.management.UsageThresholdSupport} -- +This property should be present if the VM supports setting a +usage threshold and monitoring when it is matched or exceeded. +@end itemize + +The methods are as follows (all take a pool name as their +first parameter): + +@itemize @bullet +@item @code{(getCollectionUsage(String))} -- Returns a +@code{java.lang.management.MemoryUsage} object, containing the +memory usage statistics following a garbage collection cycle +for the specified pool. This may also return @code{null} if +the pool isn't an appropriate pool for this particular task. +@item @code{(getCollectionUsageThreshold(String))} -- Returns +the pool's collection usage threshold, if supported. +@item @code{(getCollectionUsageThresholdCount(String))} -- Returns +the number of times the specified pool has matched or exceeded +its collection usage threshold, if supported. +@item @code{(getMemoryManagerNames(String))} -- Returns a list +of names of memory managers which manage the specified pool. +@item @code{(getPeakUsage(String))} -- Returns a +@code{java.lang.management.MemoryUsage} object for the peak +usage level of the specified pool. +@item @code{(getType(String))} -- Returns a string containing +either @code{"HEAP"} or @code{"NON_HEAP"} which indicates the type of +memory used by the specified pool. +@item @code{(getUsage(String))} -- Returns a +@code{java.lang.management.MemoryUsage} object for the current +usage level of the specified pool. +@item @code{(getUsageThreshold(String))} -- Returns +the pool's usage threshold, if supported. +@item @code{(getUsageThresholdCount(String))} -- Returns +the number of times the specified pool has matched or exceeded +its usage threshold, if supported. +@item @code{(isValid(String))} -- Returns true if the pool +is still in use by the virtual machine. +@item @code{(resetPeakUsage(String))} -- Resets the peak usage +levels to the current usage levels for the specified pool. +@item @code{(setCollectionUsageThreshold(String, long))} -- Sets +the pool's collection usage threshold, if supported. +@item @code{(setUsageThreshold(String, long))} -- Sets +the pool's usage threshold, if supported. +@end itemize + +@node gnu.java.lang.management.VMMemoryManagerMXBeanImpl,gnu.java.lang.management.VMGarbageCollectorMXBeanImpl,gnu.java.lang.management.VMMemoryPoolMXBeanImpl,gnu.java.lang.management +@subsection @code{gnu.java.lang.management.VMMemoryManagerMXBeanImpl} + +The @code{gnu.java.lang.management.MemoryManagerMXBeanImpl} provides an +implementation of the optional @code{java.lang.management.MemoryManagerMXBean} +interface, and is supported by VM functionality in the form of +@code{gnu.java.lang.management.VMMemoryManagerMXBeanImpl}. Providing +this interface requires implementing two methods (each takes the name +of the manager as the first argument): + +@itemize @bullet +@item @code{(getMemoryPoolNames(String))} -- Returns a list of the +memory pools that the manager maintains. A default implementation +which scans the results of @code{getMemoryManagerNames()} for each +pool is provided. +@item @code{(isValid(String))} -- Returns true if the specified +manager is still valid, i.e., it is still in use by the virtual machine. +@end itemize + +@node gnu.java.lang.management.VMGarbageCollectorMXBeanImpl,,gnu.java.lang.management.VMMemoryManagerMXBeanImpl,gnu.java.lang.management +@subsection @code{gnu.java.lang.management.VMGarbageCollectorMXBeanImpl} + +The @code{gnu.java.lang.management.GarbageCollectorMXBeanImpl} provides an +implementation of the optional @code{java.lang.management.GarbageCollectorMXBean} +interface, and is supported by VM functionality in the form of +@code{gnu.java.lang.management.VMGarbageCollectorMXBeanImpl}. Providing +this interface requires implementing two methods (each takes the name +of the garbage collector as the first argument): + +@itemize @bullet +@item @code{(getCollectionCount(String))} -- Returns the number of +times the specified garbage collector has run. +@item @code{(getCollectionTime(String))} -- Returns the accumulated +number of milliseconds for which the garbage collector has run. +@end itemize + +Note that each garbage collector is also a memory manager, and so an +implementation of the @code{gnu.java.lang.management.VMMemoryManagerMXBeanImpl} +methods for its name should also be provided. + +@node java.lang.management, Classpath Callbacks, gnu.java.lang.management, Classpath Hooks +@section @code{java.lang.management} + +@code{gnu.java.lang.management} provides the VM interfaces for the GNU +implementations of the management beans. + +@menu +* java.lang.management.VMManagementFactory:: +@end menu + +@node java.lang.management.VMManagementFactory,,,java.lang.management +@subsection @code{java.lang.management.VMManagementFactory} + +This VM interface provides the names of the memory pools, memory managers +and garbage collectors for use by the @code{java.lang.management.ManagementFactory} +in creating lists of appropriate beans for these types of managed object. + +The methods are as follows: + +@itemize @bullet +@item @code{(getMemoryPoolNames())} -- Returns a list of the names +of the current memory pools in use by the virtual machine. +@item @code{(getMemoryManagerNames())} -- Returns a list of the names +of the current memory managers in use by the virtual machine. This +should not include those that are also garbage collectors. +@item @code{(getGarbageCollectorNames())} -- Returns a list of the names +of the current garbage collectors in use by the virtual machine. +@end itemize + +@node Classpath Callbacks, , java.lang.management, Classpath Hooks +Some of the classes you implement for the VM will need to call back to +package-private methods in Classpath: + +@itemize @bullet +@item @code{java.lang.ThreadGroup.addThread(Thread)} +Call this method from @code{Thread} when a new @code{Thread} is created, to add it to +the group. + +@item @code{java.lang.ThreadGroup.removeThread(Thread)} +Call this method from @code{Thread} when a @code{Thread} is stopped or destroyed. + +@item @code{gnu.java.lang.management.MemoryMXBeanImpl.fireThresholdExceededNotification(String, long, long, long, long)} +If the monitoring of memory usage thresholds is supported, this method +should be called when the normal usage of a memory pool crosses the +threshold, in order to emit a notification. Another notification +should not be emitted until there is an intermittent period where the +usage is again below the threshold. The parameters are the memory +pool name, the usage levels (init, used, committed and max) and the +number of times the threshold has been crossed. + +@item @code{gnu.java.lang.management.MemoryMXBeanImpl.fireCollectionThresholdExceededNotification(String, long, long, long, long)} +If the monitoring of memory usage thresholds is supported, this method +should be called when the usage of a memory pool after a garbage +collection cycle crosses the threshold, in order to emit a +notification. Another notification should not be emitted until there +is an intermittent period where the usage is again below the +threshold. The parameters are the memory pool name, the usage levels +(init, used, committed and max) and the number of times the threshold +has been crossed. + +@end itemize + +@node VM Hooks, JNI Implementation, Classpath Hooks, Top +@comment node-name, next, previous, up +@chapter VM Hooks + +VMs need to do some dirty work; there are some things in the VM that +unfortunately are dependent on the internal structure of various +classes. This is a guide to all of the things the VM itself needs to +know about classes. + +Some of the core classes, while being implemented by GNU Classpath, +provide space for state (in the form of a @code{vmdata} object) to be +stored by the VM, and can not be constructed normally. + +@itemize @bullet +@item java.lang.Class +@item java.lang.ClassLoader +@end itemize + +The default implementations of some VM classes also follow this methodology, +when it is intended that most VMs will keep the default. + +@itemize @bullet +@item java.lang.VMThread +@item java.lang.VMThrowable +@end itemize + +Several core classes must be completely implemented by the VM for Classpath to +work, although reference implementations are provided. These classes are: + +@itemize @bullet +@item java.lang.reflect.Constructor +@item java.lang.reflect.Method +@item java.lang.reflect.Field +@end itemize + +The following issues are of note; + +@itemize @bullet +@item @code{java.lang.Class} @* +The GNU Classpath implementation of @code{java.lang.Class} provides an +object for storing the internal state of the class maintained by the VM. +This is the only known place where this matters. The class is +constructed with this data by the VM@. Some VMs do not create the +@code{Class} object at the point where the class is defined; instead, +they wait until a @code{Class} object is actually used. + +@item Array Classes @* +When you are creating an array class, you should set the +@code{ClassLoader} of the array class to the @code{ClassLoader} of its +component type. Whenever you add a class to a @code{ClassLoader}, you +need to notify the @code{ClassLoader} and add the new @code{Class} to +its internal cache of classes. To do this, call +@code{ClassLoader.addVMCreatedClass(Class)}. @emph{Note: this is +written in anticipation of 1.2 support and does not apply just yet.} + +@item Primordial Class Loader @* +When the primordial class loader loads a class, it needs to tell +Classpath what it has done in order for security stuff to work right. +To do this, call the static method +@code{ClassLoader.newPrimordialClass(Class)}. + +Even the first few core classes need to do this; in order to do it, +simply call this method @emph{after} the initial class loading has been +done. No harm will come, as long as you follow the guidelines in the +@pxref{Initialization} section. + +@emph{Note: this is written in anticipation of 1.2 support and does not +apply just yet.} + +@item Top-level Exception Handler @* +Exceptions take care of themselves in Classpath; all you need to do in +the top-level exception handler is call @code{Throwable.printStackTrace()}. + +@item Security and Traces @* +There will eventually be a feature in the 1.2 security that keeps the +@code{AccessController} from having to evaluate @emph{all} of the +@code{ProtectionDomain}s every time a security check is made. I think a common +case is a single method doing a lot of things that require security +checks. However, I don't want to bog down the method stack too much, so +this feature of the VM will have the @code{AccessController} for a thread +calling out to the VM to tell it how high it was on the stack when it +made the last security request. Every time the stack goes lower than +that number, the VM will decrement the number. The @code{AccessController} +will remember what the accumulated protection status was at every stack +level (an @code{AccessControlContext}) and use that aggregated information to +do the check. I am not sure, however, whether the savings are +substantial enough to outweigh the integer check and set after every +method call. I will investigate. + +@item Threading @* +I figured I'd put this here because a VM guy might be wondering about it. +We implement @code{ThreadGroup}, but that class is almost entirely +VM-independent. The root @code{ThreadGroup}, a static field called +@code{ThreadGroup.root}, should be initialized by Classpath, but if you wish to +reinitialize it yourself, there should be no harm. + +@end itemize + +@node JNI Implementation, JVMTI Implementation, VM Hooks, Top +@comment node-name, next, previous, up +@chapter JNI Implementation + +Classpath comes with its own implementation of @file{jni.h}. This +file can be customized by the VM in a few ways, by defining macros +that affect the interpretation of the file. These macros are all +intended for use by a VM which uses GNU Classpath and which wants to +use a single copy of @file{jni.h} for both internal and external use. + +@itemize @bullet +@item _CLASSPATH_VM_JNI_TYPES_DEFINED +Some VMs like to define JNI ``object'' types in a special way. If +this macro is defined, the Classpath @file{jni.h} will avoid defining +these types. By default, these types are defined in @file{jni.h}. +The full list of types and macros treated this way is: @samp{jobject}, +@samp{jclass}, @samp{jstring}, @samp{jthrowable}, @samp{jweak}, +@samp{jarray}, @samp{jobjectArray}, @samp{jbyteArray}, +@samp{jshortArray}, @samp{jintArray}, @samp{jlongArray}, +@samp{jbooleanArray}, @samp{jcharArray}, @samp{jfloatArray}, +@samp{jdoubleArray}, @samp{JNIEnv}, @samp{JavaVM}, @samp{JNI_TRUE} +(macro), @samp{JNI_FALSE} (macro). + +@item _CLASSPATH_VM_INTERNAL_TYPES_DEFINED +If the VM has its own definitions for @samp{jfieldID} and +@samp{jmethodID}, then it should define this macro. Otherwise, +@file{jni.h} will provide definitions for these types. + +@item _CLASSPATH_JNIIMPEXP +Three functions -- @samp{JNI_GetDefaultJavaVMInitArgs}, +@samp{JNI_CreateJavaVM}, and @samp{JNI_GetCreatedJavaVMs} -- must be +marked as @samp{JNIIMPORT} when seen by user code, but most likely +should be marked as @samp{JNIEXPORT} when defined in the VM +implementation. This macro can be defined to one or the other by the +VM as appropriate. If this macro is not defined, it defaults to +@samp{JNIIMPORT}. + +@item _CLASSPATH_JNIENV_CONTENTS +A VM can add fields to the @samp{JNIEnv} structure by defining this to +be a sequence of field declarations. + +@end itemize + +@node JVMTI Implementation, Miscellaneous VM Requirements, JNI Implementation, Top +@comment node-name, next, previous, up +@chapter JVMTI Implementation + +Classpath comes with its own implementation of @file{jvmti.h}. This +file can be customized by the VM in a few ways by defining macros that +affect the interpretation of the file. These macros are all intended +for use for use by a VM which uses GNU Classpath and which wants to +use a single copy of @file{jvmti.h} for both internal and external use. + +@itemize @bullet +@item _CLASSPATH_VM_JVMTI_TYPES_DEFINED +Some VMs like to define JVMTI ``object'' types in a special way. If +this macro is defined, the Classpath @file{jvmti.h} will avoid defining +these types. By default these types are defined in @file{jvmti.h}. +The full list of types and macros treated this way is: @samp{jthread}, +@samp{jthreadGroup}, @samp{jlocation}, and @samp{jrawMonitorID}. By +default @samp{jrawMonitorID} is defined as an opaque pointer which +must be defined by the VM. + +@item _CLASSPATH_JVMTIENV_CONTENTS +A VM can add fields to the @samp{jvmtiEnv} structure by defining this +to be a sequence of field declarations. + +@end itemize + +@node Miscellaneous VM Requirements, , JVMTI Implementation, Top +@comment node-name, next, previous, up +@chapter Miscellaneous VM Requirements + +Classpath places a few requirements on the VM that uses it. + +@menu +* JNI Version:: +* VM Threading Model:: +* Boot Library Path Property:: +@end menu + +@node JNI Version, VM Threading Model, Miscellaneous VM Requirements, Miscellaneous VM Requirements +@comment node-name, next, previous, up +@section JNI Version + +Classpath currently uses only JNI 1.1, except for one JNI 1.2 function +in the JNI Invocation API: GetEnv(). And GetEnv() is only used in the +now deprecated ``portable native sync'' code. + +A future direction will probably be to require that all VMs provide +JNI 1.2. If this poses problems, please raise them on the classpath +mailing list. + +@node VM Threading Model, Boot Library Path Property, JNI Version, Miscellaneous VM Requirements +@comment node-name, next, previous, up +@section VM Threading Model + +VM authors can implement a number of different threading models. When +native code is also threaded there is the potential for one threading +model to deadlock the other. The +@uref{http://java.sun.com/docs/books/jni/html/other.html#29406,Java +Native Interface Programmer's Guide and Specification} suggests +consulting VM documentation in such situations. Classpath uses +existing libraries, for example the AWT peers can use the GTK+ +graphics library. As these libraries assume a different threading +model, there is the potential for the native code to deadlock a VM. + +The different threading models available to a VM author are: +@enumerate +@item +@i{Native threads}: Map a Java thread to an underlying operating system +thread (normally a POSIX compatible pthread). This approach reduces +the potential for deadlock as there is only one thread scheduling +mechanism. +@item +@i{Green threads 1}: Green threads are threads scheduled by the VM, +typically by switching swapping registers. In early VMs green threads +were seen as advantageous as they didn't require the operating system +to reschedule, save and swap all of a threads registers. The green +thread 1 model switches thread on an externally created event, such as +a timer interrupt. An example of a VM using this approach is Kaffe +configured with its jthreads model. +@item +@i{Green threads 2}: The essential difference with this model is to +not switch threads on an event, but at fixed points in the code being +executed by the VM@. Points chosen could be backward branches (loops) +or method calls. This approach can be advantageous to nonconservative +garbage collectors, as non-running threads would be at known points +and can have fixed register maps. It can also reduce the number of +registers it is necessary to swap when switching threads. +@item +@i{M:N threading}: a flaw to green threading is that it is unable to +use multiple processors. @i{M}:@i{N} threading fixes this problem by +running groups of green threads on multiple underlying native +threads. An example of a VM using this approach is the Jikes RVM, +which uses @i{M}:@i{N} threading combined with the green thread 2 +model. +@end enumerate + +An example of the problem of mixing threading models is: +@itemize @bullet +@item +A Java thread calls a native method. The native method acquires a lock. +@item +The native method calls back into the VM. +@item +An event triggers the VM to reschedule the currently running thread. +@item +A new VM thread, executing on the same underlying native thread, calls +a native method. +@item +The native method tries to acquire the lock already acquired earlier. As +the lock is busy the thread waits and allows the operating system to +reschedule native threads. +@item +The operating system reschedules the VM thread again, but the lock is +still busy and in some threading models will remain busy forever +(the VM is deadlocked). +@end itemize + +VMs that don't use the underlying operating system thread scheduling +mechanism need to avoid deadlock. One now deprecated approach was to +build Classpath and VMs on top of a wrapper thread library (aka +portable native sync). The wrapper thread library used was GLIB's +@dfn{gthreads}. This approach has been deprecated because: +@enumerate +@item +The wrapper library is only in use by some native libraries. For +example, GTK+ uses the gthread library but QT does not. +@item +The wrapper library can't be in use prior to the VM starting as the VM +must replace the wrapper libraries functions with its own. This +prevents the VM from running as a plugin in an application that +already uses the wrapper library. +@end enumerate + +An alternative approach is for the VM to detect deadlocked native code +and swap Java threads off of that native thread. The VM can't, +however, swap two blocked native threads that are potentially +deadlocking each other on a lock. The lock will be associated with the +native thread. To prevent this from happening the VM must hijack +functions that operate on locks. This is done by redefining the lock +functions inside the VM and configuring the linker so that it uses the +VMs symbol in preference to that of the external thread support +library. The VM's lock function can then reschedule Java threads if it +must wait for the lock. + +@node Boot Library Path Property, , VM Threading Model, Miscellaneous VM Requirements +@comment node-name, next, previous, up +@section Boot Library Path Property + +As of GNU Classpath 0.15 a system property named @code{gnu.classpath.boot.library.path} +can be set by the VM to specify the directories which contain GNU Classpath's native +libraries. Usually this value is given at configuration time and is then hardcoded +in the VM@. However for development purposes it is handy to switch to another installation +by overriding the properties' value on the command line. + +A VM that does not support this feature can simply ignore the property. + +For compatibility reasons we suggest to set the default value of @code{java.library.path} +to the value of the @code{LD_LIBRARY_PATH} environment if it exists on your platform. + +@bye + + + |