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| author | The Android Open Source Project <initial-contribution@android.com> | 2009-03-03 19:28:47 -0800 |
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| committer | The Android Open Source Project <initial-contribution@android.com> | 2009-03-03 19:28:47 -0800 |
| commit | f6c387128427e121477c1b32ad35cdcaa5101ba3 (patch) | |
| tree | 2aa25fa8c8c3a9caeecf98fd8ac4cd9b12717997 /docs/debugmon.html | |
| parent | f72d5de56a522ac3be03873bdde26f23a5eeeb3c (diff) | |
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diff --git a/docs/debugmon.html b/docs/debugmon.html new file mode 100644 index 000000000..cf56ef514 --- /dev/null +++ b/docs/debugmon.html @@ -0,0 +1,736 @@ +<HTML> + + +<head> + <title>Dalvik VM Debug Monitor</title> + <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> + <link href="http://www.google.com/favicon.ico" type="image/x-icon" + rel="shortcut icon"> + <link href="../android.css" type="text/css" rel="stylesheet"> + <script language="JavaScript1.2" type="text/javascript"> +function highlight(name) { + if (document.getElementById) { + tags = [ 'span', 'div', 'tr', 'td' ]; + for (i in tags) { + elements = document.getElementsByTagName(tags[i]); + if (elements) { + for (j = 0; j < elements.length; j++) { + elementName = elements[j].getAttribute("id"); + if (elementName == name) { + elements[j].style.backgroundColor = "#C0F0C0"; + } else if (elementName && elementName.indexOf("rev") == 0) { + elements[j].style.backgroundColor = "#FFFFFF"; + } + } + } + } + } +} + </script> +</head> +<body onload="prettyPrint()"> + +<h1><a name="My_Project_"></a>Dalvik VM<br>Debug Monitor</h1> + +<!-- Status is one of: Draft, Current, Needs Update, Obsolete --> +<p style="text-align:center"><strong>Status:</strong><em>Draft</em> +<small>(as of March 6, 2007)</small></p> +<address> +[authors] +<address> + +<!-- last modified date can be different to the "Status date." It automatically +updates +whenever the file is modified. --> +<i>Modified:</i> + <!-- this script automatically sets the modified date,you don't need to modify +it --> + <script type=text/javascript> + <!-- + var lm = new Date(document.lastModified); + document.write(lm.toDateString()); + //--> + </script> +</address> + +<p><br> +<HR> + +<h2>Introduction</h2> + +<p>It's extremely useful to be able to monitor the live state of the +VM. For Android, we need to monitor multiple VMs running on a device +connected through USB or a wireless network connection. This document +describes a debug monitor server that interacts with multiple VMs, and +an API that VMs and applications can use to provide information +to the monitor. + +<p>Some things we can monitor with the Dalvik Debug Monitor ("DDM"): +<ul> + <li> Thread states. Track thread creation/exit, busy/idle status. + <li> Overall heap status, useful for a heap bitmap display or + fragmentation analysis. +</ul> + +<p>It is possible for something other than a VM to act as a DDM client, but +that is a secondary goal. Examples include "logcat" log extraction +and system monitors for virtual memory usage and load average. + +<p>It's also possible for the DDM server to be run on the device, with +the information presented through the device UI. However, the initial goal +is to provide a display tool that takes advantage of desktop tools and +screen real estate. + +<p>This work is necessary because we are unable to use standard JVMTI-based +tools with Dalvik. JVMTI relies on bytecode insertion, which is not +currently possible because Dalvik doesn't support Java bytecode. + +<p>The DDM server is written in the Java programming language +for portability. It uses a desktop +UI toolkit (SWT) for its interface. + + +<h2>Protocol</h2> + +<p>To take advantage of existing infrastructure we are piggy-backing the +DDM protocol on top of JDWP (the Java Debug Wire Protocol, normally spoken +between a VM and a debugger). To a +non-DDM client, the DDM server just looks like a debugger. + +<p>The JDWP protocol is very close to what we want to use. In particular: +<ul> + <li>It explicitly allows for vendor-defined packets, so there is no + need to "bend" the JDWP spec. + <li>Events may be posted from the VM at arbitrary points. Such + events do not elicit a response from the debugger, meaning the client + can post data and immediately resume work without worrying about the + eventual response. + <li>The basic protocol is stateless and asynchronous. Request packets + from the debugger side include a serial number, which the VM includes + in the response packet. This allows multiple simultaneous + conversations, which means the DDM traffic can be interleaved with + debugger traffic. +</ul> + +<p>There are a few issues with using JDWP for our purposes: +<ul> + <li>The VM only expects one connection from a debugger, so you couldn't + attach the monitor and a debugger at the same time. This will be + worked around by connecting the debugger to the monitor and passing the + traffic through. (We're already doing the pass-through with "jdwpspy"; + requires some management of our request IDs though.) This should + be more convenient than the current "guess the port + number" system when we're attached to a device. + <li>The VM behaves differently when a debugger is attached. It will + run more slowly, and any objects passed to the monitor or debugger are + immune to GC. We can work around this by not enabling the slow path + until non-DDM traffic is observed. We also want to have a "debugger + has connected/disconnected" message that allows the VM to release + debugger-related resources without dropping the net connection. + <li>Non-DDM VMs should not freak out when DDM connects. There are + no guarantees here for 3rd-party VMs (e.g. a certain mainstream VM, + which crashes instantly), but our older JamVM can be + configured to reject the "hello" packet. +</ul> + + +<h3>Connection Establishment</h3> + +<p>There are two basic approaches: have the server contact the VMs, and +have the VMs contact the server. The former is less "precise" than the +latter, because you have to scan for the clients, but it has some +advantages. + +<p>There are three interesting scenarios: +<ol> + <li>The DDM server is started, then the USB-attached device is booted + or the simulator is launched. + <li>The device or simulator is already running when the DDM server + is started. + <li>The DDM server is running when an already-started device is + attached to USB. +</ol> +<p>If we have the VMs connect to the DDM server on startup, we only handle +case #1. If the DDM server scans for VMs when it starts, we only handle +case #2. Neither handles case #3, which is probably the most important +of the bunch as the device matures. +<p>The plan is to have a drop-down menu with two entries, +"scan workstation" and "scan device". +The former causes the DDM server to search for VMs on "localhost", the +latter causes it to search for VMs on the other side of an ADB connection. +The DDM server will scan for VMs every few seconds, either checking a +range of known VM ports (e.g. 8000-8040) or interacting with some sort +of process database on the device. Changing modes causes all existing +connections to be dropped. +<p>When the DDM server first starts, it will try to execute "adb usb" +to ensure that the ADB server is running. (Note it will be necessary +to launch the DDM server from a shell with "adb" in the path.) If this +fails, talking to the device will still be possible so long as the ADB +daemon is already running. + +<h4>Connecting a Debugger</h4> + +<p>With the DDM server sitting on the JDWP port of all VMs, it will be +necessary to connect the debugger through the DDM server. Each VM being +debugged will have a separate port being listened to by the DDM server, +allowing you to connect a debugger to one or more VMs simultaneously. + +<p>In the common case, however, the developer will only want to debug +a single VM. One port (say 8700) will be listened to by the DDM server, +and anything connecting to it will be connected to the "current VM" +(selected in the UI). This should allow developers to focus on a +single application, which may otherwise shift around in the ordering, without +having to adjust their IDE settings to a different port every time they +restart the device. + + +<h3>Packet Format</h3> + +<p>Information is sent in chunks. Each chunk starts with: +<pre> +u4 type +u4 length +</pre> +and contains a variable amount of type-specific data. +Unrecognized types cause an empty response from the client and +are quietly ignored by the server. [Should probably return an error; +need an "error" chunk type and a handler on the server side.] + +<p>The same chunk type may have different meanings when sent in different +directions. For example, the same type may be used for both a query and +a response to the query. For sanity the type must always be used in +related transactions. + +<p>This is somewhat redundant with the JDWP framing, which includes a +4-byte length and a two-byte type code ("command set" and "command"; a +range of command set values is designated for "vendor-defined commands +and extensions"). Using the chunk format allows us to remain independent +of the underlying transport, avoids intrusive integration +with JDWP client code, and provides a way to send multiple chunks in a +single transmission unit. [I'm taking the multi-chunk packets into +account in the design, but do not plan to implement them unless the need +arises.] + +<p>Because we may be sending data over a slow USB link, the chunks may be +compressed. Compressed chunks are written as a chunk type that +indicates the compression, followed by the compressed length, followed +by the original chunk type and the uncompressed length. For zlib's deflate +algorithm, the chunk type is "ZLIB". + +<p>Following the JDWP model, packets sent from the server to the client +are always acknowledged, but packets sent from client to server never are. +The JDWP error code field is always set to "no error"; failure responses +from specific requests must be encoded into the DDM messages. + +<p>In what follows "u4" is an unsigned 32-bit value and "u1" is an +unsigned 8-bit value. Values are written in big-endian order to match +JDWP. + + +<h3>Initial Handshake</h3> + +<p>After the JDWP handshake, the server sends a HELO chunk to the client. +If the client's JDWP layer rejects it, the server assumes that the client +is not a DDM-aware VM, and does not send it any further DDM queries. +<p>On the client side, upon seeing a HELO it can know that a DDM server +is attached and prepare accordingly. The VM should not assume that a +debugger is attached until a non-DDM packet arrives. + +<h4>Chunk HELO (server --> client)</h4> +<p>Basic "hello" message. +<pre> +u4 DDM server protocol version +</pre> + + +<h4>Chunk HELO (client --> server, reply only)</h4> +Information about the client. Must be sent in response to the HELO message. +<pre> +u4 DDM client protocol version +u4 pid +u4 VM ident string len (in 16-bit units) +u4 application name len (in 16-bit units) +var VM ident string (UTF-16) +var application name (UTF-16) +</pre> + +<p>If the client does not wish to speak to the DDM server, it should respond +with a JDWP error packet. This is the same behavior you'd get from a VM +that doesn't support DDM. + + +<h3>Debugger Management</h3> +<p>VMs usually prepare for debugging when a JDWP connection is established, +and release debugger-related resources when the connection drops. We want +to open the JDWP connection early and hold it open after the debugger +disconnects. +<p>The VM can tell when a debugger attaches, because it will start seeing +non-DDM JDWP traffic, but it can't identify the disconnect. For this reason, +we need to send a packet to the client when the debugger disconnects. +<p>If the DDM server is talking to a non-DDM-aware client, it will be +necessary to drop and re-establish the connection when the debugger goes away. +(This also works with DDM-aware clients; this packet is an optimization.) + +<h4>Chunk DBGD (server --> client)</h4> +<p>Debugger has disconnected. The client responds with a DBGD to acknowledge +receipt. No data in request, no response required. + + +<h3>VM Info</h3> +<p>Update the server's info about the client. + +<h4>Chunk APNM (client --> server)</h4> + +<p>If a VM's application name changes -- possible in our environment because +of the "pre-initialized" app processes -- it must send up one of these. +<pre> +u4 application name len (in 16-bit chars) +var application name (UTF-16) +</pre> + +<h4>Chunk WAIT (client --> server)</h4> + +<p>This tells DDMS that one or more threads are waiting on an external +event. The simplest use is to tell DDMS that the VM is waiting for a +debugger to attach. +<pre> +u1 reason (0 = wait for debugger) +</pre> +If DDMS is attached, the client VM sends this up when waitForDebugger() +is called. If waitForDebugger() is called before DDMS attaches, the WAIT +chunk will be sent up at about the same time as the HELO response. + + +<h3>Thread Status</h3> + +<p>The client can send updates when their status changes, or periodically +send thread state info, e.g. 2x per +second to allow a "blinkenlights" display of thread activity. + +<h4>Chunk THEN (server --> client)</h4> + +<p>Enable thread creation/death notification. +<pre> +u1 boolean (true=enable, false=disable) +</pre> +<p>The response is empty. The client generates THCR packets for all +known threads. (Note the THCR packets may arrive before the THEN +response.) + +<h4>Chunk THCR (client --> server)</h4> +<p>Thread Creation notification. +<pre> +u4 VM-local thread ID (usually a small int) +u4 thread name len (in 16-bit chars) +var thread name (UTF-16) +</pre> + +<h4>Chunk THDE (client --> server)</h4> +<p>Thread Death notification. +<pre> +u4 VM-local thread ID +</pre> + +<h4>Chunk THST (server --> client)</h4> + +<p>Enable periodic thread activity updates. +Threads in THCR messages are assumed to be in the "initializing" state. A +THST message should follow closely on the heels of THCR. +<pre> +u4 interval, in msec +</pre> +<p>An interval of 0 disables the updates. This is done periodically, +rather than every time the thread state changes, to reduce the amount +of data that must be sent for an actively running VM. + +<h4>Chunk THST (client --> server)</h4> +<p>Thread Status, describing the state of one or more threads. This is +most useful when creation/death notifications are enabled first. The +overall layout is: +<pre> +u4 count +var thread data +</pre> +Then, for every thread: +<pre> +u4 VM-local thread ID +u1 thread state +u1 suspended +</pre> +<p>"thread state" must be one of: +<ul> <!-- don't use ol, we may need (-1) or sparse --> + <li> 1 - running (now executing or ready to do so) + <li> 2 - sleeping (in Thread.sleep()) + <li> 3 - monitor (blocked on a monitor lock) + <li> 4 - waiting (in Object.wait()) + <li> 5 - initializing + <li> 6 - starting + <li> 7 - native (executing native code) + <li> 8 - vmwait (waiting on a VM resource) +</ul> +<p>"suspended" will be 0 if the thread is running, 1 if not. +<p>[Any reason not to make "suspended" be the high bit of "thread state"? +Do we need to differentiate suspend-by-GC from suspend-by-debugger?] +<p>[We might be able to send the currently-executing method. This is a +little risky in a running VM, and increases the size of the messages +considerably, but might be handy.] + + +<h3>Heap Status</h3> + +<p>The client sends what amounts to a color-coded bitmap to the server, +indicating which stretches of memory are free and which are in use. For +compactness the bitmap is run-length encoded, and based on multi-byte +"allocation units" rather than byte counts. + +<p>In the future the server will be able to correlate the bitmap with more +detailed object data, so enough information is provided to associate the +bitmap data with virtual addresses. + +<p>Heaps may be broken into segments within the VM, and due to memory +constraints it may be desirable to send the bitmap in smaller pieces, +so the protocol allows the heap data to be sent in several chunks. +To avoid ambiguity, the client is required +to send explicit "start" and "end" messages during an update. + +<p>All messages include a "heap ID" that can be used to differentiate +between multiple independent virtual heaps or perhaps a native heap. The +client is allowed to send information about different heaps simultaneously, +so all heap-specific information is tagged with a "heap ID". + +<h4>Chunk HPIF (server --> client)</h4> +<p>Request heap info. +<pre> +u1 when to send +</pre> +<p>The "when" values are: +<pre> +0: never +1: immediately +2: at the next GC +3: at every GC +</pre> + +<h4>Chunk HPIF (client --> server, reply only)</h4> +<p>Heap Info. General information about the heap, suitable for a summary +display. +<pre> +u4 number of heaps +</pre> +For each heap: +<pre> +u4 heap ID +u8 timestamp in ms since Unix epoch +u1 capture reason (same as 'when' value from server) +u4 max heap size in bytes (-Xmx) +u4 current heap size in bytes +u4 current number of bytes allocated +u4 current number of objects allocated +</pre> +<p>[We can get some of this from HPSG, more from HPSO.] +<p>[Do we need a "heap overhead" stat here, indicating how much goes to +waste? e.g. (8 bytes per object * number of objects)] + +<h4>Chunk HPSG (server --> client)</h4> +<p>Request transmission of heap segment data. +<pre> +u1 when to send +u1 what to send +</pre> +<p>The "when" to send will be zero to disable transmission, 1 to send +during a GC. Other values are currently undefined. (Could use to pick +which part of the GC to send it, or cause periodic transmissions.) +<p>The "what" field is currently 0 for HPSG and 1 for HPSO. +<p>No reply is expected. + +<h4>Chunk NHSG (server --> client)</h4> +<p>Request transmission of native heap segment data. +<pre> +u1 when to send +u1 what to send +</pre> +<p>The "when" to send will be zero to disable transmission, 1 to send +during a GC. Other values are currently undefined. +<p>The "what" field is currently ignored. +<p>No reply is expected. + +<h4>Chunk HPST/NHST (client --> server)</h4> +<p>This is a Heap Start message. It tells the server to discard any +existing notion of what the client's heap looks like, and prepare for +new information. HPST indicates a virtual heap dump and must be followed +by zero or more HPSG/HPSO messages and an HPEN. NHST indicates a native +heap dump and must be followed by zero or more NHSG messages and an NHEN. + +<p>The only data item is: +<pre> +u4 heap ID +</pre> + +<h4>Chunk HPEN/NHEN (client --> server)</h4> +<p>Heap End, indicating that all information about the heap has been sent. +A HPST will be paired with an HPEN and an NHST will be paired with an NHEN. + +<p>The only data item is: +<pre> +u4 heap ID +</pre> + +<h4>Chunk HPSG (client --> server)</h4> +<p>Heap segment data. Each chunk describes all or part of a contiguous +stretch of heap memory. +<pre> +u4 heap ID +u1 size of allocation unit, in bytes (e.g. 8 bytes) +u4 virtual address of segment start +u4 offset of this piece (relative to the virtual address) +u4 length of piece, in allocation units +var usage data +</pre> +<p>The "usage data" indicates the status of each allocation unit. The data +is a stream of pairs of bytes, where the first byte indicates the state +of the allocation unit, and the second byte indicates the number of +consecutive allocation units with the same state. +<p>The bits in the "state" byte have the following meaning: +<pre> ++---------------------------------------+ +| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | ++---------------------------------------+ +| P | U0 | K2 | K1 | K0 | S2 | S1 | S0 | ++---------------------------------------+ +</pre> +<ul> + <li>'S': solidity + <ul> + <li>0=free + <li>1=has hard reference + <li>2=has soft reference + <li>3=has weak reference + <li>4=has phantom reference + <li>5=pending finalization + <li>6=marked, about to be swept + </ul> + <li>'K': kind + <ul> + <li>0=object + <li>1=class object + <li>2=array of byte/boolean + <li>3=array of char/short + <li>4=array of Object/int/float + <li>5=array of long/double + </ul> + <li>'P': partial flag (not used for HPSG) + <li>'U': unused, must be zero +</ul> + +<p>The use of the various 'S' types depends on when the information is +sent. The current plan is to send it either immediately after a GC, +or between the "mark" and "sweep" phases of the GC. For a fancy generational +collector, we may just want to send it up periodically. + +<p>The run-length byte indicates the number of allocation units minus one, so a +length of 255 means there are 256 consecutive units with this state. In +some cases, e.g. arrays of bytes, the actual size of the data is rounded +up the nearest allocation unit. +<p>For HPSG, the runs do not end at object boundaries. It is not possible +to tell from this bitmap whether a run contains one or several objects. +(But see HPSO, below.) +<p>[If we find that we have many long runs, we can overload the 'P' flag +or dedicate the 'U' flag to indicate that we have a 16-bit length instead +of 8-bit. We can also use a variable-width integer scheme for the length, +encoding 1-128 in one byte, 1-16384 in two bytes, etc.] +<p>[Alternate plan for 'K': array of byte, array of char, array of Object, +array of miscellaneous primitive type] +<p>To parse the data, the server runs through the usage data until either +(a) the end of the chunk is reached, or (b) all allocation units have been +accounted for. (If these two things don't happen at the same time, the +chunk is rejected.) +<p>Example: suppose a VM has a heap at 0x10000 that is 0x2000 bytes long +(with an 8-byte allocation unit size, that's 0x0400 units long). +The client could send one chunk (allocSize=8, virtAddr=0x10000, offset=0, +length=0x0400) or two (allocSize=8, virtAddr=0x10000, offset=0, length=0x300; +then allocSize=8, virtAddr=0x10000, offset=0x300, length=0x100). +<p>The client must encode the entire heap, including all free space at +the end, or the server will not have an accurate impression of the amount +of memory in the heap. This refers to the current heap size, not the +maximum heap size. + +<h4>Chunk HPSO (client --> server)</h4> +<p>This is essentially identical to HPSG, but the runs are terminated at +object boundaries. If an object is larger than 256 allocation units, the +"partial" flag is set in all runs except the last. +<p>The resulting unpacked bitmap is identical, but the object boundary +information can be used to gain insights into heap layout. +<p>[Do we want to have a separate message for this? Maybe just include +a "variant" flag in the HPST packet. Another possible form of output +would be one that indicates the age, in generations, of each block of +memory. That would provide a quick visual indication of "permanent vs. +transient residents", perhaps with a 16-level grey scale.] + +<h4>Chunk NHSG (client --> server)</h4> +<p>Native heap segment data. Each chunk describes all or part of a +contiguous stretch of native heap memory. The format is the same as +for HPSG, except that only solidity values 0 (= free) and 1 (= hard +reference) are used, and the kind value is always 0 for free chunks +and 7 for allocated chunks, indicating a non-VM object. +<pre> +u4 heap ID +u1 size of allocation unit, in bytes (e.g. 8 bytes) +u4 virtual address of segment start +u4 offset of this piece (relative to the virtual address) +u4 length of piece, in allocation units +var usage data +</pre> + +<h3>Generic Replies</h3> + +The client-side chunk handlers need a common way to report simple success +or failure. By convention, an empty reply packet indicates success. + +<h4>Chunk FAIL (client --> server, reply only)</h4> +<p>The chunk includes a machine-readable error code and a +human-readable error message. Server code can associate the failure +with the original request by comparing the JDWP packet ID. +<p>This allows a standard way of, for example, rejecting badly-formed +request packets. +<pre> +u4 error code +u4 error message len (in 16-bit chars) +var error message (UTF-16) +</pre> + +<h3>Miscellaneous</h3> + +<h4>Chunk EXIT (server --> client)</h4> +<p>Cause the client to exit with the specified status, using System.exit(). +Useful for certain kinds of testing. +<pre> +u4 exit status +</pre> + +<h4>Chunk DTRC (server --> client)</h4> +<p>[TBD] start/stop dmtrace; can send the results back over the wire. For +size reasons we probably need "sending", "data", "key", "finished" as +4 separate chunks/packets rather than one glob. + + +<h2>Client API</h2> + +<p>The API is written in the Java programming language +for convenience. The code is free to call native methods if appropriate. + +<h3>Chunk Handler API</h3> + +<p>The basic idea is that arbitrary code can register handlers for +specific chunk types. When a DDM chunk with that type arrives, the +appropriate handler is invoked. The handler's return value provides the +response to the server. + +<p>There are two packages. android.ddm lives in the "framework" library, +and has all of the chunk handlers and registration code. It can freely +use Android classes. org.apache.harmony.dalvik.ddmc lives in the "core" +library, and has +some base classes and features that interact with the VM. Nothing should +need to modify the org.apache.harmony.dalvik.ddmc classes. + +<p>The DDM classes pass chunks of data around with a simple class: + +<pre class=prettyprint> +class Chunk { + int type; + byte[] data; + int offset, length; +}; +</pre> + +<p>The chunk handlers accept and return them: +<pre class=prettyprint> +public Chunk handleChunk(Chunk request) +</pre> +<p>The code is free to parse the chunk and generate a response in any +way it chooses. Big-endian byte ordering is recommended but not mandatory. +<p>Chunk handlers will be notified when a DDM server connects or disconnects, +so that they can perform setup and cleanup operations: +<pre class=prettyprint> +public void connected() +public void disconnected() +</pre> + +<p>The method processes the request, formulates a response, and returns it. +If the method returns null, an empty JDWP success message will be returned. +<p>The request/response interaction is essentially asynchronous in the +protocol. The packets are linked together with the JDWP message ID. +<p>[We could use ByteBuffer here instead of byte[], but it doesn't gain +us much. Wrapping a ByteBuffer around an array is easy. We don't want +to pass the full packet in because we could have multiple chunks in one +request packet. The DDM code needs to collect and aggregate the responses +to all chunks into a single JDWP response packet. Parties wanting to +write multiple chunks in response to a single chunk should send a null +response back and use "sendChunk()" to send the data independently.] + +<h3>Unsolicited event API</h3> + +<p>If a piece of code wants to send a chunk of data to the server at some +arbitrary time, it may do so with a method provided by +org.apache.harmony.dalvik.DdmServer: + +<pre class=prettyprint> +public static void sendChunk(Chunk chunk) +</pre> + +<p>There is no response or status code. No exceptions are thrown. + + +<h2>Server API</h2> + +<p>This is similar to the client side in many ways, but makes extensive +use of ByteBuffer in a perhaps misguided attempt to use java.nio.channels +and avoid excessive thread creation and unnecessary data copying. + +<p>Upon receipt of a packet, the server will identify it as one of: +<ol> + <li>Message to be passed through to the debugger + <li>Response to an earlier request + <li>Unsolicited event packet +</ol> +<p>To handle (2), when messages are sent from the server to the client, +the message must be paired with a callback method. The response might be +delayed for a while -- or might never arrive -- so the server can't block +waiting for responses from the client. +<p>The chunk handlers look like this: +<pre class=prettyprint> +public void handleChunk(Client client, int type, + ByteBuffer data, boolean isReply, int msgId) +</pre> +<p>The arguments are: +<dl> + <dt>client + <dd>An object representing the client VM that send us the packet. + <dt>type + <dd>The 32-bit chunk type. + <dt>data + <dd>The data. The data's length can be determined by calling data.limit(). + <dt>isReply + <dd>Set to "true" if this was a reply to a message we sent earlier, + "false" if the client sent this unsolicited. + <dt>msgId + <dd>The JDWP message ID. Useful for connecting replies with requests. +</dl> +<p>If a handler doesn't like the contents of a packet, it should log an +error message and return. If the handler doesn't recognize the packet at +all, it can call the superclass' handleUnknownChunk() method. + +<p>As with the client, the server code can be notified when clients +connect or disconnect. This allows the handler to send initialization +code immediately after a connect, or clean up after a disconnect. +<p>Data associated with a client can be stored in a ClientData object, +which acts as a general per-client dumping around for VM and UI state. + + +<P><BR> + +<HR> + +<address>Copyright © 2007 The Android Open Source Project</address> + +</body> +</HTML> |