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Inject
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<H2>
<FONT SIZE="-1">
javax.inject</FONT>
<BR>
Annotation Type Inject</H2>
<HR>
<DL>
<DT><PRE><FONT SIZE="-1">@Target(value={METHOD,CONSTRUCTOR,FIELD})
@Retention(value=RUNTIME)
@Documented
</FONT>public @interface <B>Inject</B></DL>
</PRE>
<P>
Identifies injectable constructors, methods, and fields. May apply to static
as well as instance members. An injectable member may have any access
modifier (private, package-private, protected, public). Constructors are
injected first, followed by fields, and then methods. Fields and methods
in superclasses are injected before those in subclasses. Ordering of
injection among fields and among methods in the same class is not specified.
<p>Injectable constructors are annotated with <code>@Inject</code> and accept
zero or more dependencies as arguments. <code>@Inject</code> can apply to at most
one constructor per class.
<p><tt><blockquote style="padding-left: 2em; text-indent: -2em;">@Inject
<i>ConstructorModifiers<sub>opt</sub></i>
<i>SimpleTypeName</i>(<i>FormalParameterList<sub>opt</sub></i>)
<i>Throws<sub>opt</sub></i>
<i>ConstructorBody</i></blockquote></tt>
<p><code>@Inject</code> is optional for public, no-argument constructors when no
other constructors are present. This enables injectors to invoke default
constructors.
<p><tt><blockquote style="padding-left: 2em; text-indent: -2em;">
@Inject<sub><i>opt</i></sub>
<i>Annotations<sub>opt</sub></i>
public
<i>SimpleTypeName</i>()
<i>Throws<sub>opt</sub></i>
<i>ConstructorBody</i></blockquote></tt>
<p>Injectable fields:
<ul>
<li>are annotated with <code>@Inject</code>.
<li>are not final.
<li>may have any otherwise valid name.</li></ul>
<p><tt><blockquote style="padding-left: 2em; text-indent: -2em;">@Inject
<i>FieldModifiers<sub>opt</sub></i>
<i>Type</i>
<i>VariableDeclarators</i>;</blockquote></tt>
<p>Injectable methods:
<ul>
<li>are annotated with <code>@Inject</code>.</li>
<li>are not abstract.</li>
<li>do not declare type parameters of their own.</li>
<li>may return a result</li>
<li>may have any otherwise valid name.</li>
<li>accept zero or more dependencies as arguments.</li></ul>
<p><tt><blockquote style="padding-left: 2em; text-indent: -2em;">@Inject
<i>MethodModifiers<sub>opt</sub></i>
<i>ResultType</i>
<i>Identifier</i>(<i>FormalParameterList<sub>opt</sub></i>)
<i>Throws<sub>opt</sub></i>
<i>MethodBody</i></blockquote></tt>
<p>The injector ignores the result of an injected method, but
non-<code>void</code> return types are allowed to support use of the method in
other contexts (builder-style method chaining, for example).
<p>Examples:
<pre>
public class Car {
// Injectable constructor
@Inject public Car(Engine engine) { ... }
// Injectable field
@Inject private Provider<Seat> seatProvider;
// Injectable package-private method
@Inject void install(Windshield windshield, Trunk trunk) { ... }
}</pre>
<p>A method annotated with <code>@Inject</code> that overrides another method
annotated with <code>@Inject</code> will only be injected once per injection
request per instance. A method with <i>no</i> <code>@Inject</code> annotation
that overrides a method annotated with <code>@Inject</code> will not be
injected.
<p>Injection of members annotated with <code>@Inject</code> is required. While an
injectable member may use any accessibility modifier (including
<tt>private</tt>), platform or injector limitations (like security
restrictions or lack of reflection support) might preclude injection
of non-public members.
<h3>Qualifiers</h3>
<p>A <A HREF="../../javax/inject/Qualifier.html" title="annotation in javax.inject">qualifier</A> may annotate an injectable field
or parameter and, combined with the type, identify the implementation to
inject. Qualifiers are optional, and when used with <code>@Inject</code> in
injector-independent classes, no more than one qualifier should annotate a
single field or parameter. The qualifiers are bold in the following example:
<pre>
public class Car {
@Inject private <b>@Leather</b> Provider<Seat> seatProvider;
@Inject void install(<b>@Tinted</b> Windshield windshield,
<b>@Big</b> Trunk trunk) { ... }
}</pre>
<p>If one injectable method overrides another, the overriding method's
parameters do not automatically inherit qualifiers from the overridden
method's parameters.
<h3>Injectable Values</h3>
<p>For a given type T and optional qualifier, an injector must be able to
inject a user-specified class that:
<ol type="a">
<li>is assignment compatible with T and</li>
<li>has an injectable constructor.</li>
</ol>
<p>For example, the user might use external configuration to pick an
implementation of T. Beyond that, which values are injected depend upon the
injector implementation and its configuration.
<h3>Circular Dependencies</h3>
<p>Detecting and resolving circular dependencies is left as an exercise for
the injector implementation. Circular dependencies between two constructors
is an obvious problem, but you can also have a circular dependency between
injectable fields or methods:
<pre>
class A {
@Inject B b;
}
class B {
@Inject A a;
}</pre>
<p>When constructing an instance of <code>A</code>, a naive injector
implementation might go into an infinite loop constructing an instance of
<code>B</code> to set on <code>A</code>, a second instance of <code>A</code> to set on
<code>B</code>, a second instance of <code>B</code> to set on the second instance of
<code>A</code>, and so on.
<p>A conservative injector might detect the circular dependency at build
time and generate an error, at which point the programmer could break the
circular dependency by injecting <A HREF="../../javax/inject/Provider.html" title="interface in javax.inject"><CODE>Provider<A></CODE></A> or <code>Provider<B></code> instead of <code>A</code> or <code>B</code> respectively. Calling <A HREF="../../javax/inject/Provider.html#get()"><CODE>get()</CODE></A> on the provider directly from the constructor or
method it was injected into defeats the provider's ability to break up
circular dependencies. In the case of method or field injection, scoping
one of the dependencies (using <A HREF="../../javax/inject/Singleton.html" title="annotation in javax.inject">singleton scope</A>, for
example) may also enable a valid circular relationship.
<P>
<P>
<DL>
<DT><B>See Also:</B><DD><A HREF="../../javax/inject/Qualifier.html" title="annotation in javax.inject"><CODE>@Qualifier</CODE></A>,
<A HREF="../../javax/inject/Provider.html" title="interface in javax.inject"><CODE>Provider</CODE></A></DL>
<P>
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