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javax.inject
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<H2>
Package javax.inject
</H2>
This package specifies a means for obtaining objects in such a way as to
maximize reusability, testability and maintainability compared to
traditional approaches such as constructors, factories, and service
locators (e.g., JNDI). This process, known as <i>dependency
injection</i>, is beneficial to most nontrivial applications.
<P>
<B>See:</B>
<BR>
<A HREF="#package_description"><B>Description</B></A>
<P>
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<B>Interface Summary</B></FONT></TH>
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<TD WIDTH="15%"><B><A HREF="../../javax/inject/Provider.html" title="interface in javax.inject">Provider<T></A></B></TD>
<TD>Provides instances of <code>T</code>.</TD>
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<B>Annotation Types Summary</B></FONT></TH>
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<TD WIDTH="15%"><B><A HREF="../../javax/inject/Inject.html" title="annotation in javax.inject">Inject</A></B></TD>
<TD>Identifies injectable constructors, methods, and fields.</TD>
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<TD WIDTH="15%"><B><A HREF="../../javax/inject/Named.html" title="annotation in javax.inject">Named</A></B></TD>
<TD>String-based <A HREF="../../javax/inject/Qualifier.html" title="annotation in javax.inject">qualifier</A>.</TD>
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<TD WIDTH="15%"><B><A HREF="../../javax/inject/Qualifier.html" title="annotation in javax.inject">Qualifier</A></B></TD>
<TD>Identifies qualifier annotations.</TD>
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<TD WIDTH="15%"><B><A HREF="../../javax/inject/Scope.html" title="annotation in javax.inject">Scope</A></B></TD>
<TD>Identifies scope annotations.</TD>
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<TD WIDTH="15%"><B><A HREF="../../javax/inject/Singleton.html" title="annotation in javax.inject">Singleton</A></B></TD>
<TD>Identifies a type that the injector only instantiates once.</TD>
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<P>
<A NAME="package_description"><!-- --></A><H2>
Package javax.inject Description
</H2>
<P>
This package specifies a means for obtaining objects in such a way as to
maximize reusability, testability and maintainability compared to
traditional approaches such as constructors, factories, and service
locators (e.g., JNDI). This process, known as <i>dependency
injection</i>, is beneficial to most nontrivial applications.
<p>Many types depend on other types. For example, a <tt>Stopwatch</tt> might
depend on a <tt>TimeSource</tt>. The types on which a type depends are
known as its <i>dependencies</i>. The process of finding an instance of a
dependency to use at run time is known as <i>resolving</i> the dependency.
If no such instance can be found, the dependency is said to be
<i>unsatisfied</i>, and the application is broken.
<p>In the absence of dependency injection, an object can resolve its
dependencies in a few ways. It can invoke a constructor, hard-wiring an
object directly to its dependency's implementation and life cycle:
<pre> class Stopwatch {
final TimeSource timeSource;
Stopwatch () {
timeSource = <b>new AtomicClock(...)</b>;
}
void start() { ... }
long stop() { ... }
}</pre>
<p>If more flexibility is needed, the object can call out to a factory or
service locator:
<pre> class Stopwatch {
final TimeSource timeSource;
Stopwatch () {
timeSource = <b>DefaultTimeSource.getInstance()</b>;
}
void start() { ... }
long stop() { ... }
}</pre>
<p>In deciding between these traditional approaches to dependency
resolution, a programmer must make trade-offs. Constructors are more
concise but restrictive. Factories decouple the client and implementation
to some extent but require boilerplate code. Service locators decouple even
further but reduce compile time type safety. All three approaches inhibit
unit testing. For example, if the programmer uses a factory, each test
against code that depends on the factory will have to mock out the factory
and remember to clean up after itself or else risk side effects:
<pre> void testStopwatch() {
<b>TimeSource original = DefaultTimeSource.getInstance();
DefaultTimeSource.setInstance(new MockTimeSource());
try {</b>
// Now, we can actually test Stopwatch.
Stopwatch sw = new Stopwatch();
...
<b>} finally {
DefaultTimeSource.setInstance(original);
}</b>
}</pre>
<p>In practice, supporting this ability to mock out a factory results in
even more boilerplate code. Tests that mock out and clean up after multiple
dependencies quickly get out of hand. To make matters worse, a programmer
must predict accurately how much flexibility will be needed in the future
or else suffer the consequences. If a programmer initially elects to use a
constructor but later decides that more flexibility is required, the
programmer must replace every call to the constructor. If the programmer
errs on the side of caution and write factories up front, it may result in
a lot of unnecessary boilerplate code, adding noise, complexity, and
error-proneness.
<p><i>Dependency injection</i> addresses all of these issues. Instead of
the programmer calling a constructor or factory, a tool called a
<i>dependency injector</i> passes dependencies to objects:
<pre> class Stopwatch {
final TimeSource timeSource;
<b>@Inject Stopwatch(TimeSource TimeSource)</b> {
this.TimeSource = TimeSource;
}
void start() { ... }
long stop() { ... }
}</pre>
<p>The injector further passes dependencies to other dependencies until it
constructs the entire object graph. For example, suppose the programmer
asked an injector to create a <tt>StopwatchWidget</tt> instance:
<pre> /** GUI for a Stopwatch */
class StopwatchWidget {
@Inject StopwatchWidget(Stopwatch sw) { ... }
...
}</pre>
<p>The injector might:
<ol>
<li>Find a <tt>TimeSource</tt>
<li>Construct a <tt>Stopwatch</tt> with the <tt>TimeSource</tt>
<li>Construct a <tt>StopwatchWidget</tt> with the <tt>Stopwatch</tt>
</ol>
<p>This leaves the programmer's code clean, flexible, and relatively free
of dependency-related infrastructure.
<p>In unit tests, the programmer can now construct objects directly
(without an injector) and pass in mock dependencies. The programmer no
longer needs to set up and tear down factories or service locators in each
test. This greatly simplifies our unit test:
<pre> void testStopwatch() {
Stopwatch sw = new Stopwatch(new MockTimeSource());
...
}</pre>
<p>The total decrease in unit-test complexity is proportional to the
product of the number of unit tests and the number of dependencies.
<p><b>This package provides dependency injection annotations that enable
portable classes</b>, but it leaves external dependency configuration up to
the injector implementation. Programmers annotate constructors, methods,
and fields to advertise their injectability (constructor injection is
demonstrated in the examples above). A dependency injector identifies a
class's dependencies by inspecting these annotations, and injects the
dependencies at run time. Moreover, the injector can verify that all
dependencies have been satisfied at <i>build time</i>. A service locator,
by contrast, cannot detect unsatisfied dependencies until run time.
<p>Injector implementations can take many forms. An injector could
configure itself using XML, annotations, a DSL (domain-specific language),
or even plain Java code. An injector could rely on reflection or code
generation. An injector that uses compile-time code generation may not even
have its own run time representation. Other injectors may not be able to
generate code at all, neither at compile nor run time. A "container", for
some definition, can be an injector, but this package specification aims to
minimize restrictions on injector implementations.
<P>
<P>
<DL>
<DT><B>See Also:</B><DD><A HREF="../../javax/inject/Inject.html" title="annotation in javax.inject"><CODE>@Inject</CODE></A></DL>
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<font size='-1'>Copyright (C) 2009 <a href='http://code.google.com/p/atinject/'>The JSR-330 Expert Group</a>. Licensed under the <a href='http://www.apache.org/licenses/LICENSE-2.0'>Apache License</a>, Version 2.0.</font>
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