Package javax.inject
Many types depend on other types. For example, a Stopwatch might depend on a TimeSource. The types on which a type depends are known as its dependencies. The process of finding an instance of a dependency to use at run time is known as resolving the dependency. If no such instance can be found, the dependency is said to be unsatisfied, and the application is broken.
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:
class Stopwatch {
final TimeSource timeSource;
Stopwatch () {
timeSource = new AtomicClock(...);
}
void start() { ... }
long stop() { ... }
}
If more flexibility is needed, the object can call out to a factory or service locator:
class Stopwatch {
final TimeSource timeSource;
Stopwatch () {
timeSource = DefaultTimeSource.getInstance();
}
void start() { ... }
long stop() { ... }
}
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:
void testStopwatch() {
TimeSource original = DefaultTimeSource.getInstance();
DefaultTimeSource.setInstance(new MockTimeSource());
try {
// Now, we can actually test Stopwatch.
Stopwatch sw = new Stopwatch();
...
} finally {
DefaultTimeSource.setInstance(original);
}
}
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.
Dependency injection addresses all of these issues. Instead of the programmer calling a constructor or factory, a tool called a dependency injector passes dependencies to objects:
class Stopwatch {
final TimeSource timeSource;
@Inject Stopwatch(TimeSource TimeSource) {
this.TimeSource = TimeSource;
}
void start() { ... }
long stop() { ... }
}
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 StopwatchWidget instance:
/** GUI for a Stopwatch */
class StopwatchWidget {
@Inject StopwatchWidget(Stopwatch sw) { ... }
...
}
The injector might:
- Find a TimeSource
- Construct a Stopwatch with the TimeSource
- Construct a StopwatchWidget with the Stopwatch
This leaves the programmer's code clean, flexible, and relatively free of dependency-related infrastructure.
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:
void testStopwatch() {
Stopwatch sw = new Stopwatch(new MockTimeSource());
...
}
The total decrease in unit-test complexity is proportional to the product of the number of unit tests and the number of dependencies.
This package provides dependency injection annotations that enable portable classes, 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 build time. A service locator, by contrast, cannot detect unsatisfied dependencies until run time.
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.
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