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: *
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. * * @see javax.inject.Inject @Inject */ package javax.inject;