理解 javax.inject 示例代码？

Question

I would very much appreciate someone helping me understand this javax example code and why the @inject annotation is actually useful/what it does. The code below comes from: Code From this Source

class Stopwatch {
 final TimeSource timeSource;
 @Inject Stopwatch(TimeSource TimeSource) {
   this.TimeSource = TimeSource;
 }
 void start() { ... }
 long stop() { ... }
}

First off, I think (I hope) there is a typo and the "TimeSource" parameter should really be "timeSource", since the field in the Stopwatch class is CamelCase.

What I really don't understand though is what the @inject annotation is actually doing!? Is it equivalent to this constructor?:

class Stopwatch {
 final TimeSource timeSource;
 Stopwatch(TimeSource timeSource) {
 this.timeSource = timeSource;
 }
}

Above is how I would handle adding the dependency, so I hope I'm correct that's how the @inject keyword does it... Or am I way off??

Answer 1

If you have understood Spring's @Autowired annotation then, the code above is no different. @Autowired is a Spring specific annotation and @Inject comes from the Java Dependency Injection specification

class Stopwatch {
 final TimeSource timeSource;
 @Inject Stopwatch(TimeSource TimeSource) {
   this.TimeSource = TimeSource;
 }
 void start() { ... }
 long stop() { ... }
}

Here, @Inject is indicating that when the instance of Stopwatch is created, it should be instantiated via that constructor and pass in an instance that is assignable to TimeSource

Instead of the programmer calling a constructor or factory, a tool called a dependency injector passes dependencies to objects

So, as stated here, the dependency injector will make sure to inject an instance of TimeSource while creating an instance of Stopwatch .

Answer 2

Late answer but may help someone else. The @Inject annotation does nothing on its own.

You need a framework that will act on the annotation.

eg Guice

Injector injector = Guice.createInjector();
YourObject yo = injector.getInstance(YourObject.class);

The annotation is in javax to provide a standard name, so in theory I could swap Guice for something else.