Guice系列之用户指南（二）

原文地址：https://code.google.com/p/google-guice/wiki/BindingAnnotations

BindingAnnotations（绑定注释）：一个类型可能会有多个实现类，在绑定时加上注解，这样可以确定在依赖注入时用的具体是哪个实现类。
具体有2种，一种是自定义注解，另一种是@Named。

先看自定义注解，贴代码：

import com.google.inject.BindingAnnotation;
import java.lang.annotation.Target;
import java.lang.annotation.Retention;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
import static java.lang.annotation.ElementType.PARAMETER;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;

@BindingAnnotation 
@Target({FIELD, PARAMETER, METHOD }) 
@Retention(RUNTIME)
public @interface Tom {

}

public interface Animal {
	void say();
}

public class Cat implements Animal{

	@Override
	public void say() {
		System.out.println("i am a cat");
	}

}

public class Dog implements Animal{

	@Override
	public void say() {
		System.out.println("i am a dog");
	}

}

public interface HelloService {
	void sayHello();
}

import com.google.inject.Inject;

public class HelloServiceImpl implements HelloService {

	Animal animal;
	
	@Inject
	public HelloServiceImpl(@Tom Animal animal) {
		this.animal = animal;
	}
	
	@Override
	public void sayHello() {
		System.out.println(animal.getClass().getSimpleName());
		animal.say();
	}
}

import com.google.inject.AbstractModule;

public class HelloServiceModule extends AbstractModule {

	@Override
	protected void configure() {
		bind(Animal.class).annotatedWith(Tom.class).to(Cat.class);
		bind(Animal.class).to(Dog.class);
		bind(HelloService.class).to(HelloServiceImpl.class);
	}

}

public class Test {
	public static void main(String[] args) {
		Injector injector = Guice.createInjector(new HelloServiceModule());
		HelloService helloService = injector.getInstance(HelloService.class);
		helloService.sayHello();
	}
}

执行结果：
Cat
i am a cat

当把HelloServiceImpl的构造函数里的@Tom去掉后，执行结果：
Dog
i am a dog

发现，多个实现类绑定到一个类型时，后者覆盖前者。

第二种，用@Named注解，贴代码：

import com.google.inject.Inject;
import com.google.inject.name.Named;

public class HelloServiceImpl2 implements HelloService {

	Animal animal;
	
	@Inject
	public HelloServiceImpl2(@Named("Snoopy") Animal animal) {
		this.animal = animal;
	}
	
	@Override
	public void sayHello() {
		System.out.println(animal.getClass().getSimpleName());
		animal.say();
	}
}

import com.google.inject.AbstractModule;
import com.google.inject.name.Names;

public class HelloServiceModule2 extends AbstractModule {

	@Override
	protected void configure() {
		bind(Animal.class).annotatedWith(Names.named("Snoopy")).to(Dog.class);
		bind(Animal.class).to(Cat.class);
		bind(HelloService.class).to(HelloServiceImpl2.class);
	}

}

import com.google.inject.Guice;
import com.google.inject.Injector;

public class Test2 {
	public static void main(String[] args) {
		Injector injector = Guice.createInjector(new HelloServiceModule2());
		HelloService helloService = injector.getInstance(HelloService.class);
		helloService.sayHello();
	}
}

执行结果：
Dog
i am a dog

练习代码在这里