Guice系列之用户指南（二）-白红宇

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