那些关于Autowired你可能不知道的事

先上代码

public interface TalkService {
    void talk(String content);
}

@Service(value = "withSisterTalkService")
public class WithSisterTalkService implements TalkService {
    @Override
    public void talk(String content) {
        System.out.println(this.getClass().getName() + ":" + content);
    }
}

@Service(value = "withGirlFriendTalkService")
public class WithGirlFriendTalkService implements TalkService {
    @Override
    public void talk(String content) {
        System.out.println(this.getClass().getName() + ":" + content);
    }
}

@Service
public class TalkServiceStrategyContext implements TalkService {
 
    private Map<String, TalkService> strategyMap = new ConcurrentHashMap<>();
 
    @Autowired
    public TalkServiceStrategyContext(Map<String, TalkService> strategyMap) {
        this.strategyMap.clear();
        this.strategyMap.putAll(strategyMap);
    }
 
    @Override
    public void talk(String content) {
 
    }
}

@Autowired
private Map<String, TalkService> talkServiceMap;
 
@GetMapping(value = "doTest")
public String doTest() {
    Set<String> strings = talkServiceMap.keySet();
    for (String string : strings) {
        System.out.println(string + ":" + talkServiceMap.get(string).toString());
    }
    return this.getClass().getName();
}

其访问测试controller后，打印的信息如下：

talkServiceStrategyContext:com.haiyang.onlinejava.complier.service.impl.TalkServiceStrategyContext@2f0b1419
withGirlFriendTalkService:com.haiyang.onlinejava.complier.service.impl.WithGirlFriendTalkService@1cf19a02
withSisterTalkService:com.haiyang.onlinejava.complier.service.impl.WithSisterTalkService@1ef3c76d

看了后感觉很奇怪，在上方只定义了一个map<String,TalkService>的map,居然它就能自动找到实现了TalkService的所有bean,并将service的beanName作为了key，感觉还是牛逼啊，spring的注解居然还能这样用。

然后简单看了下Autowired的源码，其javaDoc文档里也有说明：

package org.springframework.beans.factory.annotation;
 
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
 
/**
 * Marks a constructor, field, setter method or config method as to be
 * autowired by Spring's dependency injection facilities.
 *
 * <p>Only one constructor (at max) of any given bean class may carry this
 * annotation, indicating the constructor to autowire when used as a Spring
 * bean. Such a constructor does not have to be public.
 *
 * <p>Fields are injected right after construction of a bean, before any
 * config methods are invoked. Such a config field does not have to be public.
 *
 * <p>Config methods may have an arbitrary name and any number of arguments;
 * each of those arguments will be autowired with a matching bean in the
 * Spring container. Bean property setter methods are effectively just
 * a special case of such a general config method. Such config methods
 * do not have to be public.
 *
 * <p>In the case of multiple argument methods, the 'required' parameter is
 * applicable for all arguments.
 *
 * <p>In case of a {@link java.util.Collection} or {@link java.util.Map}
 * dependency type, the container will autowire all beans matching the
 * declared value type. In case of a Map, the keys must be declared as
 * type String and will be resolved to the corresponding bean names.
 *
 * <p>Note that actual injection is performed through a
 * {@link org.springframework.beans.factory.config.BeanPostProcessor
 * BeanPostProcessor} which in turn means that you <em>cannot</em>
 * use {@code @Autowired} to inject references into
 * {@link org.springframework.beans.factory.config.BeanPostProcessor
 * BeanPostProcessor} or
 * {@link org.springframework.beans.factory.config.BeanFactoryPostProcessor BeanFactoryPostProcessor}
 * types. Please consult the javadoc for the {@link AutowiredAnnotationBeanPostProcessor}
 * class (which, by default, checks for the presence of this annotation).
 *
 * @author Juergen Hoeller
 * @author Mark Fisher
 * @since 2.5
 * @see AutowiredAnnotationBeanPostProcessor
 * @see Qualifier
 * @see Value
 */
@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Autowired {
 
	/**
	 * Declares whether the annotated dependency is required.
	 * <p>Defaults to {@code true}.
	 */
	boolean required() default true;
 
}

关注这句：

In case of a java.util.Collection or java.util.Map dependency type, the container will autowire all beans matching the declared value type. In case of a Map, the keys must be declared as type String and will be resolved to the corresponding bean names.

读不懂没关系，翻译一下

对于java.util.Collection或java.util.Map依赖类型，容器将自动装配所有与声明的值类型匹配的bean。对于Map，键必须声明为String类型，并解析为相应的bean名称。

总结

它大致是说Autowired当使用在Collection里时，会将所申明类的所有实现类都放在那个指定的Collection里；如果Autowired和map使用的话呢，它将它bean的名称作为key,所有的bean作为value.