Spring @Autowired 注入集合类型的问题

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从spring 2.5版本开始，spring提供了基于注解方式的依赖注入。在容器的xml配置文件中，添加如下的配置

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<context:annotation-config />  
<context:component-scan base-package="com.example" />  

即可扫描com.example包及其子包下所有使用特定注解注明的类，创建他们的实例并完成他们之间的依赖注入。非常的方便，大大方便了系统开发的配置。

昨天一个同事在使用@Autowired自动注入依赖的集合bean时碰到了问题。定义了Manager，并声明自动注入一个Set<String>

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package com.example;  
  
import java.util.Set;  
  
import org.springframework.beans.factory.annotation.Autowired;  
import org.springframework.context.ApplicationListener;  
import org.springframework.context.event.ApplicationContextEvent;  
import org.springframework.context.event.ContextRefreshedEvent;  
import org.springframework.stereotype.Component;  
  
@Component  
public class Manager implements ApplicationListener<ApplicationContextEvent> {  
  
    @Autowired  
    private Set<String> locations;  
  
    @Override  
    public void onApplicationEvent(ApplicationContextEvent event) {  
        if (event instanceof ContextRefreshedEvent) {  
            for (String loc : locations) {  
                System.out.println("location -> " + loc);  
            }  
        }  
    }  
}  

容器配置文件如下

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<?xml version="1.0" encoding="UTF-8"?>  
<beans xmlns="http://www.springframework.org/schema/beans"  
    xmlns:context="http://www.springframework.org/schema/context"  
    xmlns:util="http://www.springframework.org/schema/util" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
    xsi:schemaLocation="  
            http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.1.xsd  
            http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.1.xsd  
            http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-4.1.xsd">  
  
    <context:annotation-config />  
  
    <context:component-scan base-package="com.example" />  
  
    <bean id="resourcePackage" class="java.lang.String">  
        <constructor-arg value="com.example.resource" />  
    </bean>  
  
    <bean id="resourceLocation" class="java.lang.String">  
        <constructor-arg value="classpath:config" />  
    </bean>  
  
    <util:set id="locations" value-type="java.lang.String">  
        <value>com.example.module</value>  
        <value>com.example.common.entity</value>  
    </util:set>  
</beans>  

期望的是注入配置文件中id为locations的set，而事实却是容器启动后，输出的却是
location -> com.example.resource
location -> classpath:config

很奇怪，debug跟踪源码，在org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DependencyDescriptor, String, Set<String>, TypeConverter) 方法中发现了原因

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if (type.isArray()) {  
    Class<?> componentType = type.getComponentType();  
    DependencyDescriptor targetDesc = new DependencyDescriptor(descriptor);  
    targetDesc.increaseNestingLevel();  
    Map<String, Object> matchingBeans = findAutowireCandidates(beanName, componentType, targetDesc);  
    if (matchingBeans.isEmpty()) {  
        if (descriptor.isRequired()) {  
            raiseNoSuchBeanDefinitionException(componentType, "array of " + componentType.getName(), descriptor);  
        }  
        return null;  
    }  
    if (autowiredBeanNames != null) {  
        autowiredBeanNames.addAll(matchingBeans.keySet());  
    }  
    TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());  
    Object result = converter.convertIfNecessary(matchingBeans.values(), type);  
    if (this.dependencyComparator != null && result instanceof Object[]) {  
        Arrays.sort((Object[]) result, adaptDependencyComparator(matchingBeans));  
    }  
    return result;  
}  
else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {  
    Class<?> elementType = descriptor.getCollectionType();  
    if (elementType == null) {  
        if (descriptor.isRequired()) {  
            throw new FatalBeanException("No element type declared for collection [" + type.getName() + "]");  
        }  
        return null;  
    }  
    DependencyDescriptor targetDesc = new DependencyDescriptor(descriptor);  
    targetDesc.increaseNestingLevel();  
    Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType, targetDesc);  
    if (matchingBeans.isEmpty()) {  
        if (descriptor.isRequired()) {  
            raiseNoSuchBeanDefinitionException(elementType, "collection of " + elementType.getName(), descriptor);  
        }  
        return null;  
    }  
    if (autowiredBeanNames != null) {  
        autowiredBeanNames.addAll(matchingBeans.keySet());  
    }  
    TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());  
    Object result = converter.convertIfNecessary(matchingBeans.values(), type);  
    if (this.dependencyComparator != null && result instanceof List) {  
        Collections.sort((List<?>) result, adaptDependencyComparator(matchingBeans));  
    }  
    return result;  
}  
else if (Map.class.isAssignableFrom(type) && type.isInterface()) {  
    Class<?> keyType = descriptor.getMapKeyType();  
    if (keyType == null || !String.class.isAssignableFrom(keyType)) {  
        if (descriptor.isRequired()) {  
            throw new FatalBeanException("Key type [" + keyType + "] of map [" + type.getName() +  
                    "] must be assignable to [java.lang.String]");  
        }  
        return null;  
    }  
    Class<?> valueType = descriptor.getMapValueType();  
    if (valueType == null) {  
        if (descriptor.isRequired()) {  
            throw new FatalBeanException("No value type declared for map [" + type.getName() + "]");  
        }  
        return null;  
    }  
    DependencyDescriptor targetDesc = new DependencyDescriptor(descriptor);  
    targetDesc.increaseNestingLevel();  
    Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType, targetDesc);  
    if (matchingBeans.isEmpty()) {  
        if (descriptor.isRequired()) {  
            raiseNoSuchBeanDefinitionException(valueType, "map with value type " + valueType.getName(), descriptor);  
        }  
        return null;  
    }  
    if (autowiredBeanNames != null) {  
        autowiredBeanNames.addAll(matchingBeans.keySet());  
    }  
    return matchingBeans;  
}  
else {  
    Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);  
    if (matchingBeans.isEmpty()) {  
        if (descriptor.isRequired()) {  
            raiseNoSuchBeanDefinitionException(type, "", descriptor);  
        }  
        return null;  
    }  
    if (matchingBeans.size() > 1) {  
        String primaryBeanName = determineAutowireCandidate(matchingBeans, descriptor);  
        if (primaryBeanName == null) {  
            throw new NoUniqueBeanDefinitionException(type, matchingBeans.keySet());  
        }  
        if (autowiredBeanNames != null) {  
            autowiredBeanNames.add(primaryBeanName);  
        }  
        return matchingBeans.get(primaryBeanName);  
    }  
    // We have exactly one match.  
    Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();  
    if (autowiredBeanNames != null) {  
        autowiredBeanNames.add(entry.getKey());  
    }  
    return entry.getValue();  
}  

对于@Autowired声明的数组、集合类型，spring并不是根据beanName去找容器中对应的bean，而是把容器中所有类型与集合（数组）中元素类型相同的bean构造出一个对应集合，注入到目标bean中。对应到上问配置文件中，就是把容器中所有类型为java.lang.String的bean放到新建的Set中，然后注入到Manager bean中。也就是把resourcePackage和resourceLoaction这两个String注入了，导致上面的输出结果。

在spring reference中也发现相关说明。

@Autowired

If you intend to express annotation-driven injection by name, do not primarily use @Autowired, even if is technically capable of referring to a bean name through @Qualifier values. Instead, use the JSR-250 @Resource annotation, which is semantically defined to identify a specific target component by its unique name, with the declared type being irrelevant for the matching process.
As a specific consequence of this semantic difference, beans that are themselves defined as a collection or map type cannot be injected through @Autowired, because type matching is not properly applicable to them. Use @Resource for such beans, referring to the specific collection or map bean by unique name.
@Autowired applies to fields, constructors, and multi-argument methods, allowing for narrowing through qualifier annotations at the parameter level. By contrast, @Resource is supported only for fields and bean property setter methods with a single argument. As a consequence, stick with qualifiers if your injection target is a constructor or a multi-argument method.

从上面的说明中找到解决办法就是注入集合类型不要使用@Autowired，而使用@Resource注解。同时Spring官方也是不推荐使用@Autowired的。

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