從spring 2.5版本開始，spring提供了基於註解方式的依賴注入。在容器的xml配置檔案中，新增如下的配置

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

即可掃描com.example包及其子包下所有使用特定註解註明的類，建立他們的例項並完成他們之間的依賴注入。非常的方便，大大方便了系統開發的配置。

昨天一個同事在使用@Autowired自動注入依賴的集合bean時碰到了問題。定義了Manager，並宣告自動注入一個Set<String>

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);
			}
		}
	}
}
 

容器配置檔案如下

<?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>
 

期望的是注入配置檔案中id為locations的set，而事實卻是容器啟動後，輸出的卻是
location -> com.example.resource
location -> classpath:config

很奇怪，debug跟蹤原始碼，在org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DependencyDescriptor, String, Set<String>, TypeConverter) 方法中發現了原因

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();
}

對於@Autowired宣告的陣列、集合型別，spring並不是根據beanName去找容器中對應的bean，而是把容器中所有型別與集合（陣列）中元素型別相同的bean構造出一個對應集合，注入到目標bean中。對應到上問配置檔案中，就是把容器中所有型別為java.lang.String的bean放到新建的Set中，然後注入到Manager bean中。也就是把resourcePackage和resourceLoaction這兩個String注入了，導致上面的輸出結果。

在spring reference中也發現相關說明。

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.

從上面的說明中找到解決辦法就是注入集合型別不要使用@Autowired，而使用@Resource註解。同時Spring官方也是不推薦使用@Autowired的。