Spring @Autowired 注入集合类型的问题

我不是女神ヾ 2021-09-14 12:18 406阅读 0赞

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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1.  <context:annotation-config />  
2.  <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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1.  package com.example;  
2.    
3.  import java.util.Set;  
4.    
5.  import org.springframework.beans.factory.annotation.Autowired;  
6.  import org.springframework.context.ApplicationListener;  
7.  import org.springframework.context.event.ApplicationContextEvent;  
8.  import org.springframework.context.event.ContextRefreshedEvent;  
9.  import org.springframework.stereotype.Component;  
10.   
11. @Component  
12. public class Manager implements ApplicationListener<ApplicationContextEvent> \{  
13.   
14.     @Autowired  
15.     private Set<String> locations;  
16.   
17.     @Override  
18.     public void onApplicationEvent(ApplicationContextEvent event) \{  
19.         if (event instanceof ContextRefreshedEvent) \{  
20.             for (String loc : locations) \{  
21.                 System.out.println("location -> " + loc);  
22.             \}  
23.         \}  
24.     \}  
25. \}

容器配置文件如下

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1.  <?xml version="1.0" encoding="UTF-8"?>  
2.  <beans xmlns="http://www.springframework.org/schema/beans"  
3.      xmlns:context="http://www.springframework.org/schema/context"  
4.      xmlns:util="http://www.springframework.org/schema/util" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
5.      xsi:schemaLocation="  
6.              http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.1.xsd  
7.              http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.1.xsd  
8.              http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-4.1.xsd">  
9.    
10.     <context:annotation-config />  
11.   
12.     <context:component-scan base-package="com.example" />  
13.   
14.     <bean id="resourcePackage" class="java.lang.String">  
15.         <constructor-arg value="com.example.resource" />  
16.     </bean>  
17.   
18.     <bean id="resourceLocation" class="java.lang.String">  
19.         <constructor-arg value="classpath:config" />  
20.     </bean>  
21.   
22.     <util:set id="locations" value-type="java.lang.String">  
23.         <value>com.example.module</value>  
24.         <value>com.example.common.entity</value>  
25.     </util:set>  
26. </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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1.  if (type.isArray()) \{  
2.      Class<?> componentType = type.getComponentType();  
3.      DependencyDescriptor targetDesc = new DependencyDescriptor(descriptor);  
4.      targetDesc.increaseNestingLevel();  
5.      Map<String, Object> matchingBeans = findAutowireCandidates(beanName, componentType, targetDesc);  
6.      if (matchingBeans.isEmpty()) \{  
7.          if (descriptor.isRequired()) \{  
8.              raiseNoSuchBeanDefinitionException(componentType, "array of " + componentType.getName(), descriptor);  
9.          \}  
10.         return null;  
11.     \}  
12.     if (autowiredBeanNames != null) \{  
13.         autowiredBeanNames.addAll(matchingBeans.keySet());  
14.     \}  
15.     TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());  
16.     Object result = converter.convertIfNecessary(matchingBeans.values(), type);  
17.     if (this.dependencyComparator != null && result instanceof Object\[\]) \{  
18.         Arrays.sort((Object\[\]) result, adaptDependencyComparator(matchingBeans));  
19.     \}  
20.     return result;  
21. \}  
22. else if (Collection.class.isAssignableFrom(type) && type.isInterface()) \{  
23.     Class<?> elementType = descriptor.getCollectionType();  
24.     if (elementType == null) \{  
25.         if (descriptor.isRequired()) \{  
26.             throw new FatalBeanException("No element type declared for collection \[" + type.getName() + "\]");  
27.         \}  
28.         return null;  
29.     \}  
30.     DependencyDescriptor targetDesc = new DependencyDescriptor(descriptor);  
31.     targetDesc.increaseNestingLevel();  
32.     Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType, targetDesc);  
33.     if (matchingBeans.isEmpty()) \{  
34.         if (descriptor.isRequired()) \{  
35.             raiseNoSuchBeanDefinitionException(elementType, "collection of " + elementType.getName(), descriptor);  
36.         \}  
37.         return null;  
38.     \}  
39.     if (autowiredBeanNames != null) \{  
40.         autowiredBeanNames.addAll(matchingBeans.keySet());  
41.     \}  
42.     TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());  
43.     Object result = converter.convertIfNecessary(matchingBeans.values(), type);  
44.     if (this.dependencyComparator != null && result instanceof List) \{  
45.         Collections.sort((List<?>) result, adaptDependencyComparator(matchingBeans));  
46.     \}  
47.     return result;  
48. \}  
49. else if (Map.class.isAssignableFrom(type) && type.isInterface()) \{  
50.     Class<?> keyType = descriptor.getMapKeyType();  
51.     if (keyType == null || !String.class.isAssignableFrom(keyType)) \{  
52.         if (descriptor.isRequired()) \{  
53.             throw new FatalBeanException("Key type \[" + keyType + "\] of map \[" + type.getName() +  
54.                     "\] must be assignable to \[java.lang.String\]");  
55.         \}  
56.         return null;  
57.     \}  
58.     Class<?> valueType = descriptor.getMapValueType();  
59.     if (valueType == null) \{  
60.         if (descriptor.isRequired()) \{  
61.             throw new FatalBeanException("No value type declared for map \[" + type.getName() + "\]");  
62.         \}  
63.         return null;  
64.     \}  
65.     DependencyDescriptor targetDesc = new DependencyDescriptor(descriptor);  
66.     targetDesc.increaseNestingLevel();  
67.     Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType, targetDesc);  
68.     if (matchingBeans.isEmpty()) \{  
69.         if (descriptor.isRequired()) \{  
70.             raiseNoSuchBeanDefinitionException(valueType, "map with value type " + valueType.getName(), descriptor);  
71.         \}  
72.         return null;  
73.     \}  
74.     if (autowiredBeanNames != null) \{  
75.         autowiredBeanNames.addAll(matchingBeans.keySet());  
76.     \}  
77.     return matchingBeans;  
78. \}  
79. else \{  
80.     Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);  
81.     if (matchingBeans.isEmpty()) \{  
82.         if (descriptor.isRequired()) \{  
83.             raiseNoSuchBeanDefinitionException(type, "", descriptor);  
84.         \}  
85.         return null;  
86.     \}  
87.     if (matchingBeans.size() > 1) \{  
88.         String primaryBeanName = determineAutowireCandidate(matchingBeans, descriptor);  
89.         if (primaryBeanName == null) \{  
90.             throw new NoUniqueBeanDefinitionException(type, matchingBeans.keySet());  
91.         \}  
92.         if (autowiredBeanNames != null) \{  
93.             autowiredBeanNames.add(primaryBeanName);  
94.         \}  
95.         return matchingBeans.get(primaryBeanName);  
96.     \}  
97.     // We have exactly one match.  
98.     Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();  
99.     if (autowiredBeanNames != null) \{  
100.         autowiredBeanNames.add(entry.getKey());  
101.     \}  
102.     return entry.getValue();  
103. \}

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][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。

[view plain]: http://blog.csdn.net/qianlibie/article/details/44079647#
[Autowired]: http://docs.spring.io/spring/docs/4.1.0.RELEASE/spring-framework-reference/html/beans.html#beans-autowired-annotation