Java-Spring各种依赖注入注解的区别.doc

Spring对于Bean的依赖注入，支持多种注解方式：Resourcejavax.annotationJSR250 (Common Annotations for Java)Injectjavax.injectJSR330 (Dependency Injection for Java)Autowiredorg.springframework.bean.factorySpring直观上看起来，Autowired是Spring提供的注解，其他几个都是JDK本身内建的注解，Spring对这些注解也进行了支持。但是使用起来这三者到底有什么区别呢？笔者经过方法的测试，发现一些有意思的特性。区别总结如下：一、Autowired有个required属性，可以配置为false，这种情况下如果没有找到对应的bean是不会抛异常的。Inject和Resource没有提供对应的配置，所以必须找到否则会抛异常。二、 Autowired和Inject基本是一样的，因为两者都是使用AutowiredAnnotationBeanPostProcessor来处理依赖注入。但是Resource是个例外，它使用的是CommonAnnotationBeanPostProcessor来处理依赖注入。当然，两者都是BeanPostProcessor。Autowired和Inject- 默认 autowired by type- 可以 通过Qualifier 显式指定 autowired by qualifier name。- 如果 autowired by type 失败（找不到或者找到多个实现），则退化为autowired by field nameResource- 默认 autowired by field name- 如果 autowired by field name失败，会退化为 autowired by type- 可以 通过Qualifier 显式指定 autowired by qualifier name- 如果 autowired by qualifier name失败，会退化为 autowired by field name。但是这时候如果 autowired by field name失败，就不会再退化为autowired by type了。TIPS Qualified name VS Bean name在Spring设计中，Qualified name并不等同于Bean name，后者必须是唯一的，但是前者类似于tag或者group的作用，对特定的bean进行分类。可以达到getByTag(group)的效果。对于XML配置的bean，可以通过id属性指定bean name（如果没有指定，默认使用类名首字母小写），通过标签指定qualifier name： 如果是通过注解方式，那么可以通过Qualifier注解指定qualifier name，通过Named或者Component（Service，Repository等）的value值指定bean name：Component(lamborghini)Qualifier(luxury)public class Lamborghini implements Car 或者ComponentNamed(lamborghini)Qualifier(luxury)public class Lamborghini implements Car 同样，如果没有指定bean name，那么Spring会默认是用类名首字母小写(Lamborghini=lamborghini)。三、 通过Anotation注入依赖的方式在XML注入方式之前进行。如果对同一个bean的依赖同时使用了两种注入方式，那么XML的优先。但是不同担心通过Anotation注入的依赖没法注入XML中配置的bean，依赖注入是在bean的注册之后进行的。四、目前的autowired by type方式（笔者用的是3.2.3.RELEASE版本），Spring的AutowiredAnnotationBeanPostProcessor实现都是有”bug”的，也就是说Autowired和Inject都是有坑的（称之为坑，不称之为bug是因为貌似是故意的。）。这是来源于线上的一个bug，也是这边文章的写作原因。现场如下：application-context.xml中有如下定义： 其中static-field应用的常量定义在如下类中：package me.arganzheng.study.spring.autowired;public interface Constants public interface Language public static final String EN = CommonConstants.LANG_ENGLISH; public static final String JP = CommonConstants.LANG_JAPANESE; public static final String IND = CommonConstants.LANG_INDONESIAN; public static final String PT = CommonConstants.LANG_PORTUGUESE; public static final String TH = CommonConstants.LANG_THAI; public static final String EN_RIN = CommonConstants.LANG_ENGLISH_INDIA; public static final String AR = CommonConstants.LANG_Arabic; 然后如果我们在代码中如下声明依赖：public class AutowiredTest extends BaseSpringTestCase Autowired private Map languageChangesMap; Test public void testAutowired() notNull(languageChangesMap); System.out.println(languageChangesMap.getClass().getSimpleName(); System.out.println(languageChangesMap); Guess what，诡异的事情发生了！运行结果如下：LinkedHashMapen=CommonConstants.LANG_ENGLISH, ja=CommonConstants.LANG_JAPANESE, ind=CommonConstants.LANG_INDONESIAN, pt=CommonConstants.LANG_PORTUGUESE, th=CommonConstants.LANG_THAI, ar=CommonConstants.LANG_Arabic, en-rIn=CommonConstants.LANG_ENGLISH_INDIA也就是说Map严重: Caught exception while allowing TestExecutionListener org.springframework.test.context.support.DependencyInjectionTestExecutionListener5c51ee0a to prepare test instance me.arganzheng.study.spring.autowired.AutowiredTest6e301e0org.springframework.beans.factory.BeanCreationException: Error creating bean with name me.arganzheng.study.spring.autowired.AutowiredTest: Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private java.util.Map me.arganzheng.study.spring.autowired.AutowiredTest.languageChangesMap; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type java.lang.String found for dependency map with value type java.lang.String: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: org.springframework.beans.factory.annotation.Autowired(required=true) .Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type java.lang.String found for dependency map with value type java.lang.String: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: org.springframework.beans.factory.annotation.Autowired(required=true) at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:986) at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:843) at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:768) at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:486) . 28 moredebug了一下，发现确实是Spring的一个bug。在DefaultListableBeanFactory的这个方法出问题了：protected Object doResolveDependency(DependencyDescriptor descriptor, Class type, String beanName, Set autowiredBeanNames, TypeConverter typeConverter) throws BeansException . 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; Map matchingBeans = findAutowireCandidates(beanName, valueType, descriptor); 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; . 关键在这一句：Map严重: Caught exception while allowing TestExecutionListener org.springframework.test.context.support.DependencyInjectionTestExecutionListener9476189 to prepare test instance me.arganzheng.study.spring.autowired.AutowiredTest2d546e21.Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type java.lang.String found for dependency map with value type java.lang.String: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: org.springframework.beans.factory.annotation.Autowired(required=true), org.springframework.beans.factory.annotation.Qualifier(value=languageChangesMap) at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:986) at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:843) at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:768) at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:486) . 28 moredebug了一下，发现跟没有指定qualifie name是一样的执行路径。不是指定了bean name了吗？为什么还是autowired by type呢？仔细查看了一下才发现。DefaultListableBeanFactory的doResolveDependency方法对首先对类型做了区别：protected Object doResolveDependency(DependencyDescriptor descriptor, Class type, String beanName, Set autowiredBeanNames, TypeConverter typeConverter) throws BeansException Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor); if (value != null) if (value instanceof String) String strVal = resolveEmbeddedValue(String) value); BeanDefinition bd = (beanName != null & containsBean(beanName) ? getMergedBeanDefinition(beanName) : null); value = evaluateBeanDefinitionString(strVal, bd); TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter(); return (descriptor.getField() != null ? converter.convertIfNecessary(value, type, descriptor.getField() : converter.convertIfNecessary(value, type, descriptor.getMethodParameter(); if (type.isArray() Class componentType = type.getComponentType(); Map matchingBeans = findAutowireCandidates(beanName, componentType, descriptor); 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(); return converter.convertIfNecessary(matchingBeans.values(), type); 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; Map matchingBeans = findAutowireCandidates(beanName, elementType, descriptor); 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(); return converter.convertIfNecessary(matchingBeans.values(), type); 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; Map matchingBeans = findAutowireCandidates(beanName, valueType, descriptor); 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 matchingBeans = findAutowireCandidates(beanName, type, descriptor); if (matchingBeans.isEmpty() if (descriptor.isRequired() raiseNoSuchBeanDefinitionException(type, , descriptor); return null; if (matchingBeans.size() 1) String primaryBeanName = determinePrimaryCandidate(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 entry = matchingBeans.entrySet().iterator().next(); if (autowiredBeanNames != null) autowiredBeanNames.add(entry.getKey(); return entry.getValue(); 如果是Array，Collection或者Map，则根据集合类中元素的类型来进行autowired by type（Map使用value的类型）。为什么这么特殊处理呢？原来，Spring是为了达到这样的目的：让你可以一次注入所有符合类型的实现，也就是说可以这样子注入：Autowiredprivate List cars;如果你的car有多个实现，那么都会注入进来，不会再报org.springframework.beans.factory.NoSuchBeanDefinitionException: No unique bean of type me.arganzheng.study.spring.autowired.Car is defined: expected single matching bean but found 2: audi, toyota.然而，上面的情况如果你用Resource则不会有这个问题：public class AutowiredTest extends BaseSpringTestCase Resource Qualifier(languageChangesMap) private Map languageChangesMap; Test public void testAutowired() assertNotNull(languageChangesMap); System.out.println(languageChangesMap.getClass().getSimpleName(); System.out.println(languageChangesMap); 正常运行：LinkedHashMappt=pt, br=pt, jp=ja, ja=ja, ind=ind, id=ind, en-rin=en-rIn, in=en-rIn, en=en, gb=en, th=th, ar=ar, eg=ar当然，你如果不指定Qualifier(“languageChangesMap”)，同时field name不是languageChangesMap，那么还是一样报错的。Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type java.lang.String found for dependency map with value type java.lang.String: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: javax.annotation.Resource(shareable=true, mappedName=, description=, name=, type=class java.lang.Object, authenticationType=CONTAINER, lookup=)at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:986)at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:843)at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDepen