java 注解value动态获取 java注解autowired

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mob64ca140e76c8 2023-09-11 06:46:18

文章标签 java 注解value动态获取 @autowired注解作用 @qualifier注解 spring java 文章分类 Java 后端开发

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 name@Resource- 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

@Component@Named("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：

<?xml version="1.0" encoding="UTF-8"?>

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：

LinkedHashMap{en=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.DependencyInjectionTestExecutionListener@5c51ee0a] to prepare test instance [me.arganzheng.study.spring.autowired.AutowiredTest@6e301e0]org.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 more

debug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.DependencyInjectionTestExecutionListener@9476189] to prepare test instance [me.arganzheng.study.spring.autowired.AutowiredTest@2d546e21]...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 more

debug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,

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