详解Java Spring各种依赖注入注解的区别

2024-03-20 10:08:04 125

注解注入顾名思义就是通过注解来实现注入，Spring和注入相关的常见注解有Autowired、Resource、Qualifier、Service、Controller、Repository、Component。

Autowired是自动注入，自动从spring的上下文找到合适的bean来注入

Resource用来指定名称注入

Qualifier和Autowired配合使用，指定bean的名称
Service，Controller，Repository分别标记类是Service层类，Controller层类，数据存储层的类，spring扫描注解配置时，会标记这些类要生成bean。

Component是一种泛指，标记类是组件，spring扫描注解配置时，会标记这些类要生成bean。

Spring对于Bean的依赖注入，支持多种注解方式：

@Resource
javax.annotation
JSR250(CommonAnnotationsforJava)
@Inject
javax.inject
JSR330(DependencyInjectionforJava)
@Autowired
org.springframework.bean.factory
Spring

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autowiredbytype
-可以通过@Qualifier显式指定autowiredbyqualifiername。
-如果autowiredbytype失败（找不到或者找到多个实现），则退化为autowiredbyfieldname
@Resource
-默认autowiredbyfieldname
-如果autowiredbyfieldname失败，会退化为autowiredbytype
-可以通过@Qualifier显式指定autowiredbyqualifiername
-如果autowiredbyqualifiername失败，会退化为autowiredbyfieldname。但是这时候如果autowiredbyfieldname失败，就不会再退化为autowiredbytype了。

TIPSQualifiednameVSBeanname

在Spring设计中，Qualifiedname并不等同于Beanname，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beanname（如果没有指定，默认使用类名首字母小写），通过标签指定qualifiername：

<beanid="lamborghini"class="me.arganzheng.study.spring.autowired.Lamborghini">
<qualifiervalue="luxury"/>
<!--injectanydependenciesrequiredbythisbean-->
</bean>

如果是通过注解方式，那么可以通过@Qualifier注解指定qualifiername，通过@Named或者@Component（@Service，@Repository等）的value值指定beanname：

@Component("lamborghini")
@Qualifier("luxury")
publicclassLamborghiniimplementsCar{
}

或者

@Component
@Named("lamborghini")
@Qualifier("luxury")
publicclassLamborghiniimplementsCar{
}

同样，如果没有指定beanname，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autowiredbytype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：

<xmlversion="1.0"encoding="UTF-8"?>
<beansxmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:aop="http://www.springframework.org/schema/aop"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:util="http://www.springframework.org/schema/util"
xsi:schemaLocation="
http://www.springframework.org/schema/beanshttp://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/aophttp://www.springframework.org/schema/aop/spring-aop-3.0.xsd
http://www.springframework.org/schema/contexthttp://www.springframework.org/schema/context/spring-context-3.0.xsd
http://www.springframework.org/schema/utilhttp://www.springframework.org/schema/util/spring-util-2.5.xsd">
<context:annotation-config/>
<context:component-scanbase-package="me.arganzheng.study"/>
<util:constantid="en"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.EN"/>
<util:constantid="ja"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.JP"/>
<util:constantid="ind"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.IND"/>
<util:constantid="pt"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.PT"/>
<util:constantid="th"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.TH"/>
<util:constantid="ar"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.AR"/>
<util:constantid="en-rIn"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.EN_RIN"/>
<util:mapid="languageChangesMap"key-type="java.lang.String"
value-type="java.lang.String">
<entrykey="pt"value="pt"/>
<entrykey="br"value="pt"/>
<entrykey="jp"value="ja"/>
<entrykey="ja"value="ja"/>
<entrykey="ind"value="ind"/>
<entrykey="id"value="ind"/>
<entrykey="en-rin"value="en-rIn"/>
<entrykey="in"value="en-rIn"/>
<entrykey="en"value="en"/>
<entrykey="gb"value="en"/>
<entrykey="th"value="th"/>
<entrykey="ar"value="ar"/>
<entrykey="eg"value="ar"/>
</util:map>
</beans>

其中static-field应用的常量定义在如下类中：

packageme.arganzheng.study.spring.autowired;
publicinterfaceConstants{
publicinterfaceLanguage{
publicstaticfinalStringEN="CommonConstants.LANG_ENGLISH";
publicstaticfinalStringJP="CommonConstants.LANG_JAPANESE";
publicstaticfinalStringIND="CommonConstants.LANG_INDONESIAN";
publicstaticfinalStringPT="CommonConstants.LANG_PORTUGUESE";
publicstaticfinalStringTH="CommonConstants.LANG_THAI";
publicstaticfinalStringEN_RIN="CommonConstants.LANG_ENGLISH_INDIA";
publicstaticfinalStringAR="CommonConstants.LANG_Arabic";
}
}

然后如果我们在代码中如下声明依赖：

publicclassAutowiredTestextendsBaseSpringTestCase{
@Autowired
privateMap<String,String>languageChangesMap;
@Test
publicvoidtestAutowired(){
notNull(languageChangesMap);
System.out.println(languageChangesMap.getClass().getSimpleName());
System.out.println(languageChangesMap);
}
}

Guesswhat，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:CaughtexceptionwhileallowingTestExecutionListener

[org.springframework.test.context.support.DependencyInjectionTestExecutionListener@5c51ee0a]topreparetestinstance[me.arganzheng.study.spring.autowired.AutowiredTest@6e301e0]
org.springframework.beans.factory.BeanCreationException:Errorcreatingbeanwithname'me.arganzheng.study.spring.autowired.AutowiredTest':Injectionofautowireddependenciesfailed;nestedexceptionisorg.springframework.beans.factory.BeanCreationException:Couldnotautowirefield:privatejava.util.Mapme.arganzheng.study.spring.autowired.AutowiredTest.languageChangesMap;nestedexceptionisorg.springframework.beans.factory.NoSuchBeanDefinitionException:Noqualifyingbeanoftype[java.lang.String]foundfordependency[mapwithvaluetypejava.lang.String]:expectedatleast1beanwhichqualifiesasautowirecandidateforthisdependency.Dependencyannotations:{@org.springframework.beans.factory.annotation.Autowired(required=true)}
...
edby:org.springframework.beans.factory.NoSuchBeanDefinitionException:Noqualifyingbeanoftype[java.lang.String]foundfordependency[mapwithvaluetypejava.lang.String]:expectedatleast1beanwhichqualifiesasautowirecandidateforthisdependency.Dependencyannotations:{@org.springframework.beans.factory.annotation.Autowired(required=true)}
atorg.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:986)
atorg.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:843)
atorg.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:768)
atorg.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:486)
...28more

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：

protectedObjectdoResolveDependency(DependencyDescriptordescriptor,Class<?>type,StringbeanName,
Set<String>autowiredBeanNames,TypeConvertertypeConverter)throwsBeansException{
...
elseif(Map.class.isAssignableFrom(type)&&type.isInterface()){
Class<?>keyType=descriptor.getMapKeyType();
if(keyType==null||!String.class.isAssignableFrom(keyType)){
if(descriptor.isRequired()){
thrownewFatalBeanException("Keytype["+keyType+"]ofmap["+type.getName()+
"]mustbeassignableto[java.lang.String]");
}
returnnull;
}
Class<?>valueType=descriptor.getMapValueType();
if(valueType==null){
if(descriptor.isRequired()){
thrownewFatalBeanException("Novaluetypedeclaredformap["+type.getName()+"]");
}
returnnull;
}
Map<String,Object>matchingBeans=findAutowireCandidates(beanName,valueType,descriptor);
if(matchingBeans.isEmpty()){
if(descriptor.isRequired()){
raiseNoSuchBeanDefinitionException(valueType,"mapwithvaluetype"+valueType.getName(),descriptor);
}
returnnull;
}
if(autowiredBeanNames!=null){
autowiredBeanNames.addAll(matchingBeans.keySet());
}
returnmatchingBeans;
}
...
}

关键在这一句：Map

严重:CaughtexceptionwhileallowingTestExecutionListener

[org.springframework.test.context.support.DependencyInjectionTestExecutionListener@9476189]topreparetestinstance[me.arganzheng.study.spring.autowired.AutowiredTest@2d546e21]
...
Causedby:org.springframework.beans.factory.NoSuchBeanDefinitionException:Noqualifyingbeanoftype[java.lang.String]foundfordependency[mapwithvaluetypejava.lang.String]:expectedatleast1beanwhichqualifiesasautowirecandidateforthisdependency.Dependencyannotations:{@org.springframework.beans.factory.annotation.Autowired(required=true),@org.springframework.beans.factory.annotation.Qualifier(value=languageChangesMap)}
atorg.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:986)
atorg.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:843)
atorg.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:768)
atorg.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:486)
...28more

debug了一下，发现跟没有指定qualifiename是一样的执行路径。不是指定了beanname了吗？为什么还是autowiredbytype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：

protectedObjectdoResolveDependency(DependencyDescriptordescriptor,Class<?>type,StringbeanName,
Set<String>autowiredBeanNames,TypeConvertertypeConverter)throwsBeansException{
Objectvalue=getAutowireCandidateResolver().getSuggestedValue(descriptor);
if(value!=null){
if(valueinstanceofString){
StringstrVal=resolveEmbeddedValue((String)value);
BeanDefinitionbd=(beanName!=null&&containsBean(beanName)?getMergedBeanDefinition(beanName):null);
value=evaluateBeanDefinitionString(strVal,bd);
}
TypeConverterconverter=(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<String,Object>matchingBeans=findAutowireCandidates(beanName,componentType,descriptor);
if(matchingBeans.isEmpty()){
if(descriptor.isRequired()){
raiseNoSuchBeanDefinitionException(componentType,"arrayof"+componentType.getName(),descriptor);
}
returnnull;
}
if(autowiredBeanNames!=null){
autowiredBeanNames.addAll(matchingBeans.keySet());
}
TypeConverterconverter=(typeConverter!=null?typeConverter:getTypeConverter());
returnconverter.convertIfNecessary(matchingBeans.values(),type);
}
elseif(Collection.class.isAssignableFrom(type)&&type.isInterface()){
Class<?>elementType=descriptor.getCollectionType();
if(elementType==null){
if(descriptor.isRequired()){
thrownewFatalBeanException("Noelementtypedeclaredforcollection["+type.getName()+"]");
}
returnnull;
}
Map<String,Object>matchingBeans=findAutowireCandidates(beanName,elementType,descriptor);
if(matchingBeans.isEmpty()){
if(descriptor.isRequired()){
raiseNoSuchBeanDefinitionException(elementType,"collectionof"+elementType.getName(),descriptor);
}
returnnull;
}
if(autowiredBeanNames!=null){
autowiredBeanNames.addAll(matchingBeans.keySet());
}
TypeConverterconverter=(typeConverter!=null?typeConverter:getTypeConverter());
returnconverter.convertIfNecessary(matchingBeans.values(),type);
}
elseif(Map.class.isAssignableFrom(type)&&type.isInterface()){
Class<?>keyType=descriptor.getMapKeyType();
if(keyType==null||!String.class.isAssignableFrom(keyType)){
if(descriptor.isRequired()){
thrownewFatalBeanException("Keytype["+keyType+"]ofmap["+type.getName()+
"]mustbeassignableto[java.lang.String]");
}
returnnull;
}
Class<?>valueType=descriptor.getMapValueType();
if(valueType==null){
if(descriptor.isRequired()){
thrownewFatalBeanException("Novaluetypedeclaredformap["+type.getName()+"]");
}
returnnull;
}
Map<String,Object>matchingBeans=findAutowireCandidates(beanName,valueType,descriptor);
if(matchingBeans.isEmpty()){
if(descriptor.isRequired()){
raiseNoSuchBeanDefinitionException(valueType,"mapwithvaluetype"+valueType.getName(),descriptor);
}
returnnull;
}
if(autowiredBeanNames!=null){
autowiredBeanNames.addAll(matchingBeans.keySet());
}
returnmatchingBeans;
}
else{
Map<String,Object>matchingBeans=findAutowireCandidates(beanName,type,descriptor);
if(matchingBeans.isEmpty()){
if(descriptor.isRequired()){
raiseNoSuchBeanDefinitionException(type,"",descriptor);
}
returnnull;
}
if(matchingBeans.size()>1){
StringprimaryBeanName=determinePrimaryCandidate(matchingBeans,descriptor);
if(primaryBeanName==null){
thrownewNoUniqueBeanDefinitionException(type,matchingBeans.keySet());
}
if(autowiredBeanNames!=null){
autowiredBeanNames.add(primaryBeanName);
}
returnmatchingBeans.get(primaryBeanName);
}
//Wehaveexactlyonematch.
Map.Entry<String,Object>entry=matchingBeans.entrySet().iterator().next();
if(autowiredBeanNames!=null){
autowiredBeanNames.add(entry.getKey());
}
returnentry.getValue();
}
}

如果是Array，Collection或者Map，则根据集合类中元素的类型来进行autowiredbytype（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：

@Autowired
privateList<Car>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:
Nouniquebeanoftype[me.arganzheng.study.spring.autowired.Car]isdefined:
expectedsinglematchingbeanbutfound2:[audi,toyota].

然而，上面的情况如果你用@Resource则不会有这个问题：

publicclassAutowiredTestextendsBaseSpringTestCase{
@Resource
@Qualifier("languageChangesMap")
privateMap<String,String>languageChangesMap;
@Test
publicvoidtestAutowired(){
assertNotNull(languageChangesMap);
System.out.println(languageChangesMap.getClass().getSimpleName());
System.out.println(languageChangesMap);
}
}

正常运行：

LinkedHashMap
{pt=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fieldname不是languageChangesMap，那么还是一样报错的。

Causedby:org.springframework.beans.factory.NoSuchBeanDefinitionException:Noqualifyingbeanoftype[java.lang.String]foundfordependency[mapwithvaluetypejava.lang.String]:expectedatleast1beanwhichqualifiesasautowirecandidateforthisdependency.Dependencyannotations:{@javax.annotation.Resource(shareable=true,mappedName=,description=,name=,type=classjava.lang.Object,authenticationType=CONTAINER,lookup=)}
atorg.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:986)
atorg.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:843)
atorg.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:768)
atorg.springframework.context.annotation.CommonAnnotationBeanPostProcessor.autowireResource(CommonAnnotationBeanPostProcessor.java:438)
atorg.springframework.context.annotation.CommonAnnotationBeanPostProcessor.getResource(CommonAnnotationBeanPostProcessor.java:416)
atorg.springframework.context.annotation.CommonAnnotationBeanPostProcessor$ResourceElement.getResourceToInject(CommonAnnotationBeanPostProcessor.java:550)
atorg.springframework.beans.factory.annotation.InjectionMetadata$InjectedElement.inject(InjectionMetadata.java:150)
atorg.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)
atorg.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessPropertyValues(CommonAnnotationBeanPostProcessor.java:303)
...26more

而且，@Resource也可以实现上面的List接收所有实现：

publicclassAutowiredTestextendsBaseSpringTestCase{
@Resource
@Qualifier("languageChangesMap")
privateMap<String,String>languageChangesMap;
@Resource
privateList<Car>cars;
@Test
publicvoidtestAutowired(){
assertNotNull(languageChangesMap);
System.out.println(languageChangesMap.getClass().getSimpleName());
System.out.println(languageChangesMap);
assertNotNull(cars);
System.out.println(cars.getClass().getSimpleName());
System.out.println(cars);
}
}

运行的妥妥的：

LinkedHashMap
{pt=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}
ArrayList

[me.arganzheng.study.spring.autowired.Audi@579584da,me.arganzheng.study.spring.autowired.Toyota@19453122]
这是因为@Resource注解使用的是CommonAnnotationBeanPostProcessor处理器，跟AutowiredAnnotationBeanPostProcessor不是同一个作者[/偷笑]。这里就不分析了，感兴趣的同学可以自己看代码研究一下。

最终结论如下：

1、@Autowired和@Inject

autowiredbytype可以通过@Qualifier显式指定autowiredbyqualifiername（非集合类。注意：不是autowiredbybeanname！）

如果autowiredbytype失败（找不到或者找到多个实现），则退化为autowiredbyfieldname（非集合类）

2、@Resource

默认autowiredbyfieldname
如果autowiredbyfieldname失败，会退化为autowiredbytype
可以通过@Qualifier显式指定autowiredbyqualifiername
如果autowiredbyqualifiername失败，会退化为autowiredbyfieldname。但是这时候如果autowiredbyfieldname失败，就不会再退化为autowiredbytype了
测试工程保存在GitHub上，是标准的maven工程，感兴趣的同学可以clone到本地运行测试一下。

补充

有同事指出Spring官方文档上有这么一句话跟我的结有点冲突：

However,althoughyoucanusethisconventiontorefertospecificbeansbyname,@Autowiredisfundamentallyabouttype-driveninjectionwithoptionalsemanticqualifiers.Thismeansthatqualifiervalues,evenwiththebeannamefallback,alwayshavenarrowingsemanticswithinthesetoftypematches;theydonotsemanticallyexpressareferencetoauniquebeanid.

也就是说@Autowired即使加了@Qualifier注解，其实也是autowiredbytype。@Qualifier只是一个限定词，过滤条件而已。重新跟进了一下代码，发现确实是这样子的。Spring设计的这个@Qualifiername并不等同于beanname。他有点类似于一个tag。不过如果这个tag是唯一的化，那么其实效果上等同于beanname。实现上，Spring是先getByType，得到listcandicates，然后再根据qualifiername进行过滤。

再定义一个兰博基尼，这里使用@Qualifier指定：

packageme.arganzheng.study.spring.autowired;
importorg.springframework.beans.factory.annotation.Qualifier;
importorg.springframework.stereotype.Component;
@Component
@Qualifier("luxury")
publicclassLamborghiniimplementsCar{
}

再定义一个劳斯莱斯，这里故意用@Named指定：

packageme.arganzheng.study.spring.autowired;
importjavax.inject.Named;
importorg.springframework.stereotype.Component;
@Component
@Named("luxury")
publicclassRollsRoyceimplementsCar{
}

测试一下注入定义的豪华车：

packageme.arganzheng.study.spring.autowired;
importstaticjunit.framework.Assert.assertNotNull;
importjava.util.List;
importme.arganzheng.study.BaseSpringTestCase;
importorg.junit.Test;
importorg.springframework.beans.factory.annotation.Autowired;
importorg.springframework.beans.factory.annotation.Qualifier;
/**
*
*@authorzhengzhibin
*
*/
publicclassAutowiredTestextendsBaseSpringTestCase{
@Autowired
@Qualifier("luxury")
privateList<Car>luxuryCars;
@Test
publicvoidtestAutowired(){
assertNotNull(luxuryCars);
System.out.println(luxuryCars.getClass().getSimpleName());
System.out.println(luxuryCars);
}
}

运行结果如下：

ArrayList
[me.arganzheng.study.spring.autowired.Lamborghini@66b875e1,me.arganzheng.study.spring.autowired.RollsRoyce@58433b76]

补充：Autowiringmodes

Spring支持四种autowire模式，当使用XML配置方式时，你可以通过autowire属性指定。

no.(Default)Noautowiring.Beanreferencesmustbedefinedviaarefelement.Changingthedefaultsettingisnotrecommendedforlargerdeployments,becausespecifyingcollaboratorsexplicitlygivesgreatercontrolandclarity.Tosomeextent,itdocumentsthestructureofasystem.
byName.Autowiringbypropertyname.Springlooksforabeanwiththesamenameasthepropertythatneedstobeautowired.Forexample,ifabeandefinitionissettoautowirebyname,anditcontainsamasterproperty(thatis,ithasasetMaster(..)method),Springlooksforabeandefinitionnamedmaster,andusesittosettheproperty.
byType.Allowsapropertytobeautowiredifexactlyonebeanofthepropertytypeexistsinthecontainer.Ifmorethanoneexists,afatalexceptionisthrown,whichindicatesthatyoumaynotusebyTypeautowiringforthatbean.Iftherearenomatchingbeans,nothinghappens;thepropertyisnotset.
constructor.AnalogoustobyType,butappliestoconstructorarguments.Ifthereisnotexactlyonebeanoftheconstructorargumenttypeinthecontainer,afatalerrorisraised.

如果使用@Autowired、@Inject或者@Resource注解的时候，则稍微复杂一些，会有一个失败退化过程，并且引入了Qualifier。不过基本原理是一样。

详解Java Spring各种依赖注入注解的区别

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