詳解Java Spring各種依賴注入註解的差異

註解注入顧名思義就是透過註解來實現注入，Spring和注入相關的常見註解有Autowired、Resource、Qualifier、Service、Controller、Repository、Component。

Autowired是自動注入，自動從spring的上下文找到合適的bean來注入

Resource用來指定名稱注入

Qualifier和Autowired配合使用，指定bean的名稱
Service，Controller，Repository Service層類，Controller層類，資料儲存層的類，spring掃描註解配置時，會標記這些類別要產生bean。

Component是一種泛指，標記類別是元件，spring掃描註解配置時，會標記這些類別要產生bean。

Spring對於Bean的依賴注入，支持多種註解方式：

@Resource
javax.annotation
JSR250 (Common Annotations for Java)
@Inject
javax.inject
JSR330 (Dependency Injection for Java)
@Autowired
org.springframework.bean.factory
Spring

直觀上看起來，@Autowired是Spring提供的註解，其他幾個都是JDK本身內建的註解，Spring對這些註解也進行了支持。但是使用起來這三者到底有什麼差別呢？筆者經過方法的測試，發現一些有趣的特性。

區別總結如下：

一、@Autowired有個required屬性，可以配置為false，這種情況下如果沒有找到對應的bean是不會拋異常的。 @Inject和@Resource沒有提供對應的配置，所以必須找到否則會拋異常。

二、 @Autowired和@Inject基本上是一樣的，因為兩者都是使用AutowiredAnnotationBeanPostProcessor來處理 依賴注入。但是@Resource是個例外，它使用的是CommonAnnotationBeanPostProcessor來處理依賴注入。當然，兩者 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設計中，Qualified name並不等同於Bean name，後者必須是唯一的，但是前者類似於tag或者group的作用，對特定的bean進行分類。可以達到getByTag(group)的效果。對於XML配置的bean，可以透過id屬性指定bean name（如果沒有指定，預設使用類別名稱首字母小寫），透過標籤指定qualifier name：

<bean id="lamborghini" class="me.arganzheng.study.spring.autowired.Lamborghini">
<qualifier value="luxury"/>
<!-- inject any dependencies required by this bean -->
</bean>

如果是透過註解方式，那麼可以透過註解@Qualifier註解指定qualifier name，透過@Named或@Component（@Service，@Repository等）的value值指定bean name：

@Component("lamborghini")
@Qualifier("luxury")
public class Lamborghini implements Car {
}

或

@Component
@Named("lamborghini")
@Qualifier("luxury")
public class Lamborghini implements Car {
}

同樣，如果沒有指定「預設值類別名首字母小寫(Lamborghini=>lamborghini)。

三、 透過Anotation注入依賴的方式在XML注入方式之前進行。如果對同一個bean的依賴同時使用了兩種注入方式，那麼XML的優先順序。但不同擔心透過Anotation注入的依賴沒法注入XML中配置的bean，依賴注入是在bean的註冊之後進行的。

四、目前的autowired by type方式（筆者用的是3.2.3.RELEASE版本），Spring的AutowiredAnnotationBeanPostProcessor 實作都是有」bug」的，也就是說@Autowired和@Inject都是有坑的（稱之為坑，不稱為bug是因為貌似是故意的。這是來自線上 的一個bug，也是這邊文章的寫作原因。現場如下：

application-context.xml中有如下定義：

<xml version="1.0" encoding="UTF-8"?>
<beans xmlns="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/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-3.0.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd
http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-2.5.xsd">
<context:annotation-config />
<context:component-scan base-package="me.arganzheng.study" />
<util:constant id="en"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.EN" />
<util:constant id="ja"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.JP" />
<util:constant id="ind"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.IND" />
<util:constant id="pt"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.PT" />
<util:constant id="th"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.TH" />
<util:constant id="ar"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.AR" />
<util:constant id="en-rIn"
static-field="me.arganzheng.study.spring.autowired.Constants.Language.EN_RIN" />
<util:map id="languageChangesMap" key-type="java.lang.String"
value-type="java.lang.String">
<entry key="pt" value="pt" />
<entry key="br" value="pt" />
<entry key="jp" value="ja" />
<entry key="ja" value="ja" />
<entry key="ind" value="ind" />
<entry key="id" value="ind" />
<entry key="en-rin" value="en-rIn" />
<entry key="in" value="en-rIn" />
<entry key="en" value="en" />
<entry key="gb" value="en" />
<entry key="th" value="th" />
<entry key="ar" value="ar" />
<entry key="eg" value="ar" />
</util:map>
</beans>

其中static-field應用的常數定義在如下類中：

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";
}
}

然後如果我們在如下類中：

public class AutowiredTest extends BaseSpringTestCase {
@Autowired
private Map<String, String> languageChangesMap;
@Test
public void testAutowired() {
notNull(languageChangesMap);
System.out.println(languageChangesMap.getClass().getSimpleName());
System.out.println(languageChangesMap);
}
}

Guess what，詭異的事情發生了！

運行結果如下：

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}

也就是說Map

嚴重: 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)}
...
ed 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

delid，發現了一下確實是Springdering的一個bug了一下，發現了一個確實是Spdering的一個bug，發現一個確實是Springdering的一個bug，發現了一個確實是Springdering的一個bug。在DefaultListableBeanFactory的這個方法出問題了：

protected Object doResolveDependency(DependencyDescriptor descriptor, Class<?> type, String beanName,
Set<String> 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<String, Object> 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;
}
...
}

關鍵在這一句：Map

嚴重: 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

不是指定了bean name了嗎？為什麼還是autowired by type呢？仔細查看了一下才發現。 DefaultListableBeanFactory的doResolveDependency方法對首先對類型做 了區別：

protected Object doResolveDependency(DependencyDescriptor descriptor, Class<?> type, String beanName,
Set<String> 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<String, Object> 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<String, Object> 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<String, Object> 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<String, Object> 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<String, Object> entry = matchingBeans.entrySet().iterator().next();
if (autowiredBeanNames != null) {
autowiredBeanNames.add(entry.getKey());
}
return entry.getValue();
}
}

如果是Array，Collection或Map，則根據集合類別中元素的類型來進行autowired by type（Map使用value的類型）。為什麼這麼特殊處理呢？原來，Spring是為了達到這樣的目的：讓你可以一次注入所有符合類型的實現，也就是說可以這樣子注入：

@Autowired 

private List cars;

如果你的car有多個實現，那麼都會注入進來，不會再報

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，上面的情況如果你用@Resource則不會有這個問題： 

public class AutowiredTest extends BaseSpringTestCase {
@Resource
@Qualifier("languageChangesMap")
private Map<String, String> languageChangesMap;
@Test
public void testAutowired() {
assertNotNull(languageChangesMap);
System.out.println(languageChangesMap.getClass().getSimpleName());
System.out.println(languageChangesMap);
}
}

正常運行：

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}

不指定@Qualifier(“languageChangesMap”)，同時field name不是languageChangesMap，那麼還是一樣報錯的。

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.resolveDependency(DefaultListableBeanFactory.java:768)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.autowireResource(CommonAnnotationBeanPostProcessor.java:438)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.getResource(CommonAnnotationBeanPostProcessor.java:416)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor$ResourceElement.getResourceToInject(CommonAnnotationBeanPostProcessor.java:550)
at org.springframework.beans.factory.annotation.InjectionMetadata$InjectedElement.inject(InjectionMetadata.java:150)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessPropertyValues(CommonAnnotationBeanPostProcessor.java:303)
... 26 more

而且，@Resource也可以實現上面的List接收所有實作：

public class AutowiredTest extends BaseSpringTestCase {
@Resource
@Qualifier("languageChangesMap")
private Map<String, String> languageChangesMap;
@Resource
private List<Car> cars;
@Test
public void testAutowired() {
assertNotNull(languageChangesMap);
System.out.println(languageChangesMap.getClass().getSimpleName());
System.out.println(languageChangesMap);
assertNotNull(cars);
System.out.println(cars.getClass().getSimpleName());
System.out.println(cars);
}
}

運行的妥妥的：

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

autowired by type 可以 通过@Qualifier 显式指定 autowired by qualifier name（非集合类。注意：不是autowired by bean 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（非集合类）

2、@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
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, although you can use this convention to refer to specific beans by name, @Autowired is fundamentally about type-driven injection with optional semantic qualifiers. This means that qualifier values, even with the bean name fallback, always have narrowing semantics within the set of type matches; they do not semantically express a reference to a unique bean id.

也就是说@Autowired即使加了@Qualifier注解，其实也是autowired by type。@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 @Qualifier name 并不等同于 bean name。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bean name。实现上，Spring是先getByType，得到list candicates，然后再根据qualifier name进行过滤。

再定义一个兰博基尼，这里使用@Qualifier指定：

package me.arganzheng.study.spring.autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;
@Component
@Qualifier("luxury")
public class Lamborghini implements Car {
}

再定义一个劳斯莱斯，这里故意用@Named指定：

package me.arganzheng.study.spring.autowired;
import javax.inject.Named;
import org.springframework.stereotype.Component;
@Component
@Named("luxury")
public class RollsRoyce implements Car {
}

测试一下注入定义的豪华车：

package me.arganzheng.study.spring.autowired;
import static junit.framework.Assert.assertNotNull;
import java.util.List;
import me.arganzheng.study.BaseSpringTestCase;
import org.junit.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
/**
*
* @author zhengzhibin
*
*/
public class AutowiredTest extends BaseSpringTestCase {
@Autowired
@Qualifier("luxury")
private List<Car> luxuryCars;
@Test
public void testAutowired() {
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：Autowiring modes

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) No autowiring. Bean references must be defined via a ref element. Changing the default setting is not recommended for larger deployments, because specifying collaborators explicitly gives greater control and clarity. To some extent, it documents the structure of a system.
byName. Autowiring by property name. Spring looks for a bean with the same name as the property that needs to be autowired. For example, if a bean definition is set to autowire by name, and it contains a master property (that is, it has a setMaster(..) method), Spring looks for a bean definition named master, and uses it to set the property.
byType. Allows a property to be autowired if exactly one bean of the property type exists in the container. If more than one exists, a fatal exception is thrown, which indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens; the property is not set.
constructor. Analogous to byType, but applies to constructor arguments. If there is not exactly one bean of the constructor argument type in the container, a fatal error is raised.

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。

更多详解Java Spring各种依赖注入注解的区别相关文章请关注PHP中文网！