@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援。但是使用起來這三者到底有什麼區別呢？筆者經過方法的測試，發現一些有意思的特性。

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
- 預設 autowired by type
- 可以 通過@Qualifier 顯式指定 autowired by qualifier name。
- 如果 autowired by type 失敗（找不到或者找到多個實現），則退化為autowired by field name

@Resource
- 預設 autowired by field name
- 如果 autowired by field name失敗，會退化為 autowired by type
- 可以 通過@Qualifier 顯式指定 autowired by qualifier name
- 如果 autowired by qualifier name失敗，會退化為 autowired by field name。但是這時候如果 autowired by field name失敗，就不會再退化為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，可以通過id屬性指定bean name（如果沒有指定，預設使用類名首字母小寫），通過標籤指定qualifier name：

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

如果是通過註解方式，那麼可以通過@Qualifier註解指定qualifier name，通過@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 {

}

同樣，如果沒有指定bean name，那麼Spring會預設是用類名首字母小寫(Lamborghini=>lamborghini)。

三、 通過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。但是不同擔心通過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實現都是有”bug”的，也就是說@Autowired和@Inject都是有坑的（稱之為坑，不稱之為bug是因為貌似是故意的。。）。這是來源於線上的一個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應用的常量定義在如下類中：

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";
    }
}

然後如果我們在程式碼中如下宣告依賴：

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！

執行結果如下：

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.springframewor[email protected]5c51ee0a] to prepare test instance [[email protected]]
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，發現確實是Spring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.springframewor[email protected]9476189] to prepare test instance [[email protected]]
...
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，發現跟沒有指定qualifie name是一樣的執行路徑。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方法對首先對型別做了區別：

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或者Map，則根據集合類中元素的型別來進行autowired by type（Map使用value的型別）。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型別的實現，也就是說可以這樣子注入：

@Autowired
private List<Car> 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);
    }
}

正常執行：

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}

當然，你如果不指定@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接收所有實現：

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);
    }
}

執行的妥妥的：

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
[[email protected], [email protected]]

這是因為@Resource註解使用的是CommonAnnotationBeanPostProcessor處理器，跟AutowiredAnnotationBeanPostProcessor不是同一個作者[/偷笑]。這裡就不分析了，感興趣的同學可以自己看程式碼研究一下。

最終結論如下：

1、@Autowired和@Inject

autowired by type 可以 通過@Qualifier 顯式指定 autowired by qualifier name（非集合類。注意：不是autowired by bean name！）
如果 autowired by type 失敗（找不到或者找到多個實現），則退化為autowired by field name（非集合類）

2、@Resource

預設 autowired by field name
如果 autowired by field name失敗，會退化為 autowired by type
可以 通過@Qualifier 顯式指定 autowired by qualifier name
如果 autowired by qualifier name失敗，會退化為 autowired by field name。但是這時候如果 autowired by field name失敗，就不會再退化為autowired by type了

測試工程儲存在GitHub上，是標準的maven工程，感興趣的同學可以clone到本地執行測試一下。

補充

有同事指出Spring官方文件上有這麼一句話跟我的結有點衝突：

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.

也就是說@Autowired即使加了@Qualifier註解，其實也是autowired by type。@Qualifier只是一個限定詞，過濾條件而已。重新跟進了一下程式碼，發現確實是這樣子的。Spring設計的這個 @Qualifier name 並不等同於 bean name。他有點類似於一個tag。不過如果這個tag是唯一的化，那麼其實效果上等同於bean name。實現上，Spring是先getByType，得到list candicates，然後再根據qualifier name進行過濾。

再定義一個蘭博基尼，這裡使用@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 {

}

再定義一個勞斯萊斯，這裡故意用@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 {

}

測試一下注入定義的豪華車：

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);
    }

}

執行結果如下：

ArrayList
[[email protected], [email protected]]

補充：Autowiring modes

Spring支援四種autowire模式，當使用XML配置方式時，你可以通過autowire屬性指定。

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註解的時候，則稍微複雜一些，會有一個失敗退化過程，並且引入了Qualifier。不過基本原理是一樣。