詳解Java Spring各種依賴注入注解的區(qū)別

更新時間：2016年03月30日 16:20:08 作者：徐劉根

這篇文章主要介紹了詳解Java Spring各種依賴注入注解的區(qū)別的相關(guān)資料,需要的朋友可以參考下

注解注入顧名思義就是通過注解來實現(xiàn)注入，Spring和注入相關(guān)的常見注解有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分別標記類是Service層類，Controller層類，數(shù)據(jù)存儲層的類，spring掃描注解配置時，會標記這些類要生成bean。

Component是一種泛指，標記類是組件，spring掃描注解配置時，會標記這些類要生成bean。

Spring對于Bean的依賴注入，支持多種注解方式：

@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提供的注解，其他幾個都是JDK本身內(nèi)建的注解，Spring對這些注解也進行了支持。但是使用起來這三者到底有什么區(qū)別呢？筆者經(jīng)過方法的測試，發(fā)現(xiàn)一些有意思的特性。

區(qū)別總結(jié)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(yīng)的bean是不會拋異常的。@Inject和@Resource沒有提供對應(yīng)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(dāng)然，兩者都是BeanPostProcessor。

@Autowired和@Inject 
- 默認 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(xiàn)），則退化為autowired by field name 
@Resource 
- 默認 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(shè)計中，Qualified name并不等同于Bean name，后者必須是唯一的，但是前者類似于tag或者group的作用，對特定的bean進行分類?？梢赃_到getByTag(group)的效果。對于XML配置的bean，可以通過id屬性指定bean name（如果沒有指定，默認使用類名首字母小寫），通過標簽指定qualifier name：

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

如果是通過注解方式，那么可以通過@Qualifier注解指定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，那么Spring會默認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，那么XML的優(yōu)先。但是不同擔(dān)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的注冊之后進行的。

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(xiàn)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。。）。這是來源于線上的一個bug，也是這邊文章的寫作原因?，F(xiàn)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(yīng)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"; 
} 
}

然后如果我們在代碼中如下聲明依賴：

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(fā)生了！

運行結(jié)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

debug了一下，發(fā)現(xiàn)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(guān)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

debug了一下，發(fā)現(xiàn)跟沒有指定qualifie name是一樣的執(zhí)行路徑。不是指定了bean name了嗎？為什么還是autowired by type呢？仔細查看了一下才發(fā)現(xiàn)。DefaultListableBeanFactory的doResolveDependency方法對首先對類型做了區(qū)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(jù)集合類中元素的類型來進行autowired by type（Map使用value的類型）。為什么這么特殊處理呢？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(xiàn)，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(xiàn)，那么都會注入進來，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}

當(dāng)然，你如果不指定@Qualifier(“l(fā)anguageChangesMap”)，同時field name不是languageChangesMap，那么還是一樣報錯的。

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(xiàn)上面的List接收所有實現(xiàn)：

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]
這是因為@Resource注解使用的是CommonAnnotationBeanPostProcessor處理器，跟 AutowiredAnnotationBeanPostProcessor不是同一個作者[/偷笑]。這里就不分析了，感興趣的同學(xué)可以自己看代碼研究一下。

最終結(jié)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?。?br />

如果 autowired by type 失敗（找不到或者找到多個實現(xiàn)），則退化為autowired by field name（非集合類）

2、@Resource

默認 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
測試工程保存在GitHub上，是標準的maven工程，感興趣的同學(xué)可以clone到本地運行測試一下。

補充

有同事指出Spring官方文檔上有這么一句話跟我的結(jié)有點沖突：

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注解，其實也是autowired by type。@Qualifier只是一個限定詞，過濾條件而已。重新跟進了一下代碼，發(fā)現(xiàn)確實是這樣子的。Spring設(shè)計的這個 @Qualifier name 并不等同于 bean name。他有點類似于一個tag。不過如果這個tag是唯一的化，那么其實效果上等同于bean name。實現(xiàn)上，Spring是先getByType，得到list candicates，然后再根據(jù)qualifier name進行過濾。

再定義一個蘭博基尼，這里使用@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 { 
}

再定義一個勞斯萊斯，這里故意用@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 { 
}

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); 
} 
}

運行結(jié)果如下：

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

補充：Autowiring modes

Spring支持四種autowire模式，當(dāng)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注解的時候，則稍微復(fù)雜一些，會有一個失敗退化過程，并且引入了Qualifier。不過基本原理是一樣。

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