发布于 2016-07-26 19:09:01 | 118 次阅读 | 评论: 0 | 来源: 网友投递

这里有新鲜出炉的精品教程,程序狗速度看过来!

Spring Framework 开源j2ee框架

Spring是什么呢?首先它是一个开源的项目,而且目前非常活跃;它是一个基于IOC和AOP的构架多层j2ee系统的框架,但它不强迫你必须在每一层 中必须使用Spring,因为它模块化的很好,允许你根据自己的需要选择使用它的某一个模块;它实现了很优雅的MVC,对不同的数据访问技术提供了统一的接口,采用IOC使得可以很容易的实现bean的装配,提供了简洁的AOP并据此实现Transcation Managment,等等


这篇文章主要介绍了详解Java Spring各种依赖注入注解的区别的相关资料,需要的朋友可以参考下

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 (Common Annotations for Java) 
@Inject 
javax.inject 
JSR330 (Dependency Injection for Java) 
@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 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:


<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"?> 
<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<String, String> 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)} 
... 
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,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<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; 
} 
... 
}

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<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,则根据集合类中元素的类型来进行autowired by type(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
private List<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: 
No unique bean of type [me.arganzheng.study.spring.autowired.Car] is defined: 
expected single matching bean but found 2: [audi, toyota]. 

然而,上面的情况如果你用@Resource则不会有这个问题:


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field name不是languageChangesMap,那么还是一样报错的。


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也可以实现上面的List接收所有实现:


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); 
} 
}

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

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。



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