学习笔记：Spring各种依赖注入对比

转自：http://www.jb51.net/article/81741.htm（谢谢楼主分享）

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。