原文链接：1. http://www.cnblogs.com/xdp-gacl/p/3495887.html 2. http://www.cnblogs.com/xiaoxi/p/5935009.html
一、把在Spring的xml文件中配置bean改为Spring的注解来配置bean
传统的Spring做法是使用.xml文件来对bean进行注入或者是配置aop、事物，这么做有两个缺点：
1、如果所有的内容都配置在.xml文件中，那么.xml文件将会十分庞大；如果按需求分开.xml文件，那么.xml文件又会非常多。总之这将导致配置文件的可读性与可维护性变得很低。
2、在开发中在.java文件和.xml文件之间不断切换，是一件麻烦的事，同时这种思维上的不连贯也会降低开发的效率。
为了解决这两个问题，Spring引入了注解，通过"@XXX"的方式，让注解与Java Bean紧密结合，既大大减少了配置文件的体积，又增加了Java Bean的可读性与内聚性。
不使用注解：
先看一个不使用注解的Spring示例，在这个示例的基础上，改成注解版本的，这样也能看出使用与不使用注解之间的区别，先定义一个老虎：
1 package com.spring.model; 2 3 public class Tiger { 4 5 private String tigerName="TigerKing"; 6 7 public String toString(){ 8 return "TigerName:"+tigerName; 9 } 10 }
再定义一个猴子：
1 package com.spring.model; 2 3 public class Monkey { 4 5 private String monkeyName = "MonkeyKing"; 6 7 public String toString(){ 8 return "MonkeyName:" + monkeyName; 9 } 10 11 }
定义一个动物园：
1 package com.spring.model; 2 3 public class Zoo { 4 private Tiger tiger; 5 private Monkey monkey; 6 7 public Tiger getTiger() { 8 return tiger; 9 } 10 public void setTiger(Tiger tiger) { 11 this.tiger = tiger; 12 } 13 public Monkey getMonkey() { 14 return monkey; 15 } 16 public void setMonkey(Monkey monkey) { 17 this.monkey = monkey; 18 } 19 20 public String toString(){ 21 return tiger + "\n" + monkey; 22 } 23 24 }
spring的配置文件这么写：
1 <?xml version="1.0" encoding="UTF-8"?> 2 <beans 3 xmlns="http://www.springframework.org/schema/beans" 4 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 5 xmlns:p="http://www.springframework.org/schema/p" 6 xmlns:context="http://www.springframework.org/schema/context" 7 xsi:schemaLocation="http://www.springframework.org/schema/beans 8 http://www.springframework.org/schema/beans/spring-beans-3.0.xsd 9 http://www.springframework.org/schema/context 10 http://www.springframework.org/schema/context/spring-context-3.0.xsd 11 "> 12 13 <bean > 14 <property name="tiger" ref="tiger" /> 15 <property name="monkey" ref="monkey" /> 16 </bean> 17 18 <bean /> 19 <bean /> 20 21 </beans>
测试方法：
1 public class TestAnnotation { 2 /** 3 * 不使用注解 4 */ 5 @Test 6 public void test(){ 7 //读取配置文件 8 ApplicationContext ctx=new ClassPathXmlApplicationContext("applicationContext2.xml"); 9 Zoo zoo=(Zoo) ctx.getBean("zoo"); 10 System.out.println(zoo.toString()); 11 } 12 }
都很熟悉，权当复习一遍了。
1、@Autowired
@Autowired顾名思义，就是自动装配，其作用是为了消除代码Java代码里面的getter/setter与bean属性中的property。当然，getter看个人需求，如果私有属性需要对外提供的话，应当予以保留。
@Autowired默认按类型匹配的方式，在容器查找匹配的Bean，当有且仅有一个匹配的Bean时，Spring将其注入@Autowired标注的变量中。
因此，引入@Autowired注解，先看一下spring配置文件怎么写：
1 <?xml version="1.0" encoding="UTF-8"?> 2 <beans 3 xmlns="http://www.springframework.org/schema/beans" 4 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 5 xmlns:p="http://www.springframework.org/schema/p" 6 xmlns:context="http://www.springframework.org/schema/context" 7 xsi:schemaLocation="http://www.springframework.org/schema/beans 8 http://www.springframework.org/schema/beans/spring-beans-3.0.xsd 9 http://www.springframework.org/schema/context 10 http://www.springframework.org/schema/context/spring-context-3.0.xsd 11 "> 12 13 <context:component-scan base-package="com.spring" /> 14 15 <bean /> 16 <bean /> 17 <bean /> 18 19 </beans>
注意第13行，使用必须告诉spring一下我要使用注解了，告诉的方式有很多，<context:component-scan base-package="xxx" />是一种最简单的，spring会自动扫描xxx路径下的注解。
看到第15行，原来zoo里面应当注入两个属性tiger、monkey，现在不需要注入了。再看下，Zoo.java也很方便，把getter/setter都可以去掉：
1 package com.spring.model; 2 3 import org.springframework.beans.factory.annotation.Autowired; 4 5 public class Zoo { 6 7 @Autowired 8 private Tiger tiger; 9 10 @Autowired 11 private Monkey monkey; 12 13 public String toString(){ 14 return tiger + "\n" + monkey; 15 } 16 17 }
这里@Autowired注解的意思就是，当Spring发现@Autowired注解时，将自动在代码上下文中找到和其匹配（默认是类型匹配）的Bean，并自动注入到相应的地方去。
有一个细节性的问题是，假如bean里面有两个property，Zoo.java里面又去掉了属性的getter/setter并使用@Autowired注解标注这两个属性那会怎么样？答案是Spring会按照xml优先的原则去Zoo.java中寻找这两个属性的getter/setter，导致的结果就是初始化bean报错。
OK，假设此时我把.xml文件的16行、17行两行给去掉，再运行，会抛出异常：
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'zoo': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.spring.model.Tiger com.spring.model.Zoo.tiger; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No matching bean of type [com.spring.model.Tiger] found for dependency: 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.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:285) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1074) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:517) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:456) at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:291) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:288) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:190) at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:580) at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:895) at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:425) at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139) at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83) at com.spring.test.TestAnnotation.test(TestAnnotation.java:16) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45) at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15) at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42) at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20) at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47) at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231) at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60) at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229) at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50) at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222) at org.junit.runners.ParentRunner.run(ParentRunner.java:300) at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:45) at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:460) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:673) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:386) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196) Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.spring.model.Tiger com.spring.model.Zoo.tiger; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No matching bean of type [com.spring.model.Tiger] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)} at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:502) at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:84) at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:282) ... 36 more Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No matching bean of type [com.spring.model.Tiger] found for dependency: 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:920) at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:789) at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:703) at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:474) ... 38 more