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、实物，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.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：

package com.spring.model;public class Tiger{　　private String  tigerName="TigerKing";　　public String toString(){　　　　return "TigerName:"+tigerName;　　}}

　　再定义一个猴子：

package com.spring.model;public class Monkey{    private String monkeyName="MonkeyKing";    public String toString(){       return "MonkeyName:"+monkeyName;     }}

　　定义一个动物园：

package com.spring.model;public class Zoo{    private  Tiger tiger;    private Monkey monkey;        public Tiger getTiger(){       return tiger;     }     public void setTiger(Tiger tiger){        this.tiger=tiger;     }    public Monkey getMonkey(){        return monkey;     }    public void setMonkey(Monkey monkey){        this.monkey=monkey;    }     public String toString(){         return tiger+"\n"+monkey;    }}

　　spring的配置文件这么写：

　　测试方法：

public class TestAnnotation{  /**    *不使用注解    */      @Test   public void test(){      //读取配置文件      ApplicationContext ctx=new ClassPathXmlApplicationContext("applicationContext2.xml");      Zoo zoo=(Zoo) ctx.getBean("zoo");     System.out.print(zoo.toString());  }}

　　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。@Autowirred默认按类型匹配的方式，在容器查找匹配的Bean，当且仅有一个匹配的Bean时，Spring将其注入@AutoWired标注的变量中。

　　因此，引入@AutoWired注解，先看下Spring配置文件怎么写：

1 
      2 
     
      12     13     
      14     15     
      16     
      17     
      18 19 
     

　　注意第13行，使用必须告诉spring一下我要使用注解了，<context:component-scan base-package="XXX"/>是一种简单的，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tigger、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：

package com.spring.model;import org.springframework.beans.factory.annotation.Autowired;public class Zoo{    @Autowired    private Tiger tiger;     @Autowired    private Monkey monkey; public String toString(){        return tiger+"\n"+monkey;    }     }

　　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.
     
      (ClassPathXmlApplicationContext.java:139)    at org.springframework.context.support.ClassPathXmlApplicationContext.
      
       (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 moreCaused 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
      
     

　　因为，@Autowired注解要去寻找的是一个Bean，Tiger和Monkey的Bean定义都给去掉了，自然就不是一个Bean了，Spring容器找不到也很好理解。那么，如果属性找不到我不想让Spring容器抛出异常，而就是显示null，可以吗？可以的，其实异常信息里面也给出了提示了，就是将@Autowired注解的required属性设置为false即可：

package com.spring.model;import org.springframework.beans.factory.annotation.Autowired;public class Zoo {        @Autowired(required=false)    private Tiger tiger;        @Autowired(required=false)    private Monkey monkey;        public String toString(){        return tiger + "\n" + monkey;    }    }

此时，找不到tiger、monkey两个属性，Spring容器不再抛出异常而是认为这两个属性为null。

2、Qualifier（指定注入Bean的名称）

　　如果容器中有一个以上匹配的Bean，则可以通过@Qualifier注解限定Bean的名称，看下面的例子：

定义一个Car接口：

package com.spring.service;public interface ICar {        public String getCarName();}

　　两个实现类BMWCar和BenzCar：

package com.spring.service.impl;import com.spring.service.ICar;public class BMWCar implements ICar{        public String getCarName(){        return "BMW car";    }}

package com.spring.service.impl;import com.spring.service.ICar;public class BenzCar implements ICar{        public String getCarName(){        return "Benz car";    }}

　　再写一个CarFactory，引用car（这里先不用@Qualifier注解）：

package com.spring.model;import org.springframework.beans.factory.annotation.Autowired;import com.spring.service.ICar;public class CarFactory {        @Autowired    private ICar car;        public String toString(){        return car.getCarName();    }    }

　　配置文件：

1 
      2 
     
      12     13     
      14     15     
      16     
      17     
      18     
      19     20 
     

　　测试方法：

/** * Autowired注解配合Qualifier注解 */@Testpublic void test1(){    //读取配置文件    ApplicationContext ctx=new ClassPathXmlApplicationContext("applicationContext2.xml");    CarFactory carFactory=(CarFactory) ctx.getBean("carFactory");    System.out.println(carFactory.toString());}

　　运行一下，不用说，一定是报错的，Car接口有两个实现类，Spring并不知道应当引用哪个实现类。

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'carFactory': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.spring.service.ICar com.spring.model.CarFactory.car; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException:No unique bean of type [com.spring.service.ICar] is defined: expected single matching bean but found 2: [bmwCar, benz]    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.
     
      (ClassPathXmlApplicationContext.java:139)    at org.springframework.context.support.ClassPathXmlApplicationContext.
      
       (ClassPathXmlApplicationContext.java:83)    at com.spring.test.TestAnnotation.test1(TestAnnotation.java:25)    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$2.evaluate(ParentRunner.java:222)    at org.junit.runners.ParentRunner.run(ParentRunner.java:300)    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:45)    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:460)    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:673)    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:386)    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196)Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.spring.service.ICar com.spring.model.CarFactory.car; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No unique bean of type [com.spring.service.ICar] is defined: expected single matching bean but found 2: [bmwCar, benz]    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$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 moreCaused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No unique bean of type [com.spring.service.ICar] is defined: expected single matching bean but found 2: [bmwCar, benz]    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:796)    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
      
     

出现这种情况通常有两种解决办法：

(1)、在配置文件中删除其中一个实现类，Spring会自动去base-package下寻找Car接口的实现类，发现Car接口只有一个实现类，便会直接引用这个实现类。

(2)、实现类就是有多个该怎么办？此时可以使用@Qualifier注解来指定Bean的名称：

package com.spring.model;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.beans.factory.annotation.Qualifier;import com.spring.service.ICar;public class CarFactory {        @Autowired    @Qualifier("bmwCar")    private ICar car;        public String toString(){        return car.getCarName();    }    }

此处会注入名为"bmwCar"的Bean。

3、Resource

@Resource注解与@Autowired注解作用非常相似，这个就简单说了，看例子：

package com.spring.model;import javax.annotation.Resource;public class Zoo1 {        @Resource(name="tiger")    private Tiger tiger;        @Resource(type=Monkey.class)    private Monkey monkey;        public String toString(){        return tiger + "\n" + monkey;    }}

这是详细一些的用法，说一下@Resource的装配顺序：

(1)、@Resource后面没有任何内容，默认通过name属性去匹配bean，找不到再按type去匹配

(2)、指定了name或者type则根据指定的类型去匹配bean

(3)、指定了name和type则根据指定的name和type去匹配bean，任何一个不匹配都将报错

然后，区分一下@Autowired和@Resource两个注解的区别：

(1)、@Autowired默认按照byType方式进行bean匹配，@Resource默认按照byName方式进行bean匹配

(2)、@Autowired是Spring的注解，@Resource是J2EE的注解，这个看一下导入注解的时候这两个注解的包名就一清二楚了

Spring属于第三方的，J2EE是Java自己的东西，因此，建议使用@Resource注解，以减少代码和Spring之间的耦合。