Spring 系列之Spring常用注解总结

时间：2021-08-29 20:36:09

　　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：

<?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:p="http://www.springframework.org/schema/p"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-3.0.xsd
    ">
    
     <bean id="zoo" class="com.spring.model.Zoo" >
        <property name="tiger" ref="tiger" />
        <property name="monkey" ref="monkey" />
    </bean>
    
    <bean id="tiger" class="com.spring.model.Tiger" />
    <bean id="monkey" class="com.spring.model.Monkey" />

</beans>

　　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 <?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 id="zoo" class="com.spring.model.Zoo" />
16     <bean id="tiger" class="com.spring.model.Tiger" />
17     <bean id="monkey" class="com.spring.model.Monkey" />
18 
19 </beans>

　　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.<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

　　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 <?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     <!-- Autowired注解配合Qualifier注解 -->
16     <bean id="carFactory" class="com.spring.model.CarFactory" />
17     <bean id="bmwCar" class="com.spring.service.impl.BMWCar" />
18     <bean id="benz" class="com.spring.service.impl.BenzCar" />
19     
20 </beans>

　　测试方法：

/**
 * Autowired注解配合Qualifier注解
 */
@Test
public 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.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(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 more
Caused 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。

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