spring注解Autowired和Qualifier注解的讲解。很具体

电玩女神 2022-05-16 08:20 151阅读 0赞

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：

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.println(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 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，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：

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：

<?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
        ">
        
        <context:component-scan base-package="com.spring" />
        
        
        <bean id="carFactory" class="com.spring.model.CarFactory" />
        <bean id="bmwCar" class="com.spring.service.impl.BMWCar" />
        <bean id="benz" class="com.spring.service.impl.BenzCar" />
        
    </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。

完了

**@Autowired默认按照byType方式进行bean匹配**