Spring 系列之Spring經常使用註解總結

時間 2019-11-10

標籤 spring 系列經常使用註解總結欄目 Spring 简体版

原文原文鏈接

　　傳統的Spring作法是使用.xml文件來對bean進行注入或者是配置aop、實物，這麼作有兩個缺點：java

一、若是全部的內容都配置在.xml文件中，那麼.xml文件將會十分龐大；若是按需求分開.xml文件，那麼.xml文件又會很是多。總之這將致使配置文件的可讀性與可維護性變得很低。spring

二、在開發中.java文件和.xml文件之間不斷切換，是一件麻煩的事，同時這種思惟上的不連貫也會下降開發的效率。app

　　爲了解決這兩個問題，Spring引入了註解，經過「@XXX」的方　　式，讓註解與Java Bean緊密結合，既大大減小了配置文件的體積，又增長了Java Bean的可讀性與內聚性。eclipse

　　不使用註解：post

　　先看一個不使用註解的Spring示例，在這個示例的基礎上，改爲註解版本的，這樣也能看出使用與不使用註解之間的區別，先定義一個老虎：測試

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

　　再定義一個猴子：ui

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

　　定義一個動物園：this

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的配置文件這麼寫：lua

<?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>

　　測試方法：spa

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

　　都很熟悉，權當複習一遍了。

　　一、@AutoWired

　　@Autowired顧名思義，就是自動裝配，其做用是爲了消除Java代碼裏面的getter/setter與bean屬性中的property。固然，getter看我的需求，若是私有屬性須要對外提供的話，應當予以保留。@Autowirred默認按類型匹配的方式，在容器查找匹配的Bean，當且僅有一個匹配的Bean時，Spring將其注入@AutoWired標註的變量中。

　　所以，引入@AutoWired註解，先看下Spring配置文件怎麼寫：

 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，使用必須告訴spring一下我要使用註解了，<context:component-scan base-package="XXX"/>是一種簡單的，spring會自動掃描xxx路徑下的註解。

　　看到第15行，原來zoo裏面應當注入兩個屬性tigger、monkey，如今不須要注入了。再看下，Zoo.java也很方便，把getter/setter均可以去掉：

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;
    }
     
}

　　這裏@Autowired註解的意思就是，當Spring發現@Autowired註解時，將自動在代碼上下文中找到和其匹配（默認是類型匹配）的Bean，並自動注入到相應的地方去。

　　有一個細節性的問題是，假如bean裏面有兩個property,Zoo.java裏面又去掉了屬性的getter/setter並使用@Autowired註解標註這兩個屬性那會怎麼樣？答案是Spring會按照xml優先的原則去Zoo.java中尋找這兩個屬性的getter/setter,致使的結果就是初始化bean報錯。

　　OK，假設此時我把.xml文件的16行、17行兩行給去掉，再運行，會拋出異常：

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

　　由於，@Autowired註解要去尋找的是一個Bean，Tiger和Monkey的Bean定義都給去掉了，天然就不是一個Bean了，Spring容器找不到也很好理解。那麼，若是屬性找不到我不想讓Spring容器拋出異常，而就是顯示null，能夠嗎？能夠的，其實異常信息裏面也給出了提示了，就是將@Autowired註解的required屬性設置爲false便可：

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;
    }
    
}

此時，找不到tiger、monkey兩個屬性，Spring容器再也不拋出異常而是認爲這兩個屬性爲null。

二、Qualifier（指定注入Bean的名稱）

　　若是容器中有一個以上匹配的Bean，則能夠經過@Qualifier註解限定Bean的名稱，看下面的例子：

定義一個Car接口：

package com.spring.service;

public interface ICar {
    
    public String getCarName();
}

　　兩個實現類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";
    }
}

　　再寫一個CarFactory，引用car（這裏先不用@Qualifier註解）：

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註解配合Qualifier註解 -->
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>

　　測試方法：

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

　　運行一下，不用說，必定是報錯的，Car接口有兩個實現類，Spring並不知道應當引用哪一個實現類。

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

出現這種狀況一般有兩種解決辦法：
(1)、在配置文件中刪除其中一個實現類，Spring會自動去base-package下尋找Car接口的實現類，發現Car接口只有一個實現類，便會直接引用這個實現類。
(2)、實現類就是有多個該怎麼辦？此時可使用@Qualifier註解來指定Bean的名稱：

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();
    }
    
}

此處會注入名爲"bmwCar"的Bean。

三、Resource

@Resource註解與@Autowired註解做用很是類似，這個就簡單說了，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;
    }
}

這是詳細一些的用法，說一下@Resource的裝配順序：
(1)、@Resource後面沒有任何內容，默認經過name屬性去匹配bean，找不到再按type去匹配
(2)、指定了name或者type則根據指定的類型去匹配bean
(3)、指定了name和type則根據指定的name和type去匹配bean，任何一個不匹配都將報錯

而後，區分一下@Autowired和@Resource兩個註解的區別：(1)、@Autowired默認按照byType方式進行bean匹配，@Resource默認按照byName方式進行bean匹配(2)、@Autowired是Spring的註解，@Resource是J2EE的註解，這個看一下導入註解的時候這兩個註解的包名就一清二楚了Spring屬於第三方的，J2EE是Java本身的東西，所以，建議使用@Resource註解，以減小代碼和Spring之間的耦合。