Java注解作用和用法:@Autowired注解、@Resource注解和@Service注解

本文介绍了Spring框架中重要的三个注解:@Autowired、@Resource和@Service的用途和区别。@Autowired用于自动装配bean,减少getter/setter,@Resource按名称装配,而@Service用于标记bean并指定其ID。文章通过实例展示了如何使用这些注解,强调了注解使用对代码可读性和维护性的影响。

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什么是注解

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。

本篇文章,讲讲最重要的三个Spring注解,也就是@Autowired、@Resource和@Service,希望能通过有限的篇幅说清楚这三个注解的用法。

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:

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public class Tiger
{
private String tigerName = “TigerKing”;

public String toString()
{
    return "TigerName:" + tigerName;
}

}
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再定义一个猴子:

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public class Monkey
{
private String monkeyName = “MonkeyKing”;

public String toString()
{
    return "MonkeyName:" + monkeyName;
}

}
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定义一个动物园:

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public class Zoo
{
private Tiger tiger;
private Monkey monkey;

public void setTiger(Tiger tiger)
{
    this.tiger = tiger;
}

public void setMonkey(Monkey monkey)
{
    this.monkey = monkey;
}

public Tiger getTiger()
{
    return tiger;
}

public Monkey getMonkey()
{
    return monkey;
}

public String toString()
{
    return tiger + "\n" + monkey;
}

}
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spring的配置文件这么写:

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<?xml version="1.0" encoding="UTF-8"?>

<bean id="zoo" class="com.xrq.bean.Zoo" >
    <property name="tiger" ref="tiger" />
    <property name="monkey" ref="monkey" />
</bean>

<bean id="tiger" class="com.xrq.domain.Tiger" />
<bean id="monkey" class="com.xrq.domain.Monkey" />
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@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。

因此,引入@Autowired注解,先看一下spring配置文件怎么写:

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1 <?xml version="1.0" encoding="UTF-8"?>
2 <beans xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance”
3 xmlns=“http://www.springframework.org/schema/beans”
4 xmlns:context=“http://www.springframework.org/schema/context”
5 xsi:schemaLocation=“http://www.springframework.org/schema/beans
6 http://www.springframework.org/schema/beans/spring-beans-4.2.xsd
7 http://www.springframework.org/schema/context
8 http://www.springframework.org/schema/context/spring-context-4.2.xsd”>
9
10 <context:component-scan base-package=“com.xrq” />
11
12
13
14
15
16
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注意第10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12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:

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public class Zoo
{
@Autowired
private Tiger tiger;

@Autowired
private Monkey monkey;

public String toString()
{
    return tiger + "\n" + monkey;
}

}
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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13行、14行两行给去掉,再运行,会抛出异常:

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Exception in thread “main” 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.xrq.domain.Tiger com.xrq.bean.Zoo.ttiger; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.xrq.domain.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:334)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1214)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:543)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
at org.springframework.beans.factory.support.AbstractBeanFactory1.getObject(AbstractBeanFactory.java:305)atorg.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)atorg.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:301)atorg.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:196)atorg.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:772)atorg.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:835)atorg.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:537)atorg.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)atorg.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)atcom.xrq.test.MyTest.main(MyTest.java:13)Causedby:org.springframework.beans.factory.BeanCreationException:Couldnotautowirefield:privatecom.xrq.domain.Tigercom.xrq.bean.Zoo.ttiger;nestedexceptionisorg.springframework.beans.factory.NoSuchBeanDefinitionException:Noqualifyingbeanoftype[com.xrq.domain.Tiger]foundfordependency:expectedatleast1beanwhichqualifiesasautowirecandidateforthisdependency.Dependencyannotations:@org.springframework.beans.factory.annotation.Autowired(required=true)atorg.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor1.getObject(AbstractBeanFactory.java:305) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:301) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:196) at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:772) at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:835) at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:537) at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139) at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83) at com.xrq.test.MyTest.main(MyTest.java:13) Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.xrq.domain.Tiger com.xrq.bean.Zoo.ttiger; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.xrq.domain.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.AutowiredAnnotationBeanPostProcessor1.getObject(AbstractBeanFactory.java:305)atorg.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)atorg.springframework.beans.

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