@Autowired与@Resource的区别（二)

这篇文章打算通过一个实例来详解@AutoWired和@Resource。

我的项目中有两个测试service分别是class A 和class B

class A

@Service
public class A {
}

class B

@Service
public class B {
}

第一种情况：我先使用@Resource 注入A对象a,@Autowired注入B对象b,结果项目启动无异常

@Controller
public class C {
    @Resource
   private A a;
    @Autowired
   private B b;
}

第二种情况：我们只注入一个变量b 但让它作为A的对象，使用@Resoure方式注入

@Controller
public class C {
    @Resource
   private A b;
}

启动失败，异常信息如下:

org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'b' is expected to be of type [com.dotwin.service.A] but was actually of type [com.dotwin.service.B]
    org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:378)
    org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
    org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.autowireResource(CommonAnnotationBeanPostProcessor.java:522)
    org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.getResource(CommonAnnotationBeanPostProcessor.java:496)
    org.springframework.context.annotation.CommonAnnotationBeanPostProcessor$ResourceElement.getResourceToInject(CommonAnnotationBeanPostProcessor.java:627)
    org.springframework.beans.factory.annotation.InjectionMetadata$InjectedElement.inject(InjectionMetadata.java:169)
    org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)
    org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessPropertyValues(CommonAnnotationBeanPostProcessor.java:318)
    org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1219)
    org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:543)
    org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
    org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
    org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
    org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
    org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:751)
    org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:861)
    org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:541)
    org.springframework.web.servlet.FrameworkServlet.configureAndRefreshWebApplicationContext(FrameworkServlet.java:668)
    org.springframework.web.servlet.FrameworkServlet.createWebApplicationContext(FrameworkServlet.java:634)
    org.springframework.web.servlet.FrameworkServlet.createWebApplicationContext(FrameworkServlet.java:682)
    org.springframework.web.servlet.FrameworkServlet.initWebApplicationContext(FrameworkServlet.java:553)
    org.springframework.web.servlet.FrameworkServlet.initServletBean(FrameworkServlet.java:494)
    org.springframework.web.servlet.HttpServletBean.init(HttpServletBean.java:136)
    javax.servlet.GenericServlet.init(GenericServlet.java:158)
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
    org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:650)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
    org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803)
    org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459)
    org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.lang.Thread.run(Thread.java:748)

意思呢就是说，对象b期望注入类A的对象，但自动注入时注入了类B的对象，这里就体现出了@Resource注入是根据名称的原理，B类使用@Service注解无没有指定名称，所以生成的bean名称为b。@Resource使用名称注入先找到了bean为b的类B，然后直接注入，但我们定义的对象b实际是类A的。所以由于类型不匹配导致注入失败。

当我们把变量名更换成没有重名的bean的时候就不会出现这种问题，接下来，同种情况下我们使用@Autowired来注入。

@Controller
public class C {
    @Resource
   private A c;

}

启动正常不会报错，@Autowired是基于对象类型作注入，先查找类型为A的bean a，然后不管变量名称直接注入。

在@Autowired与@Resource的区别（一）> https://blog.youkuaiyun.com/fenfei_zqh/article/details/79585103 这篇文章中提及了在同一接口有两个实现类的情况下，@Autowired注入也可能会出现异常。但是我还是推荐使用@Autowired进行自动注入。