@Autowired與@Resource的區別(二)
阿新 • • 發佈:2018-12-09
這篇文章打算通過一個例項來詳解@AutoWired和@Resource。
我的專案中有兩個測試service分別是class A 和class B
class A
@Service
public class A {
}
class B
@Service
public class B {
}
第一種情況:我先使用@Resource 注入A物件a,@Autowired注入B物件b,結果專案啟動無異常
@Controller
public class C {
@Resource
private A a;
@Autowired
private B b;
}
第二種情況:我們只注入一個變數b 但讓它作為A的物件,使用@Resoure方式注入
@Controller
public class C {
@Resource
private A b;
}
啟動失敗,異常資訊如下:
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)
意思呢就是說,物件b期望注入類A的物件,但自動注入時注入了類B的物件,這裡就體現出了@Resource注入是根據名稱的原理,B類使用@Service註解無沒有指定名稱,所以生成的bean名稱為b。@Resource使用名稱注入先找到了bean為b的類B,然後直接注入,但我們定義的物件b實際是類A的。所以由於型別不匹配導致注入失敗。
當我們把變數名更換成沒有重名的bean的時候就不會出現這種問題,接下來,同種情況下我們使用@Autowired來注入。
@Controller
public class C {
@Resource
private A c;
}
啟動正常不會報錯,@Autowired是基於物件型別作注入,先查詢型別為A的bean a,然後不管變數名稱直接注入。
在@Autowired與@Resource的區別(一)> https://blog.csdn.net/fenfei_zqh/article/details/79585103 這篇文章中提及了在同一介面有兩個實現類的情況下,@Autowired注入也可能會出現異常。但是我還是推薦使用@Autowired進行自動注入。