訪問服務時出現Java Spring NullPointerException

Question

我正在嘗試使用Spring開發小型Web應用程序。問題是，當我試圖訪問我的控制器中的服務對象時，我得到NullPointerException。如何解決這個問題？

Severe: java.lang.NullPointerException 
    at controller.LoginController.executeLogin(LoginController.java:35) 
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) 
    at java.lang.reflect.Method.invoke(Method.java:497) 
    at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:175) 
    at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:446) 
    at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:434) 
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:945) 
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:876) 
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961) 
    at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863) 
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:707) 
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837) 
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) 
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682) 
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318) 
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160) 
    at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734) 
    at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673) 
    at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99) 
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174) 
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:416) 
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:283) 
    at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java:459) 
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:167) 
    at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:206) 
    at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:180) 
    at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:235) 
    at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119) 
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:283) 
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:200) 
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:132) 
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:111) 
    at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77) 
    at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:536) 
    at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:112) 
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:117) 
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:56) 
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:137) 
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:591) 
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:571) 
    at java.lang.Thread.run(Thread.java:745) 

LoginController.java

package controller; 

import ... 

@Controller 
public class LoginController { 
    @Autowired 
    private UserService userService; 

    @RequestMapping(value = "/login", method = RequestMethod.POST) 
    public ModelAndView executeLogin(HttpServletRequest request, HttpServletResponse response, @ModelAttribute("user") User user) { 
     ModelAndView model = null; 
     try { 
      boolean isValidUser = userService.isValidUser(user); 
      if (isValidUser) { 
       System.out.println("User Login Successful"); 
       request.setAttribute("loggedInUser", user.getLogin()); 
       model = new ModelAndView("index"); 
      } else { 
       model = new ModelAndView("login"); 
       model.addObject("user", user); 
       request.setAttribute("message", "Invalid credentials!!"); 
      } 

     } catch (Exception e) { 
      e.printStackTrace(); 
     } 

     return model; 
    } 
} 

UserService.java

package service; 

import ... 

@Service 
public class UserService { 

    public boolean isValidUser(User user) { 
     if(user.getLogin().equals("admin")) 
      return true; 
     else 
      return false; 
    } 
} 

的web.xml

<?xml version="1.0" encoding="UTF-8"?> 
<web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"> 

    <servlet> 
     <servlet-name>dispatcher</servlet-name> 
     <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> 
     <load-on-startup>1</load-on-startup> 
    </servlet> 

    <servlet-mapping> 
     <servlet-name>dispatcher</servlet-name> 
     <url-pattern>/</url-pattern> 
    </servlet-mapping> 

    <listener> 
     <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> 
    </listener> 

    <context-param> 
     <param-name>contextConfigLocation</param-name> 
     <param-value>/WEB-INF/applicationContext.xml</param-value> 
    </context-param> 

</web-app> 

調度-servlet.xml中

<?xml version='1.0' encoding='UTF-8' ?> 
<!-- was: <?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:aop="http://www.springframework.org/schema/aop" 
     xmlns:tx="http://www.springframework.org/schema/tx" 
     xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd 
     http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-4.0.xsd 
     http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.0.xsd"> 

    <bean class="org.springframework.web.servlet.mvc.support.ControllerClassNameHandlerMapping"/> 


    <bean id="viewResolver" 
      class="org.springframework.web.servlet.view.InternalResourceViewResolver" 
      p:prefix="/WEB-INF/jsp/" 
      p:suffix=".jsp" /> 


    <bean class="controller.LoginController"/> 

</beans> 

Answer 1

在 dispatcher-servlet.xml包括<context:component-scan base-package="service, controller" />和刪除<bean class="controller.LoginController"/>

你UserService是沒有得到掃描，因此它沒有得到注入到你LoginController。

通過打開component-scan，在base-package所有的類都將（在你的情況@Service和@Controller）進行掃描的annotations並註冊爲豆類。