Question

我相当新，并尝试在我的本地数据库上进行查询，但代码会引发NullPointerException异常。

这很奇怪，因为它似乎从不调用名为findNamesByCat(String cat)的选定存储库方法。它不显示位于此方法中的System.err.println("in repository");的输出。

紧跟在线之后，我调用了另一种服务的方法，并且有效!!

 List<Name> Names = myService.findNamesByCat(cat);

错误

        Mar 18, 2015 1:29:07 PM 
org.apache.catalina.core.StandardWrapperValve invoke
        ... threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause
        java.lang.NullPointerException
            at com.myproject.web.MyController.showCategoryNames(MyController.java:41)
    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:483)
        at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:221)
        at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:137)
        at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:110)
        at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:777)
        at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:706)
        at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
        at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:943)
        at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:877)
        at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
        at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
        at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:504)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)
        at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1074)
        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
        at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
        at java.lang.Thread.run(Thread.java:745)

代码

@Controller
public class myController {

  @Autowired 
  private MyService myService;

    @RequestMapping(value = "/{cat}", method = RequestMethod.GET)
    public String showCategoryNames(@PathVariable String cat, Model model) {
        System.err.println("in web"); //shows this
        List<Name> Names = myService.findNamesByCat(cat);
        System.err.println(Names.size()); //this is line 41
        model.addAttribute("Names", Names);
        model.addAttribute("cat", cat);
        return "cat";
    }


@Service
public class MyServiceImpl implements MyService {
    @Autowired
    private NameRepository nameRepository;

    @Override
    @Transactional(readOnly = true)
    public List<Name> findListOfNamesByCat(String cat) {
        System.err.println("in service"); //does not show this
        return nameRepository.findNamesByCat(cat);
    }
}


@Repository
public class NameRepositoryImp implements NameRepository {

    @Autowired
    SessionFactory sessionFactory;

    @Override
    public List<Name> findNamesByCat(String cat){
       System.err.println("in repository"); //does not show this
        try {
            Criteria crit = sessionFactory.getCurrentSession().createCriteria(
                    Name.class);
            crit.add(Restrictions.eq("category", cat));
            return crit.list();
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }

    }
}

的Servlet

    <context:annotation-config />
    <mvc:annotation-driven />
    <mvc:resources mapping="/resources/**" location="/resources/" />
    <context:component-scan base-package="com.myproject" />

的更新

我也做了以下更改，但它还没有用。

@Controller
public class myController {

    @Autowired
    @Qualifier("repositoryService")
    private MyService myService;


@Service(value = "repositoryService")
public class MyServiceImpl implements MyService {

    @Autowired
    @Qualifier("repositoryMain")
    private NameRepository nameRepository;


@Repository(value = "repositoryMain")
public class NameRepositoryImpl implements NameRepository {

    @Autowired
    SessionFactory sessionFactory;

Answer 1

您定义了@PathVariable String cat，但未在@RequestMapping中正确标记，为PathVariable＆＃34; cat＆＃34;添加{}，如下所示：

@RequestMapping(value = "/{cat}", method = RequestMethod.GET)

Answer 2

问题是，由于某些原因，我所调用的方法的主体是空的，正确的主体被其他方法的主体取代。更换机身并清洁项目后，它开始工作。感谢您尝试帮助并为混淆道歉。

Spring-MVC不调用存储库方法

的更新

2 个答案: