0

当我尝试从 DB 查询结果时，出现空指针异常。这是错误：

java.lang.NullPointerException
    at main.java.utils.WebAppInfo.getAppName(WebAppInfo.java:14)
    at main.java.views.adminView.IndexView.loadPageInfo(IndexView.java:21)
    at main.java.controller.MainAction.welcome(MainAction.java:24)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:219)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:746)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:687)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:80)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:925)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:856)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:915)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:811)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:796)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1221)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:118)
    at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:84)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:342)
    at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:113)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:342)
    at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:103)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:342)
    at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:113)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:342)
    at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:54)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:342)
    at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:45)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:342)
    at org.springframework.security.web.authentication.www.BasicAuthenticationFilter.doFilter(BasicAuthenticationFilter.java:150)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:342)
    at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:183)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:342)
    at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:105)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:342)
    at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:87)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:342)
    at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:192)
    at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:160)
    at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:346)
    at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:259)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1212)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:399)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:766)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:450)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:326)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
    at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:928)
    at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:549)
    at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
    at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:410)
    at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)

这是该错误来自的函数。

@Resource(name="applicationInfoService")
    private static ApplicationInfoService applicationInfoService;

    public static ApplicationInfo getAppName() {
        ApplicationInfo applicationInfo = applicationInfoService.getAppNameResult(); 
        return applicationInfo;
    }

如果我启动应用程序信息服务 (= new ApplicationInfoService()) 我会收到一个空指针错误，指向进行查询的服务类

@Service("applicationInfoService")
@Transactional
public class ApplicationInfoService {
    private EntityManager entityManager;

    @PersistenceContext
    public void setEntityManager(EntityManager entityManager){ 
        this.entityManager = entityManager; 
    }

    public ApplicationInfo getAppNameResult() {
        String sql = "FROM ApplicationInfo";
        Query query = entityManager.createQuery(sql);
        return (ApplicationInfo) query.getSingleResult();
    }
}

错误转到 createQuery 行，如果我打印出 EntityManager 它返回“null”。为什么我的 Entitymanager 不会被初始化。我在我的登录系统中使用几乎相同的构建，它在那里工作。当我在一个项目中使用多个 EntityManager 类时，我是否需要以某种方式配置它们？我不知道如何解决这个问题，因为我是 spring、hibernate、JPA 等的新手。

4

2 回答 2

2

Spring 不允许注入static字段，这就是你得到NullPointerException.

我可以给你推荐一个工作区。WebAppInfo在您的类中添加以下代码块：

@Resource(name="applicationInfoService")
public void setService(ApplicationInfoService service) {
    WebAppInfo.applicationInfoService = service;
}

不要忘记@Resouce从中删除注释applicationInfoService。

编辑：

上面的建议只是为了修复NullPointerException你的WebAppInfo类，因为没有被初始化一个static字段！

EntityManager但是你存在的原因可能有很多null。正如您所说，它在其他系统中对您有用。我建议你

  • persistence.xml部署后重新检查您的路径。
  • 在这里查看答案。

如果这些对您没有帮助，请提供足够的异常跟踪信息和相关代码。

于 2013-06-12T13:50:18.820 回答
0

What is the point of injecting a value into a static field? It can only have one value per class loader and it sort of defeats the purpose of the IoC environment. Even though sazzadur's answer will get you past the NullPointerException its not going to save you from strange bugs if other classes do same thing.

Instead of mixing paradigms, I would consider dispatching with the WebAppInfo class, converting ApplicationInfoService to a singleton and injecting it wherever appropriate.

于 2013-06-12T14:00:33.667 回答