每次我用 Eclipse 调试时，我都会得到一个 Nullpointer 异常？

Question

当我在 jboss 或 tomcat 上调试我的应用程序时，我在我放置第一个断点的行中得到一个 NullpointerException。我使用了 eclipse -clean，重新启动了我的机器，......没办法。只是无法摆脱它。

堆栈跟踪

    2011-02-02 15:13:34,848 WARNING [javax.enterprise.resource.webcontainer.jsf.lifecycle] (http-192.168.1.128-8080-2) #{userController.doLogin}: java.lang.NullPointerException
    javax.faces.FacesException: #{userController.doLogin}: java.lang.NullPointerException
        at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:118)
        at javax.faces.component.UICommand.broadcast(UICommand.java:387)
        at org.ajax4jsf.component.AjaxViewRoot.processEvents(AjaxViewRoot.java:329)
        at org.ajax4jsf.component.AjaxViewRoot.broadcastEventsForPhase(AjaxViewRoot.java:304)
        at org.ajax4jsf.component.AjaxViewRoot.processPhase(AjaxViewRoot.java:261)
        at org.ajax4jsf.component.AjaxViewRoot.processApplication(AjaxViewRoot.java:474)
        at com.sun.faces.lifecycle.InvokeApplicationPhase.execute(InvokeApplicationPhase.java:82)
        at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:100)
        at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
        at javax.faces.webapp.FacesServlet.service(FacesServlet.java:265)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.ajax4jsf.webapp.BaseXMLFilter.doXmlFilter(BaseXMLFilter.java:206)
        at org.ajax4jsf.webapp.BaseFilter.handleRequest(BaseFilter.java:290)
        at org.ajax4jsf.webapp.BaseFilter.processUploadsAndHandleRequest(BaseFilter.java:388)
        at org.ajax4jsf.webapp.BaseFilter.doFilter(BaseFilter.java:515)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at de.simplefact.navv.gui.common.viewstate.ViewExpiredFilter.doFilter(ViewExpiredFilter.java:76)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:235)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
        at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:190)
        at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:92)
        at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.process(SecurityContextEstablishmentValve.java:126)
        at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.invoke(SecurityContextEstablishmentValve.java:70)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
        at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:158)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:330)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:829)
        at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:598)
        at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
        at java.lang.Thread.run(Thread.java:619)
    Caused by: javax.faces.el.EvaluationException: java.lang.NullPointerException
        at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:102)
        at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
        ... 38 more
    Caused by: java.lang.NullPointerException
        at de.simplefact.navv.gui.common.controller.UserController.doLogin(UserController.java:58)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.apache.el.parser.AstValue.invoke(AstValue.java:170)
        at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:276)
        at com.sun.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:68)
        at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:88)
        ... 39 more

Answer 1

Nooooooooooooo... 如果你像我一样是个非常聪明的混蛋咳嗽，你会使用表达式进行调试。我的一个表达式是“throw new NullpointerException();”.. 所以每次我到达第一个调试标记时，都会执行这个表达式并强制应用程序崩溃。

.. 进一步思考你可能会用它来打扰你的同事。当你的同事去喝咖啡时，添加一些“throw new NullpointerException();”在 eclipse 的调试角度看他的表达，然后找出他有多聪明;-)。

Answer 2

您是否尝试在 UserController.java:58 上分配一个虚拟值来区分是否仍然存在？你的断点在哪里？也在 UserController.java:58 处吗？

顺便说一句，从堆栈跟踪开始，它表明它不是错误而是警告。你是否在调试选项上配置了 Eclipse，如何使用 NPE 处理警告消息？

Answer 3

UserController 的值为 null，因此当 JSF 计算 #{UserController.whatever} 时会抛出 NullPointerException。