【发布时间】：2011-09-30 07:04:03
【问题描述】：
我有这样的代码：
AJPAController aJPAController;
AClass aClass = aJPAController.find((Integer.parseInt(request.getParameter("id")));
try{
if(aJpaController.contains(aClass)){
response.sendRedirect("gosomewhere.com");
}
}
catch (java.lang.NullPointerException R){
response.sendRedirect("gosomewhereelse.com");
}
我正在尝试在数据库中查找一个实体，如果该实体在数据库中，那么它将执行一个操作，如果没有，它将执行另一个操作。
我在尝试先查找然后包含方法时可能存在逻辑错误.. 但我不知道。
提前感谢您的帮助。
编辑： 这么晚才回复很抱歉。我不得不处理一件紧急的事情。完整的错误堆栈跟踪是：
Description: An unhandled exception occurred during the execution of the web application. Please review the following stack trace for more information regarding the error.
Exception Details: java.lang.NullPointerException
null
Possible Source of Error:
Class Name: org.apache.jsp.user.report_005fredirect_jsp
File Name: report_005fredirect_jsp.java from
Method Name: _jspService
Line Number: 74
Source not available. Information regarding the location of the exception can be identified using the exception stack trace below.
Stack Trace:
org.apache.jsp.user.report_005fredirect_jsp._jspService(report_005fredirect_jsp.java from :74)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:109)
javax.servlet.http.HttpServlet.service(HttpServlet.java:847)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:408)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:483)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:373)
javax.servlet.http.HttpServlet.service(HttpServlet.java:847)
org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1523)
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:343)
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:215)
com.labristeknoloji.lcas.client.filters.SingleSignOutNameAddFilter.doFilter(SingleSignOutNameAddFilter.java:56)
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:256)
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:215)
org.jasig.cas.client.util.AssertionThreadLocalFilter.doFilter(AssertionThreadLocalFilter.java:40)
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:256)
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:215)
org.jasig.cas.client.util.HttpServletRequestWrapperFilter.doFilter(HttpServletRequestWrapperFilter.java:50)
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:256)
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:215)
org.jasig.cas.client.validation.AbstractTicketValidationFilter.doFilter(AbstractTicketValidationFilter.java:167)
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:256)
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:215)
com.labristeknoloji.lcas.client.filters.LCASAuthenticationFilter.doFilter(LCASAuthenticationFilter.java:111)
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:256)
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:215)
org.jasig.cas.client.session.SingleSignOutFilter.doFilter(SingleSignOutFilter.java:104)
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:256)
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:215)
com.labristeknoloji.lrms.web.filter.LocaleFilter.doFilter(LocaleFilter.java:76)
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:256)
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:215)
org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:277)
org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:188)
org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:641)
com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:97)
com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:85)
org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:185)
org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:332)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:233)
org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:229)
org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:334)
org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:814)
org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:743)
org.apache.jk.common.ChannelSocket$SocketConnection.runIt(ChannelSocket.java:936)
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:682)
java.lang.Thread.run(Thread.java:619)
【问题讨论】：
-
AJPAController 是做什么的？它的 find() 方法返回什么？
-
我一生中从未遇到过 NullPointerException。重写你的代码，这样你就不需要了：
if (response == null) ...等 -
AJPA 控制器初始化实体管理器并查找/更新/保留实体实例。 Find 方法返回一个 AClass 类型的对象。
-
实际上我的目标不是捕捉 NullPointerException。我试图在持久性上下文中找到一个实体实例。我试图弄清楚如何处理这个实体的不存在状态。您对此有何看法？
标签： java jpa entitymanager