Question

I have a code like this:

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");
}

I am trying to find an entity in database, if the entity is in the database then it will perform an action, if it does not it will perform another action.

I might have a logical mistake in trying first find and then contains method.. but I don’t know.

Thanks in advance for your help.

EDIT:
Sorry for the late reply. I had to deal with an urgent matter. Full error stack trace is:

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)

Answer 1

Just check the EntityManager.find(…) method.

It will return null if the entity does not exist in database. So it’s up to the implementation of the AJPAController.find(...) method what it is doing with that information. (Just for clarification: As I understood, AJPAController is a wrapper around an EntityManager.)