Re: Null-pointer exception from response.encodeUrl under Windows Tomcat 7

Jocelyn Ireson-Paine Wed, 15 Jun 2011 10:44:37 -0700

About a week ago, I reported what seemed like a sporadic null-pointerexception from response.encodeUrl. I've managed to simplify the code thatprovokes it, and will show it below. My original report, with details ofthe Tomcat version and so on, is after this mail.

The error occurred in a large system I've written that uses servlets andJSP. It enables users to analyse certain kinds of numerical data over theWeb, and uses sessions to hold information about those users currentlyusing the system. Below is a JSP page I've constructed that demonstratesthe error:

  <%@ page import="javax.servlet.*,javax.servlet.http.*,salco.*"%>


  <HTML>
  <HEAD>

  <TITLE>Test1</TITLE>
  </HEAD>

  <BODY BGCOLOR=white>

  <H1>Test1</H1>

  <%
     HttpSession this_session = request.getSession(true);
     out.println( "this_session = " + this_session + ".<BR>" );

     out.println( "Date = " + new java.util.Date() + ".<BR>" );

     ResponseHolder rh = (ResponseHolder)this_session.getAttribute(
  "response_holder" );
     out.println( "rh = " + rh + ".<BR>" );
     if ( rh == null ) {
       out.println( "rh is null.<BR>" );
       rh = new ResponseHolder( response );
       this_session.setAttribute( "response_holder", rh );
       this_session.setAttribute( "count", 0 );
       out.println( "<BR>count = 0.<BR>" );
    }
    else {
       int count = (Integer)this_session.getAttribute( "count" );
       out.println( "rh is not null.<BR>" );
       this_session.setAttribute( "count", count+1 );
       out.println( "<BR>count = " + (count+1) + ".<BR>" );
    }

    String result = "None yet";
    try {
      result = rh.response.encodeURL( "/Test1.jsp" );
      out.println( "Encoded URL OK: result = " + result + ".<BR>" );
    }
    catch ( java.lang.NullPointerException e ) {
      out.println( "Crashed while encoding URL: <BR>" );
      e.printStackTrace( new java.io.PrintWriter(out) );
    }
  %>

  </BODY>
  </HTML>

This page starts by obtaining an instance of HTTPSession by callingrequest.getSession(true). This instance will have been created when theuser logs in for their current session, by another call ofrequest.getSession(true).

The page then displays the date and time so that you can check that in mydiagnostics, the times follow one another and I'm not repeating oldoutput. It then tries to obtain an instance of a class I've defined namedResponseHolder, and does so by calling HttpSession.getAttribute.ResponseHolder is a class I wrote specially for this demo, and has onefield, an HTTPServletResponse. In real life, I store users' session datain instances with many fields, of which an HTTPServletResponse is one. Ipass these to my server-side code. This needs the HTTPServletResponse whendoing such things as encoding URLs, which is why I include it. For thepurposes of this test, the non-HTTPServletResponse fields aren't relevant,and so I've not included them in the ResponseHolder.

If the ResponseHolder instance is null, it's probably because the page isbeing loaded for the first time in the session. In that case, I create anew ResponseHolder, and call HttpSession.setAttribute to store it.

The page also maintains and displays a count. This is solely for thisdemo. As with displaying the time, it's a sanity check on the output. Forexample, it confirms that I actually have reloaded the page.

The rest of the page calls encodeURL, and displays the result. IfencodeURL crashes with a NullPointerException, it catches this anddisplays the stack trace instead.

By the way, the page has three imports. Two are for the javax.servletclasses; the third is for my project classes.


Next, here's my ResponseHolder class:
  package salco;

  import javax.servlet.*;
  import javax.servlet.http.*;

  public class ResponseHolder
  {
    public HttpServletResponse response;

    public ResponseHolder( HttpServletResponse r )
    {
      this.response = r;
    }

    public String toString()
    {
      return "[ResponseHolder " + this.response + "]";
    }
  }

As mentioned earlier, it has just one field, the HttpServletResponse.

Finally, I'll show output from three sets of transactions. To createthese, I logged into the system, thereby creating a new session asexplained above. I then loaded the JSP page, Test1.jsp, and kept reloadingit. (This was in Firefox 4.0.1; but browser and browser version areprobably not relevant. The error also occurs on Internet Explorer andGoogle Chrome.) I then copied the page shown by the browser into a textfile, and appended new pages as I reloaded them. Here, then is myoutput. It's long, so be warned; to get to the end, search for "END OFOUTPUT":


(FIRST SESSION)

Test1
this_session = org.apache.catalina.session.StandardSessionFacade@9be2b5.
Date = Wed Jun 15 17:46:45 BST 2011.
rh = null.
rh is null.

count = 0.
Encoded URL OK: result = /Test1.jsp.


Test1
this_session = org.apache.catalina.session.StandardSessionFacade@9be2b5.
Date = Wed Jun 15 17:46:55 BST 2011.
rh = [ResponseHolder org.apache.catalina.connector.ResponseFacade@9c176c].
rh is not null.

count = 1.
Crashed while encoding URL:

java.lang.NullPointerException atorg.apache.catalina.connector.Response.toAbsolute(Response.java:1594) atorg.apache.catalina.connector.Response.encodeURL(Response.java:1198) atorg.apache.catalina.connector.ResponseFacade.encodeURL(ResponseFacade.java:398)at org.apache.jsp.Test1_jsp._jspService(Test1_jsp.java:93) atorg.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) atjavax.servlet.http.HttpServlet.service(HttpServlet.java:722) atorg.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:419)atorg.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:333) atjavax.servlet.http.HttpServlet.service(HttpServlet.java:722) atorg.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)atorg.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)atorg.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)atorg.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)atorg.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)atorg.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)atorg.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:563)atorg.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)atorg.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:399)atorg.apache.coyote.http11.Http11Processor.process(Http11Processor.java:317)atorg.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:204)atorg.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:311)at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) atjava.lang.Thread.run(Unknown Source)


----

(SECOND SESSION)

Test1
this_session = org.apache.catalina.session.StandardSessionFacade@1ae9aaa.
Date = Wed Jun 15 17:50:22 BST 2011.
rh = null.
rh is null.

count = 0.
Encoded URL OK: result = /Test1.jsp.


Test1
this_session = org.apache.catalina.session.StandardSessionFacade@1ae9aaa.
Date = Wed Jun 15 17:50:35 BST 2011.

rh = [ResponseHolderorg.apache.catalina.connector.ResponseFacade@1f8bd0d].

rh is not null.

count = 1.
Encoded URL OK: result = /Test1.jsp.


Test1
this_session = org.apache.catalina.session.StandardSessionFacade@1ae9aaa.
Date = Wed Jun 15 17:50:46 BST 2011.

rh = [ResponseHolderorg.apache.catalina.connector.ResponseFacade@1f8bd0d].

rh is not null.

count = 2.
Encoded URL OK: result = /Test1.jsp.


Test1
this_session = org.apache.catalina.session.StandardSessionFacade@1ae9aaa.
Date = Wed Jun 15 17:51:44 BST 2011.

rh = [ResponseHolderorg.apache.catalina.connector.ResponseFacade@1f8bd0d].

rh is not null.

count = 3.
Encoded URL OK: result = /Test1.jsp.


Was OK up to and including count=94, then:

Test1
this_session = org.apache.catalina.session.StandardSessionFacade@1ae9aaa.
Date = Wed Jun 15 17:53:53 BST 2011.

rh = [ResponseHolderorg.apache.catalina.connector.ResponseFacade@1f8bd0d].

rh is not null.

count = 95.
Crashed while encoding URL:


----

(THIRD SESSION)

Test1
this_session = org.apache.catalina.session.StandardSessionFacade@c68a98.
Date = Wed Jun 15 18:36:38 BST 2011.
rh = null.
rh is null.

count = 0.
Encoded URL OK: result = /Test1.jsp.


Test1
this_session = org.apache.catalina.session.StandardSessionFacade@c68a98.
Date = Wed Jun 15 18:36:53 BST 2011.

rh = [ResponseHolderorg.apache.catalina.connector.ResponseFacade@121b59a].

rh is not null.

count = 1.
Encoded URL OK: result = /Test1.jsp.


Test1
this_session = org.apache.catalina.session.StandardSessionFacade@c68a98.
Date = Wed Jun 15 18:37:05 BST 2011.

rh = [ResponseHolderorg.apache.catalina.connector.ResponseFacade@121b59a].

rh is not null.

count = 2.
Encoded URL OK: result = /Test1.jsp.


Test1
this_session = org.apache.catalina.session.StandardSessionFacade@c68a98.
Date = Wed Jun 15 18:37:17 BST 2011.

rh = [ResponseHolderorg.apache.catalina.connector.ResponseFacade@121b59a].

rh is not null.

count = 3.
Encoded URL OK: result = /Test1.jsp.

Was then OK up to and including count=49, then:

Test1
this_session = org.apache.catalina.session.StandardSessionFacade@c68a98.
Date = Wed Jun 15 18:37:59 BST 2011.

rh = [ResponseHolderorg.apache.catalina.connector.ResponseFacade@121b59a].

rh is not null.

count = 50.
Crashed while encoding URL:

In this, the first session gave an error on the second use of the page,i.e. the first reload. However, the second session didn't crash until the93rd reload, and the third, until the 49th.

If this a bug, is there any chance of getting it fixed, or of getting acircumvention? If I'm doing something bad, please tell me.


Jocelyn Ireson-Paine
http://www.j-paine.org
http://www.spreadsheet-parts.org
+44 (0)7768 534 091

Jocelyn's Cartoons:
http://www.j-paine.org/blog/jocelyns_cartoons/

On Thu, 9 Jun 2011, Jocelyn Ireson-Paine wrote:

Hi,
I'm getting sporadic null-pointer exceptions from 'response.encodeUrl'. Thisis with Tomcat 7.0.14, the latest stable version to which I upgraded an hourago, and Java 1.6.0_26, which again I upgraded to at the same time, underWindows XP. I also got the error under Tomcat 5.5.33, which is why I triedupgrading. I uninstalled old Tomcats and Javas before upgrading.
Here's a trace of the error:
 java.lang.NullPointerException
atorg.apache.catalina.connector.Response.toAbsolute(Response.java:1594)atorg.apache.catalina.connector.Response.encodeURL(Response.java:1198)atorg.apache.catalina.connector.ResponseFacade.encodeUrl(ResponseFacade.java:422)
        at sharesim.ValueGame.encodeUrl(ValueGame.java:468)
The fourth line of the trace is my code.
To get some diagnostics, I encapsulated 'response.encodeUrl' in the followingmethod, and used 'println' to print to Tomcat's log. This method is the onementioned on the fourth line of the above trace.
 private final static String encodeUrl( HttpServletResponse response
                                      , String url
                                      )
 {
   System.out.println( "encodeUrl" );
   System.out.println( "response=" + response );
   System.out.println( "url=" + url );
   String result = null;
   if ( response == null )
     result = "not on Web";
   else
     result = response.encodeUrl( url );
   System.out.println( "result=" + result );
   return result;
 }
As mentioned above, the errors are sporadic. My redirection code gets calledon my server when I submit one of my forms. But sometimes, the submit works;sometimes it doesn't. Here are two successive traces from the above method:
 encodeUrl
 response=org.apache.catalina.connector.ResponseFacade@1a5ec6c
 url=/ResearcherValueGame1.jsp
 result=/ResearcherValueGame1.jsp

 encodeUrl
 response=org.apache.catalina.connector.ResponseFacade@1a5ec6c
 url=/ResearcherValueGame1.jsp
The first one worked; the second crashed rather than returning the encodedURL. The URL to be encoded is the same in both cases, and 'response' isevidently the same instance. I don't know how to dump 'response' in order toshow relevant fields (whichever they are), but am happy to try it if someonecan suggest how.
I've found a few mentions on Google of such errors, but nothing definiteabout it happening in Tomcat 7. One discussion which I thought I'd seen, butcan't find again, seemed to suggest that the problem occurred on version 5,possibly because of a missing 'synchronized'. I've not seen anything inrecent postings to this list.
I wondered whether the problem might be caused by the browser creatingmalformed cookies, or some such. (I'm using session-handling.) However, thisseems unlikely, as the error occurs regardless of whether I use Firefox,Internet Explorer, or Google Chrome.
Any ideas?

Jocelyn Ireson-Paine
http://www.j-paine.org

Jocelyn's Cartoons:
http://www.j-paine.org/blog/jocelyns_cartoons/


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Re: Null-pointer exception from response.encodeUrl under Windows Tomcat 7

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