The Java Specialists' Newsletter
Issue 1322006-09-11 Category: Tips and Tricks Java version: JDK 1.5

Welcome to the 132nd edition of The Java(tm) Specialists' Newsletter. I am sitting about 30000 feet above the Norwegian Sea on route to Oslo for what is said to be one of the finest Java conferences. JavaZone is put together by the javaBin JUG in Norway. Apparently all the Java companies in Norway shut down this week. Imagine 2 days, and almost 100 talks to attend! These Norwegians show no mercy, in line with their Viking ancestry, and have even scheduled talks during lunch time :-)

Thanks for all the kind wishes for the birth of our daughter, Evangeline Kineta Kabutz. She has only woken me up twice in the last six weeks. She is growing beautifully and will have a tough lean frame like her brother.

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Thread Dump JSP in Java 5

Ten days ago, I received a desperate phone call from a large company in Cape Town. Their Java system tended to become unstable after some time, and especially during peak periods. Since the users were processing millions of dollars on this system, they should be able to log in at any time.

We managed to solve their problem. As you probably guessed, it was due to incorrectly handled concurrency. I cannot divulge how we find such problems or how to fix it, that is our competitive advantage. Contact me offlist if your company has bugs or performance issues that you cannot solve and where an extra pair of eyes can be useful.

One of the measurements we looked at was to inspect what the threads were doing. In this case, it did not reveal much, but it can be of great value in finding other issues. For example, at another customer, we stumbled upon an infinite loop by looking at what the threads were up to.

There are several ways of doing that. If you are using Unix, you can send a "kill -3" to the process. With Windows, CTRL+Break on the console will give you that information.

This server was running on Windows (don't laugh). The application server did not allow us to start the JVM in a console window, which meant that we could not press CTRL+Break.

Another approach would have been to painstakingly step through the threads with JConsole. That was not an option to me.

One of the annoying parts of the typical thread dump is that the threads are not sorted, so it becomes a bit tricky to group them. It would also be nice to see a summary of the state in a table, to make it easier to find problems. In addition, we should be able to copy the text and diff it to see how things change between refreshes.

In good OO fashion, we separate model and view. Let's first define the model:

package com.cretesoft.tjsn.performance;

import java.io.Serializable;
import java.util.*;

public class ThreadDumpBean implements Serializable {
  private final Map<Thread, StackTraceElement[]> traces;

  public ThreadDumpBean() {
    traces = new TreeMap<Thread, StackTraceElement[]>(THREAD_COMP);
    traces.putAll(Thread.getAllStackTraces());
  }

  public Collection<Thread> getThreads() {
    return traces.keySet();
  }

  public Map<Thread, StackTraceElement[]> getTraces() {
    return traces;
  }

  /**
   * Compare the threads by name and id.
   */
  private static final Comparator<Thread> THREAD_COMP =
      new Comparator<Thread>() {
        public int compare(Thread o1, Thread o2) {
          int result = o1.getName().compareTo(o2.getName());
          if (result == 0) {
            Long id1 = o1.getId();
            Long id2 = o2.getId();
            return id1.compareTo(id2);
          }
          return result;
        }
      };
}
  

We also write a bit of JSP, making use of the Expression Language ${}.

    <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

    <jsp:useBean id="threadDump"
                 class="com.cretesoft.tjsn.performance.ThreadDumpBean"
                 scope="request"/>

    <HTML>
    <BODY>
    <h2>Thread Summary:</h2>
    <table cellpadding="5" cellspacing="5">
      <tr>
        <th>Thread</th>
        <th>State</th>
        <th>Priority</th>
        <th>Daemon</th>
      </tr>
      <c:forEach items="${threadDump.threads}" var="thr">
        <tr>
          <td><a href="#${thr.id}">${thr.name}</a></td>
          <td>${thr.state}</td>
          <td>${thr.priority}</td>
          <td>${thr.daemon}</td>
        </tr>
      </c:forEach>
    </table>

    <h2>Stack Trace of JVM:</h2>
    <c:forEach items="${threadDump.traces}" var="trace">
      <h4><a name="${trace.key.id}">${trace.key}</a></h4>
      <pre>
      <c:forEach items="${trace.value}" var="traceline">
          at ${traceline}</c:forEach>
      </pre>
    </c:forEach>

    </BODY>
    </HTML>
  

This will generate first a summary of the threads with hyperlinks to the individual stack traces. The summary shows the state of each thread.

A word of warning: you should not make this JSP page publicly accessible on your server, as it opens up the guts of the app server to the general population. Even if they cannot change the state of the threads, it might be bad enough for them to see what methods are being executed.

You might have to change the GET_STACK_TRACE_PERMISSION setting to allow yourself access into the application server.

This is a took that I will keep handy whenever I do performance tuning or difficult bug fixing on a J2EE server running Java 5.

I look forward to hearing from you how you expanded this idea to make it even more useful :)

Kind regards

Heinz

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