Thursday, January 28, 2010

Killing a particular Tomcat thread

Update: This JSP does not work on a thread that is inside some native code.  On many occasions I had a thread stuck in JNI code and it wont work.
Also in some cases thread.stop can cause jvm to hang. According to javadocs "This method is inherently unsafe. Stopping a thread with Thread.stop causes it to unlock all of the monitors that it has locked".

I have used it only in some rare occasions where I wanted to avoid a system shutdown and in some cases we ended up doing system shutdown as jvm was hung so I had a 70-80% success with it. 
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We had an interesting requirement. A tomcat thread that was spawned from an ExecutorService ThreadPool had gone Rogue and was causing lots of disk churning issues. We cant bring down the production server as that would involve downtime. Killing this thread was harmless but how to kill it, the ExecutorService variable was private and we can't call shutdownNow on it. So the solution was to generate a ThreadDump get the threaName and write a JSP to kill it. Much of the code is borrowed but as this is a unique issue , I thought of helping fellow Googlers ;).



<%@page language="java"%>
<%
    String threadNameToKill = request.getParameter("threadNameToKill");
    killThread(threadNameToKill, out);
%>
<%!
    public void killThread(String threadNameToKill, JspWriter out) throws Exception {
        ThreadGroup root = Thread.currentThread().getThreadGroup().getParent();
        while (root.getParent() != null) {
            root = root.getParent();
        }
        visit(root, 0, threadNameToKill,out);
    }

    public void visit(ThreadGroup group, int level, String threadNameToKill,JspWriter out) throws Exception {
        int numThreads = group.activeCount();
        Thread[] threads = new Thread[numThreads * 2];
        numThreads = group.enumerate(threads, false);
        for (int i = 0; i < numThreads; i++) { // Get thread
            Thread thread = threads[i];
            if (threadNameToKill.equals(thread.getName())) {
                thread.stop();
                out.write("Thread " + threadNameToKill + "Killed");
                break;
            }
        }
        int numGroups = group.activeGroupCount();
        ThreadGroup[] groups = new ThreadGroup[numGroups * 2];
        numGroups = group.enumerate(groups, false);
        for (int i = 0; i < numGroups; i++) {
            visit(groups[i], level + 1, threadNameToKill, out);
        }
    }
%>

5 comments:

  1. Nice hack ... in reading this, my first instinct was that there ought to be a standard JMX Operation for doing this. But I fired up jconsole & it looks like there is no such thing, so your approach looks to be the only option.

    I guess if this happens on a server where you don't have some kind of dynamic classloading/scripting you're out of luck!

    ReplyDelete
  2. Yes for the same reason JSPs are a blessing for us and I don't have telnet access to production machines so things like ThreadDump and SystemStats JSPs are heaven. I still haven't enabled JMX here as its a pain to enable them for Tomcat and there are tons of other interesting things to do. Will do JMX as thats a proper way when get some breathing room.

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  3. This function looks like good, but actually it seems not work, when I kill a thread, I tomcat won't re-create a new thread, and the whole tomcat system hangs after a while, no request can be processed any more.

    ReplyDelete
  4. You are right. Please check my update at top.

    ReplyDelete