The Java Developers Almanac 1.4

Home > List of Packages > java.lang [58 examples] > Threads [11 examples]

e99. Listing All Running Threads

A thread exists in a thread group and a thread group can contain other thread groups. This example visits all threads in all thread groups.

    // Find the root thread group
    ThreadGroup root = Thread.currentThread().getThreadGroup().getParent();
    while (root.getParent() != null) {
        root = root.getParent();
    }
    
    // Visit each thread group
    visit(root, 0);
    
    // This method recursively visits all thread groups under `group'.
    public static void visit(ThreadGroup group, int level) {
        // Get threads in `group'
        int numThreads = group.activeCount();
        Thread[] threads = new Thread[numThreads*2];
        numThreads = group.enumerate(threads, false);
    
        // Enumerate each thread in `group'
        for (int i=0; i<numThreads; i++) {
            // Get thread
            Thread thread = threads[i];
        }
    
        // Get thread subgroups of `group'
        int numGroups = group.activeGroupCount();
        ThreadGroup[] groups = new ThreadGroup[numGroups*2];
        numGroups = group.enumerate(groups, false);
    
        // Recursively visit each subgroup
        for (int i=0; i<numGroups; i++) {
            visit(groups[i], level+1);
        }
    }

Here's an example of some thread groups that contain some threads:

    java.lang.ThreadGroup[name=system,maxpri=10]
        Thread[Reference Handler,10,system]
        Thread[Finalizer,8,system]
        Thread[Signal Dispatcher,10,system]
        Thread[CompileThread0,10,system]
        java.lang.ThreadGroup[name=main,maxpri=10]
            Thread[main,5,main]
            Thread[Thread-1,5,main]