// 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);
    }
}