Java - Wait for third party threads to finish

Question

I have a Thread that will run a third party lib which also will run their own Threads. When the run method of my Thread finishes, third party Threads will not be finished yet.

So, what is the best way to hold my Thread until these external Threads are still running?

Answer 1

If you are an application and don't have to worry about SecurityManager restrictions, and if you are prepared to occasionally modify your code when the third-party code is updated, you can use the facilities of ThreadGroup to walk the threads and identify them by name or by the thread group that contains them.

Once you've located the threads it's a simple job to either monitor them until they are done or to use Thread.join() as appropriate.

As an example, here is some working code which dumps all threads in the JVM:

public void printThreads(PrintWriter wtr) {
    ThreadGroup     root;

    totGroups=0;
    totThreads=0;

    for(root=Thread.currentThread().getThreadGroup(); root.getParent()!=null; root=root.getParent()) {}
    wtr.println("Thread Dump:");
    printThreadGroup(wtr,root,"  ");
    wtr.println("  -- Total Groups: "+totGroups+", Total Threads: "+totThreads);
    }

public void printThreadGroup(PrintWriter wtr, ThreadGroup grp, String pfx) {
    try {
        Thread[]        thds;
        ThreadGroup[]   grps;

        totGroups++;
        wtr.println(pfx+"Group: "+grp.getName()+", "+(grp.isDaemon()?"Daemon":"Normal")+", "+(grp.isDestroyed()?"Destroyed":"Alive")+", "+grp.getMaxPriority());
        thds=new Thread[grp.activeCount()];
        grp.enumerate(thds,false);
        Arrays.sort(thds,THREAD_SORTER);
        for(int xa=0; xa<thds.length && thds[xa]!=null; xa++,totThreads++) {
            Thread          thd=thds[xa];
            wtr.println(pfx+". - ["+thd.getName()+", "+(thd.isDaemon()?"Daemon":"Normal")+", "+(thd.isAlive()?"Alive":"Not Started or Dead")+", "+thd.getPriority()+"]");
            }

        grps=new ThreadGroup[grp.activeGroupCount()];
        grp.enumerate(grps,false);
        Arrays.sort(grps,GROUP_SORTER);
        for(int xa=0; xa<grps.length && grps[xa]!=null; xa++) {
            printThreadGroup(wtr,grps[xa],(pfx+". "));
            grps[xa]=null;
            }
        }
    catch(Throwable thr) {
        wtr.println("  Cannot print threads ("+thr+")");
        }
    }

public void printStacks(PrintWriter wtr) {
    wtr.println("Thread Stack Traces:");
    try { javaMx.printStacks(wtr); } catch(Throwable thr) { wtr.println("  Cannot print stacks ("+thr+")"); }
    wtr.println("  --");
    }