java hang at the end of program

Question

I have a java program which prints out the last statement of main program and hang. I would like to know what could cause this problem thanks.

 public static void main(String[] args) {

        ...
        ...
        JSch jsch=new JSch();
    try {

        jsch.addIdentity("xxx");
        session = jsch.getSession("centos", this.ip, 22);
        session.setConfig("StrictHostKeyChecking", "no");
        session.connect();

    } catch (JSchException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    ...
    ...
        System.out.println("end of test");
    }

the program prints "end of test" and hang. what could cause this problem? I didnt use any framework. I compiled and run this in linux machine. after printing the "end of test", the linux prompt ">" doesnt show up

from ps command, I see the process is running.

i found the problem. the session conection doesnt get disconected. but why it blocks the exit? correct me if i am wrong. there is no join or wait inside main thread. can main thread get blocked?

Answer: with Stephen C 's help, finally, i found the reason. in c++, all threads are terminated after main thread exit, but in java, JVM wait for all non-Daemon thread to complete. I applied my c++ knowledge to Java but they are different in this case.

Answer 1

One possibility is that you have created another thread, and that thread hasn't terminated yet. The JVM won't shut down of its own volition until all (non-daemon) threads have terminated.

Another possibility is that you have implemented a shutdown hook, and the hook has blocked.

From ps command, I see the process is running and single thread.

That evidence is consistent with either of the above, I think.

---

Can we troubleshoot from outside (such as ps command) instead of source code.

Try using jstack (I think it is called) to get a thread stack dump.

Try attaching a debugger to the JVM.