Thread inside thread: what happens if parent thread is killed?

Question

There is something I can't understand with threads in general. In my case, using Java and Android.

Let's say I have a Thread named A, which launches thread B. If thread A stops, then thread B will continue to live. How is this possible? Who belongs to the Thread B now? To the main thread?

Thread class

public class ParentThread extends Thread {

    public void run(){
        Log.e("PARENT THREAD", "STARTS RUNNING");
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    Log.e("CHILD THREAD", "IS ALIVE");
                    try {
                        Thread.sleep(1000);
                    }
                    catch (InterruptedException exc) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
        Log.e("PARENT THREAD", "STOPS RUNNING");
    }
}

Activity

new ParentThread().start();

Logcat output

01-07 13:45:16.726 22063-22081/? E/PARENT THREAD: STARTS RUNNING
01-07 13:45:16.726 22063-22081/? E/PARENT THREAD: STOPS RUNNING
01-07 13:45:16.727 22063-22082/? E/CHILD THREAD: IS ALIVE
01-07 13:45:17.727 22063-22082/? E/CHILD THREAD: IS ALIVE
01-07 13:45:18.727 22063-22082/? E/CHILD THREAD: IS ALIVE
01-07 13:45:19.727 22063-22082/? E/CHILD THREAD: IS ALIVE
...

Threads don't "belong" to each other. They don't even have parents. — user253751, Jan 07 '16 at 12:50
You'll find this Parent/Child-Ownership construct when you're working with processes, not when you're working with threads! — ParkerHalo, Jan 07 '16 at 12:55

score 5 · Accepted Answer · answered Jan 07 '16 at 12:50

5

From docs Thread

A Thread is a concurrent unit of execution. It has its own call stack for methods being invoked, their arguments and local variables. Each application has at least one thread running when it is started, the main thread, in the main ThreadGroup. The runtime keeps its own threads in the system thread group.

answered Jan 07 '16 at 12:50

gio

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That's exactly what I wanted... I should have read the Android documentation. Thanks ! – Scaraux Jan 07 '16 at 13:01

score 4 · Answer 2 · edited Jan 07 '16 at 17:41

4

Let's say I have a Thread named A, that launches a Thread B. If the Thread A stops, the Thread B will continue to live. How is this possible?

It's easily possible - there's nothing that makes thread B stop, so it doesn't stop.

Who belongs to the Thread B now? To the main thread?

Threads don't belong to each other. The fact that it was Thread A that launched Thread B isn't recorded anywhere - as far as the system is concerned, you have two threads with no relationship between them whatsoever.

edited Jan 07 '16 at 17:41

gio

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answered Jan 07 '16 at 12:52

user253751

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1

What's wrong with that answer ? – Scaraux Jan 07 '16 at 13:11
@gio Are you sure you're not being unnecessarily sensitive there? – user253751 Jan 07 '16 at 22:07
not good example with parents – gio Jan 07 '16 at 22:23
@gio Why not? If your parents die you don't die because there's no reason for you to die, because there's no physical link between you and your parents - same as with threads. – user253751 Jan 07 '16 at 22:25

Thread inside thread: what happens if parent thread is killed?

2 Answers2