How do I make one Java thread return before some of its child threads finish?

Question

I have 2 nested threads.

First thread starts multiple instances of second thread. Each second thread has to sleep for some time (5 seconds).

I want to start the first thread and return a message to user immediately, but it seems my first thread waits until all the children of second thread to finish.

How can I achieve this? Any help?

Answer 1

It may be helpful to see code. It depends on where you are putting Thread.sleep();.

Answer 2

There are some common mistakes when dealing with java.lang.Thread.

• Calling run on the thread instead of start. This is nothing magical about the run method.
• Calling static methods on thread instances. Unfortunately this compiles. A common example is Thread.sleep. sleep is a static method and will always sleep the current thread, even if the code appears to be calling it on a different thread.

Rather than dealing with threads directly it is generally better to use a thread pool from java.util.concurrent.

Answer 3

Like someone else has pointed out with Threads you call start() which is a non-blocking call and actually gets the thread rolling. Calling run() will block until the run() method finishes. See the example code below;

public class Application {

    public static void main(String[] args) {
     FirstThread firstThread = new FirstThread();
     firstThread.start();
     System.out.println("Main Method ending");
    }
}


public class FirstThread extends Thread {

    public void run() {
     for(int i = 0; i < 3; i++) {
      SecondThread secondThread = new SecondThread(i);
      secondThread.start();
     }
     System.out.println("FirstThread is finishing");
    }
}

public class SecondThread extends Thread {

    private int i;

    public SecondThread(int i) {
     this.i = i;
    }

    public void run() {

     while(true) {
      System.out.println("Second thread number " + i + " doing stuff here...");
      // Do stuff here...

      try {
       Thread.sleep(5000); 
      }
      catch(InterruptedException ex){
       //ignore for sleeping}
      }
     }
    }
}

Which produces the output:

Main Method ending
Second thread number 0 doing stuff here...
Second thread number 1 doing stuff here...
FirstThread is finishing
Second thread number 2 doing stuff here...
Second thread number 0 doing stuff here...
Second thread number 2 doing stuff here...
Second thread number 1 doing stuff here...

Answer 4

What you should probably do, is create a single thread pool via Executors.newCachedThreadPool(). From your main thread submit Runnables (tasks) to the pool. Return to your main thread a list of Futures.

In Java there exists enough framework code that one rarely should need to deal with threads directly.

Answer 5

Hi all,

I replaced 'run' with 'start' in both first thread and second thread. It works fine now.

Thanks to all who responded with valueble suggestions.

Answer 6

U can use Synchronized keyword which will use to run one thread completely

example

public synchronized void run() //which thread to run completely { }

Answer 7

public class FirstThread extends Thread {

public synchronized void run() {
    for(int i = 0; i < 3; i++) {
            System.out.println("i="+i);
    }
    System.out.println("FirstThread is finishing");
}

} public class SecondThread extends Thread {

public synchronized void run() {
    for(int j = 0; j < 3; i++) {
            System.out.println("j="+j);
    }
    System.out.println("Second Thread is finishing");
}

}

public class Application {

public static void main(String[] args) {
    FirstThread firstThread = new FirstThread();
    SecondThread a=new SecondThread()
    firstThread.start();
    a.start();

}

output will be

} i=0 i=1 i=2 i=3 FirstThread is finishing

j=0 j=1 j=2 j=3 Second Thread is finishing