CODEWITHSUNDEEP
javamultithreading
Jani
Jani

Reputation: 21

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

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?

Upvotes: 2

Views: 1295

Answers (7)

KaliRagavendran
KaliRagavendran

Reputation: 1

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

Upvotes: 0

KaliRagavendran
KaliRagavendran

Reputation: 1

You can use the Synchronized keyword which will use to run one thread completely

example

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

Upvotes: -1

Jani
Jani

Reputation: 21

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

Thanks to all who responded with valueble suggestions.

Upvotes: 0

brianegge
brianegge

Reputation: 29872

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.

Upvotes: 1

Richard Perfect
Richard Perfect

Reputation: 208

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...

Upvotes: 0

Tom Hawtin - tackline
Tom Hawtin - tackline

Reputation: 147154

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.

Upvotes: 7

fastcodejava
fastcodejava

Reputation: 41097

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

Upvotes: 0

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