Can java thread object be invoked twice

Question

public class GetCurrentPrice implements Runnable{
   public void run(){
      // some business logic
   }
}



public class Main{
   public static void main(){
       GetCurrentPrice gcp = new GetCurrentPrice();
       Thread t = new Thread(gcp);
       while(true){
           t.start();
           //once this thread execution is complete, restart this thread.
        }
   }
}

This throws java.lang.IllegalThreadStateException. What i am trying to achieve is, run this thread once, wait for it to complete, once it is complete, run this thread again.

similar question here: http://stackoverflow.com/questions/1215548/is-it-legal-to-call-the-start-method-twice-on-the-same-thread — CSCH, Apr 10 '15 at 14:35
Haven't you found the API to check it? No problem, here it is: https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#start%28%29. Btw 4 upvotes? I absolutely don't these users here. — Tom, Apr 10 '15 at 14:57

score 5 · Accepted Answer · answered Apr 10 '15 at 14:32

5

No you cant do that. From Thread.start() API: It is never legal to start a thread more than once.

answered Apr 10 '15 at 14:32

Evgeniy Dorofeev

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You need to create a new thread the same way you created the initial thread. – MadConan Apr 10 '15 at 14:35
Another alternative is to block the thread with wait instead of finishing his execution. Then if needed you can notify on the same lock from another Thread. – Davide Lorenzo MARINO Apr 10 '15 at 14:38
2

It's a lot better to just use an executor instead of trying to do the scheduling manually. – azurefrog Apr 10 '15 at 14:55

score 4 · Answer 2 · answered Apr 10 '15 at 14:34

4

From javadoc of class java.lang.Thread

It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution.

See https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#start()

answered Apr 10 '15 at 14:34

Davide Lorenzo MARINO

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azurefrog · Answer 3 · 2015-04-10T14:50:59.623

While you can't re-run a Thread directly, you can use a ScheduledExecutorService to execute a Thread (or anything else that implements Runnable) periodically.

From the API:

The schedule methods create tasks with various delays and return a task object that can be used to cancel or check execution. The scheduleAtFixedRate and scheduleWithFixedDelay methods create and execute tasks that run periodically until cancelled.

You can just create a pool with one thread, hand it your Runnable and ask it to re-run it each time it finishes, by doing something like this:

public class GetCurrentPrice implements Runnable {
    public void run() {
        // some business logic
    }
}
public class Main {
    public static void main() {
        GetCurrentPrice gcp = new GetCurrentPrice();
        ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
        // each time thread execution completes, start a new one without delay
        scheduler.scheduleAtFixedRate(gcp, 0, 0, TimeUnit.SECONDS);
    }
}

score 2 · Answer 4 · answered Apr 10 '15 at 14:33

2

Threads in Java are not reusable.

It is not legal to try to restart the thread in Java.

answered Apr 10 '15 at 14:33

Crazyjavahacking

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score 2 · Answer 5 · edited May 23 '17 at 10:24

2

Yes, the same Thread cannot be started more than once, but the same Runnable instance can be passed to different Thread objects.

More here -> Initializing two threads with the same instance of a runnable

edited May 23 '17 at 10:24

Community

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answered Apr 10 '15 at 15:08

Tejas Unnikrishnan

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score 0 · Answer 6 · answered Apr 10 '15 at 14:35

0

No you can't start Thread more than once, You need to initialize new Runnable every time you want to start same thread.

answered Apr 10 '15 at 14:35

Hunain

385
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score 0 · Answer 7 · answered Apr 10 '15 at 15:42

Here's one way to repeat the business logic:

public class GetCurrentPrice implements Runnable {
    public void run() {
        businessLogic();
        businessLogic();
    }

    public void businessLogic() {
        // some business logic
    }
}

public class Main {
    public static void main() {
        GetCurrentPrice gcp = new GetCurrentPrice();
        new Thread(gcp).start();
    }
}

Can java thread object be invoked twice

7 Answers7