How do we come to know that the thread has finished its execution?

Question

I am using ExecutorService. Following is the code

public class A{ 
   public static void main(String args[]){
        ExecutorService executorService = Executors.newFixedThreadPool(5);
        Runnable worker = new FileUploadThread("thread");
        executorService.execute(worker);
   } 
}

public class FileuploadThread extends Thread{
    //has a parametrized constuctor

     @Override
     public void run(){
        for(int i=0; i<10000; i++){
            syso("executing...");
        }
     }
}

I want to receive an event or something in the main method when the thread completes it task. How can i do that ?

Use `Callable` and return a value. Then `Future#get` can be used. — Prashant, Jan 24 '20 at 06:57
Can you show me how ? or point out a link that i can refer ? — Pri_stack, Jan 24 '20 at 06:59
The declaration should be `FileuploadThread implements Runnable` instead of `FileuploadThread extends Thread`. It’s a mistake to allocate the resources of a `Thread` when you actually just need a `Runnable` (and a big mistake of the Java designers to let `Thread` implement `Runnable` leading to such mistakes). — Holger, Jan 24 '20 at 09:34

pbajpai · Accepted Answer · 2020-01-24T09:41:24.953

To know about the task status - You need Future instance. Now there are two points:

If you are just interested to know whether the task has been completed or not, Use executorService.submit(worker) , instead of executorService.execute(worker) method.

If you also want to get some result back after the task completion, Use Callable interface instead of Runnable. See below code:

public class A {
  public static void main(String args[]){
    ExecutorService executorService = Executors.newFixedThreadPool(5);
    Callable<String> worker = new FileUploadThread("thread");
    Future<String> workerTask = executorService.submit(worker);

    try {
        boolean isDone = workerTask.isDone();
        System.out.println("Task is done: " + isDone);

        //Wait untill task is executing
        String status = workerTask.get();

        System.out.println("Status: " + status);
        isDone = workerTask.isDone();
        System.out.println("Task is done: " + isDone);
    } catch (InterruptedException | ExecutionException e) {
        e.printStackTrace();
    }
    executorService.shutdown();
  }
}

class FileUploadThread implements Callable<String> {
  //has a parametrized constuctor
  public FileUploadThread(String thread) { }

  @Override
  public String call() throws Exception {
    for(int i=0; i<5; i++){
        System.out.println("executing..sleep for 1 sec...");
        Thread.sleep(1000);
    }
    return "DONE";
  }
}

Output:

Task is done: false
executing..sleep for 1 sec...
executing..sleep for 1 sec...
executing..sleep for 1 sec...
executing..sleep for 1 sec...
executing..sleep for 1 sec...
Status: DONE
Task is done: true

statement workerTask.get(); does this method actually waits for the thread execution? if an exeception arise in the thread how it will be handled ? — Pri_stack, Jan 24 '20 at 07:20
Yes. It does. It is explained in the javadoc. Exceptions are explained there too. — Stephen C, Jan 24 '20 at 07:21
https://stackoverflow.com/questions/2248131/handling-exceptions-from-java-executorservice-tasks for those who require the link to handle exceptions — Pri_stack, Jan 24 '20 at 07:32
@Pri_stack Just to note, you don't _have_ to switch to `Callable` in order to get a `Future`, assuming you don't need a result. There's the [`ExecutorService#submit(Runnable)`](https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/util/concurrent/ExecutorService.html#submit(java.lang.Runnable)) overload. — Slaw, Jan 24 '20 at 08:05

score 0 · Answer 2 · answered Jan 24 '20 at 06:59

When you start a thread with yourThread.start(), the system starts a new thread doing what is inside the run method. You can simply print something like this : System.out.println("FINISHED " + this.getName()) at the end of the run method.

PS : please make sure you don't place the print above in some loops, it should be the last statement in the run method. ( The .execute() method is almost the same thing with .start() )

score 0 · Answer 3 · answered Jan 24 '20 at 07:06

0

You could use yourThread.join().

answered Jan 24 '20 at 07:06

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How do we come to know that the thread has finished its execution?

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