Java ExecutorService: awaitTermination of all recursively created tasks

Question

I use an ExecutorService to execute a task. This task can recursively create other tasks which are submitted to the same ExecutorService and those

Answer 1

The only inelegant solution I could come up with is to directly use a ThreadPoolExecutor and query its getPoolSize() every once in a while. Is there really no better way do do that?

You have to use shutdown() ,awaitTermination()and shutdownNow() methods in a proper sequence.

shutdown(): Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.

awaitTermination():Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.

shutdownNow(): Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.

Recommended way from oracle documentation page of ExecutorService:

 void shutdownAndAwaitTermination(ExecutorService pool) {
   pool.shutdown(); // Disable new tasks from being submitted
   try {
     // Wait a while for existing tasks to terminate
     if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
       pool.shutdownNow(); // Cancel currently executing tasks
       // Wait a while for tasks to respond to being cancelled
       if (!pool.awaitTermination(60, TimeUnit.SECONDS))
           System.err.println("Pool did not terminate");
     }
   } catch (InterruptedException ie) {
     // (Re-)Cancel if current thread also interrupted
     pool.shutdownNow();
     // Preserve interrupt status
     Thread.currentThread().interrupt();
   }

You can replace if condition with while condition in case of long duration in completion of tasks as below:

Change

if (!pool.awaitTermination(60, TimeUnit.SECONDS))

To

 while(!pool.awaitTermination(60, TimeUnit.SECONDS)) {
     Thread.sleep(60000);
 }  

You can refer to other alternatives (except join(), which can be used with standalone thread ) in :

wait until all threads finish their work in java