ExecutorService's shutdown() doesn't wait until all threads will be finished

Question

I have a code where 4 threads run at the same time. I want to wait until all these 4 threads will be finished. And only after that to continue the app flow.

I tried

Answer 1

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();
   }

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

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.

In above example, if your tasks are taking more time to complete, you can change if condition to while condition

Replace

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

with

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