How to properly shutdown java ExecutorService

Question

I have a simple java `ExecutorService` that runs some task objects (implements `Callable`). ``` ExecutorService exec = Executors.newSingleThreadExecutor(); List tasks = new ArrayList<>(); // ... create some tasks for (CallableTask task : tasks) { Future future = exec.submit(task); result = (String) future.get(timeout, TimeUnit.SECONDS); // TASKS load some classes and invoke their methods (they may create additional threads) // ... catch interruptions and timeouts } exec.shutdownNow(); ``` After all tasks are finished (either DONE or TIMEOUT-ed), I try to shutdown the executor, but it wont stop: `exec.isTerminated() = FALSE.` I suspect that some tasks that are timeouted are not properly terminated. And yes, I know that executor's shutdown is not guaranteing anything: There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via {@link Thread#interrupt}, so any task that fails to respon…

Accepted Answer

Recommended way from Oracle API 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();
   }
```

If your pool is taking more time to shutdown, you can change

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

to

```
while (!pool.awaitTermination(60, TimeUnit.SECONDS))
```

A brief summary of the shutdown related methods

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.

awaitTermination(long timeout, TimeUnit unit) throws InterruptedException:

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

How to properly shutdown java ExecutorService

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