Need for ExecutorService.awaitTermination() after shutdown()

Question

As per Javadocs, shutdown() will wait for all submit tasks to be executed. I have two questions:

• What does submitted task mean? Do the tasks have to be submitted specifically by ExecutorService.submit() method or it included tasks submitted by ExecutorService.execute() method also?
• I have added a shutdown hook which calls ExecutorService.shutdown(). As per the docs, it should wait for all submit tasks to be executed. But it doesn't until I add a awaitTermination() call after the shutdown() call. Why doesn't it execute all tasks without awaitTermination() call?

Answer 1

What does submitted task mean? Do the tasks have to be submitted specifically by ExecutorService.submit() method or it included tasks submitted by ExecutorService.execute() method also?

A 'submitted' task in this sense is any Runnable or Callable that was passed to the ExecutorService. Although the wording of the javadoc is slightly confusing, it does not differentiate between execute() vs submit().

I have added a shutdown hook which calls ExecutorService.shutdown(). As per the docs, it should wait for all submit tasks to be executed. But it doesn't until I add a awaitTermination() call after the shutdown() call. Why doesn't it execute all tasks without awaitTermination() call?

The shutdown() operation is non-blocking (asynchronous) as are almost all of the methods on the ExecutorService API. The javadoc only states that calling shutdown() will initiate a shutdown sequence.

The key here is that the ExecutorService will try to wait for all tasks to complete, but if shutdown() is the last line of code before your main method terminates, System.exit() will be called after the end of your main method which will terminate the JVM and override the ExecutorService's attempt to wait for running tasks to complete. I would consider it undesireable for ExecutorService.shutdown() to always delay a System.exit(), and would prefer an opt-in approach to a blocking call (i.e. awaitTermination).