Controlling Shutdown and Termination of an Executor Service – Concurrency: Part II
Controlling Shutdown and Termination of an Executor Service
The ExecutorService interface provides the awaitTermination() method, which in conjunction with the shutdown methods, allows more refined control over pending tasks at shutdown.
Further control in shutting down and terminating an executor service is implemented by the awaitAndShutdownNow() method defined at (9) in Example 23.1. This method utilizes both the awaitTermination() method and the shutdownNow() method of the executor service. The executor service to shut down and the timeout to wait for pending tasks to complete are passed as parameters to the awaitAndShutdownNow() method.
The awaitAndShutdownNow() method first calls the awaitTermination() method at (10) that blocks until one of the following events occurs:
- The awaitTermination() method returns true, indicating that all tasks have completed, and thereby the executor service has been shut down and terminated.
- The awaitTermination() method returns false, indicating that there are still uncompleted tasks. The shutdownNow() method is called at (11) to cancel any uncompleted and waiting tasks. The awaitTermination() method is called again in a while loop at (12) to wait for any tasks still pending completion. This loop terminates when all tasks have completed; if the current thread is interrupted while awaiting task completion, the control goes to (13).
- The current thread is interrupted while waiting for the blocking call to the awaitTermination() method to return, in which case the InterruptedException thrown is caught by the catch block at (13). A further attempt is made in the catch block to cancel any pending tasks by calling the shutdownNow() method at (14).
The awaitAndShutdownNow() method at best makes an attempt at allowing pending tasks to complete after shutdown. It is called at (7a) and (7b) in Example 23.1 to illustrate shutting down an executor service and controlling any of its pending tasks. (For this discussion, (6b) is commented out in Example 23.1.)
The duration of the wait for pending tasks in the call to the awaitTermination() method impacts the fate of the pending tasks. The following call:
awaitAndShutdownNow(es, 2, TimeUnit.SECONDS); // (7a)
can result in the following output from the program, showing that two seconds of waiting was sufficient for all tasks to complete execution at (10), causing the executor service to terminate:
isShutdown() isTerminated()
Before execute() at (4): false false
After execute() at (4): false false
Before shutdown() at (6a): false false
pool-1-thread-2 => dice value: 4
pool-1-thread-3 => dice value: 1
pool-1-thread-1 => dice value: 2
After shutdown() at (6a): true false
After awaitTermination() at (10): true true
However, the following call:
awaitAndShutdownNow(es, 1, TimeUnit.MILLISECONDS); // (7b)
can result in the following output from the program, showing that a one-millisecond wait was insufficient for all tasks to complete execution at (10), resulting in the shutdownNow() method at (11) being called. Any pending tasks are thus interrupted and the InterruptedException is handled by each task. The awaitTermination() method returns true when called in the while loop at (12), indicating that the interrupted tasks completed their execution:
isShutdown() isTerminated()
Before execute() at (4): false false
After execute() at (4): false false
Before shutdown() at (6a): false false
pool-1-thread-2 => dice value: 3
pool-1-thread-1 => dice value: 1
pool-1-thread-3 => dice value: 1
After shutdown() at (6a): true false
Before shutdownNow() at (11): true false
After shutdownNow() at (11): true false
pool-1-thread-3: java.lang.InterruptedException: sleep interrupted
pool-1-thread-2: java.lang.InterruptedException: sleep interrupted
pool-1-thread-1: java.lang.InterruptedException: sleep interrupted
After awaitTermination() at (10): true true
