When you execute ForkJoinTask in ForkJoinPool, you can do it in a synchronous or an asynchronous way. When you do it in a synchronous way, the method that sends the task to the pool doesn't return until the task sent finishes its execution. When you do it in an asynchronous way, the method that sends the task to the executor returns immediately, so the task can continue with its execution.

You should be aware of a big difference between the two methods. When you use the synchronous methods, the task that calls one of these methods (for example, the invokeAll() method) is suspended until the tasks it sent to the pool finish their execution. This allows the ForkJoinPool class to use the work-stealing algorithm to assign a new task to the worker thread that executed the sleeping task. On the contrary, when you use the asynchronous methods (for example, the fork() method), the task continues with its execution, so the ForkJoinPool class can't use the work-stealing algorithm to increase the performance of the application. In this case, only when you call the join() or get() methods to wait for the finalization of a task, the ForkJoinPool class can use that algorithm.

In addition to the RecursiveAction and RecursiveTask classes, Java 8 introduced a new ForkJoinTask class with the CountedCompleter class. With this kind of tasks, you can include a completion action that will be executed when it is launched and there is no pending child task. This mechanism is based on a method included in the class (the onCompletion() method) and a counter of pending tasks.

This counter is initialized to zero by default and you can increment it when you need in an atomic way. Normally, you will increment this counter one by one as and when you launch a child task. Finally, when a task has finished its execution, you can try to complete the execution of the task and consequently execute the onCompletion() method. If the pending count is greater than zero, it is decremented by one. If it's zero, the onCompletion() method is executed and then the parent task is tried to be completed.

In this recipe, you will learn how to use the asynchronous methods provided by the ForkJoinPool and CountedCompleter classes for the management of tasks. You are going to implement a program that will search for files with a determined extension inside a folder and its subfolders. The CountedCompleter class you're going to implement will process the contents of a folder. For each subfolder inside that folder, it will send a new task to the ForkJoinPool class in an asynchronous way. For each file inside that folder, the task will check the extension of the file and add it to the result list if it proceeds. When a task is completed, it will insert the result lists of all its child tasks in its result task.