How do you use the ForkJoinTask API to create custom fork-join tasks?

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The ForkJoinTask API is a powerful tool for parallel programming in Java. It allows you to create custom tasks that can be executed by a fork-join pool, which is a special kind of thread pool that can dynamically adjust the number of worker threads based on the workload and the available resources. In this article, you will learn how to use the ForkJoinTask API to create custom fork-join tasks that can perform recursive, divide-and-conquer, or map-reduce operations on large data sets or complex problems.

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Gokul Jayavel

Software Engineer Co op @ Philips | Graduate Student, Teaching Assistant @ Northeastern University| Masters in Software…

1 What is a ForkJoinTask?

A ForkJoinTask is an abstract class that represents a unit of work that can be executed by a fork-join pool. It has two main methods: fork() and join(). The fork() method submits the task to the pool for asynchronous execution, and returns a reference to the task itself. The join() method waits for the task to complete and returns its result, or throws an exception if the task was cancelled or failed. A ForkJoinTask can also implement a compute() method, which defines the logic of the task and can optionally call fork() and join() on other subtasks.

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Gokul Jayavel

Software Engineer Co op @ Philips | Graduate Student, Teaching Assistant @ Northeastern University| Masters in Software Engineering | Ex-Specialist Programmer at Infosys|Full-Stack Developer|SDE|DevOps
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A ForkJoinTask is a lightweight task used in the Fork/Join framework in Java, designed for parallel execution. It represents a small piece of work that can be forked into subtasks and joined back together after completion. This framework is efficient for divide-and-conquer algorithms, where tasks are recursively split into smaller tasks until they are simple enough to be solved directly. ForkJoinTask offers methods like fork() to asynchronously execute tasks and join() to wait for their results. It enhances performance by utilizing a work-stealing algorithm, allowing idle threads to take work from busy threads.

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2 How to create a custom ForkJoinTask?

To create a custom ForkJoinTask, you need to extend one of its subclasses: RecursiveAction, RecursiveTask, or CountedCompleter. A RecursiveAction is a task that does not return a result, such as a sorting or searching algorithm. A RecursiveTask is a task that returns a result, such as a sum or a maximum value. A CountedCompleter is a task that can trigger a completion action when all its subtasks are done, such as a reduce or a combine operation. Depending on the subclass you choose, you need to override the compute() method and provide the logic of your task. You can also override other methods, such as onCompletion(), onCancel(), or getRawResult(), to customize the behavior of your task.

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Gokul Jayavel

Software Engineer Co op @ Philips | Graduate Student, Teaching Assistant @ Northeastern University| Masters in Software Engineering | Ex-Specialist Programmer at Infosys|Full-Stack Developer|SDE|DevOps
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To create a custom ForkJoinTask, extend either RecursiveTask<V> (if the task returns a result) or RecursiveAction (if it doesn't), and override the compute() method with your task's logic. For example RecursiveTask is used to sum an array in parallel by splitting the task into subtasks until they are small enough to compute directly. CountedCompleter is a specialized form in Java's Fork/Join framework, supporting tasks that trigger other tasks upon completion. It is designed for tasks that might not return a result directly but signal other tasks when completed. For example, CountedCompleter can process an array in parallel and notify its parent task when finished, using addToPendingCount(), tryComplete() methods to manage task completion.

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3 How to use a custom ForkJoinTask?

To use a custom ForkJoinTask, you need to create an instance of it and pass it to a fork-join pool. You can either use the common pool, which is a shared pool that is automatically created and managed by the JVM, or create your own pool with a custom configuration. To use the common pool, you can call ForkJoinPool.commonPool().execute(task) or ForkJoinPool.commonPool().invoke(task). To create your own pool, you can use the ForkJoinPool constructor with various parameters, such as the parallelism level, the thread factory, the exception handler, or the async mode. To execute or invoke a task on your own pool, you can call pool.execute(task) or pool.invoke(task).

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4 How to handle exceptions in a custom ForkJoinTask?

One of the challenges of parallel programming is how to handle exceptions that may occur in multiple tasks. The ForkJoinTask API provides some mechanisms to deal with exceptions in a custom ForkJoinTask. First, you can use the try-catch-finally blocks in your compute() method to catch and handle any exceptions that may occur in your task or its subtasks. Second, you can use the isCompletedAbnormally() and getException() methods to check if a task has completed abnormally and get the exception that caused it. Third, you can use the completeExceptionally() method to explicitly mark a task as completed abnormally and propagate an exception to its joiners. Fourth, you can use the ForkJoinPool.UncaughtExceptionHandler interface to define a custom handler for uncaught exceptions in your pool.

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5 How to cancel a custom ForkJoinTask?

Another challenge of parallel programming is how to cancel a custom ForkJoinTask that is no longer needed or has become obsolete. The ForkJoinTask API provides some methods to cancel a custom ForkJoinTask. First, you can use the cancel() method to request a task to be cancelled. This method returns true if the task was not started or was already completed, and false otherwise. Second, you can use the isCancelled() method to check if a task has been cancelled. Third, you can use the interrupt() method to interrupt a task that is blocking on an I/O operation or a synchronization primitive. Fourth, you can use the onCancel() method to define a custom action to perform when a task is cancelled.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you use the ForkJoinTask API to create custom fork-join tasks?

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1 What is a ForkJoinTask?

2 How to create a custom ForkJoinTask?

3 How to use a custom ForkJoinTask?

4 How to handle exceptions in a custom ForkJoinTask?

5 How to cancel a custom ForkJoinTask?

6 Here’s what else to consider

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