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This article shows you how to use Future to achieve tasks with results in Java multithreading, the content is concise and easy to understand, it can definitely brighten your eyes. I hope you can get something through the detailed introduction of this article.

Future introduction

Future represents the result of an asynchronous calculation and provides a way to check whether the calculation is complete, wait for the calculation to complete, and retrieve the result of the calculation. The cancel method of Future can cancel the execution of the task. It has a Boolean parameter, true, which means to immediately interrupt the execution of the task, and false, which means that the running task is allowed to complete. The get method of Future waits for the calculation to complete and get the result.

Runnable

Runnable is a common interface in our multithreaded development process. The Executor framework uses Runnable as its basic task representation. Runnable is a very limited interface, the run () method does not return a value and cannot throw a checked exception.

@ FunctionalInterfacepublic interface Runnable {public abstract void run ();} Callable

Unlike Runnable, Callable is a generic parameterized interface that returns the execution result of a thread and may throw an exception if something goes wrong.

Multithreaded futureFuture

The tasks performed by Executor have four lifecycle phases: create, submit, start, and finish. Because some tasks are time-consuming, there are times when you want to cancel them. In the Executor framework, tasks that have been submitted but not started can be cancelled, tasks that have started can be cancelled only if they can respond to interrupts, and canceling tasks that have been completed has no effect.

Future represents the life cycle of a task and provides methods to determine whether the task has been completed or cancelled, as well as to obtain the results of the task and cancel the task.

Public interface Future {/ / cancel the task boolean cancel (boolean mayInterruptIfRunning); / / determine whether the boolean isCancelled () has been cancelled; / / return true boolean isDone () if the task has finished; / / block until the end and return the result V get () throws InterruptedException, ExecutionException if necessary / / if necessary, it will block the specified time to wait for the end and return the result V get (long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;}

All submit methods in ExecutorService return a Future object, thus submitting a Runnable or Callable to Executor, and you can cancel the task or get the return result through the returned Future.

You can also explicitly instantiate a specified Runnable or Callable as FutureTask, and because the FutureTask class implements the Runnable and Future interfaces, it can be submitted to Executor for execution.

FutureTask inheritance relationship:

Public class FutureTask implements RunnableFuture {.} public interface RunnableFuture extends Runnable, Future {void run ();}

One difference between Future and FutureTask is that Future needs to get the result through the return value of the submit method in ExecutorService, while FutureTask does not need to set the return value when submitting the task.

Let's take a look at an example that calculates the sum of integers between 010 and returns the result:

Import java.time.LocalDateTime;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Future; / * * @ author: jhys * @ date: Created in 2021-7-6 14:43 * @ Description: * / public class FutureTest1 {public static void main (String [] args) throws Exception {ExecutorService executor = Executors.newSingleThreadExecutor (); System.out.println (LocalDateTime.now () + ": thread start") Future future = executor.submit (()-> {try {Thread.sleep (3000);} catch (InterruptedException e) {e.printStackTrace ();} System.out.println (LocalDateTime.now () + ": task start"); int sum = 0; for (int I = 0) I {Thread.sleep (3000); System.out.println (LocalDateTime.now () + ": task start"); int sum = 0; for (int I = 0; I

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