Get the thread return value. Introduction to the use of future interface and futuretask class

        Start a thread , Computer according to cpu And memory usage , Execute code in the thread at a future point in time . How do I know if the thread execution is complete , What's the result ？Future English means “ future 、 Future ”, seeing the name of a thing one thinks of its function Future Interface came into being .
        One 、Future The interface is introduced

        Usage method get Search , Block if necessary , Until it's ready .cancel Cancel the operation . Other methods are provided to determine whether the task is completed normally or cancelled . Once the calculation is done , We can't cancel the calculation . If used for cancellability Future, But do not provide available results , It can be stated that Future>, And back to null As a result of the underlying task .

Return value	Method name	Method usage description
boolean	cancel(boolean mayInterruptIfRunning)	Attempt to cancel the execution of this task . If the task has been completed 、 Has been cancelled or cannot be cancelled for other reasons , Then the attempt will fail . If it works , And calling cancel The task did not start when , The task should never run . If the task has been started , be mayInterruptIfRunning Parameter determines whether the thread executing the task should be interrupted to try to stop the task
boolean	isCancelled()	If this task is cancelled before normal completion , Then return to true
boolean	isDone()	If this task is completed, return to true
V	get()	Get thread execution results , Will be waiting , Until the thread execution is complete
V	get(long timeout, TimeUnit unit)	Get thread execution results , If no result is obtained within the specified time, an exception will be thrown

Two 、Future Code example

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;

import org.junit.Test;

public class Future_FutureTask {
	// Creating a thread pool 
	ExecutorService exec = Executors.newCachedThreadPool();

	@Test
	public void FutureTest() throws Exception {
		// return Future
		Future<?> future = exec.submit(() -> {
			System.out.println("Runnabel Interface , No return value ");
		});
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		if (future.isDone()) {
			System.out.println(" Get thread return result ：" + future.get());
		} else {
			System.out.println(" Get thread return result ：" + future.get(10L, TimeUnit.SECONDS));
		}
		System.out.println(" Whether the thread is cancelled " + future.isCancelled());

	}
}

3、 ... and 、FutureTask Introduction to implementation class

         This class provides Future The basic implementation of the interface ;FutureTask Can be used to package into  Callable  perhaps  Runnable  Object to use . because FutureTask Realized Runnable Interface , therefore FutureTask Can be submitted to Executor perform

Four 、FutureTask Code example

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.TimeUnit;

import org.junit.Test;

public class Future_FutureTask {
	//  Creating a thread pool 
	ExecutorService exec = Executors.newCachedThreadPool();

	@Test
	public void FutureTaskTest() throws Exception {
		//  Use Lambda  Create objects using expressions 
		// FutureTask<V> futureTask = new FutureTask<V>(Callable<V> callable)
		FutureTask<String> futureTask = new FutureTask<String>(() -> {
			return "FutureTask Introduction ";
		});
		exec.execute(futureTask);
		System.out.println(" obtain FutureTask Return result of :" + futureTask.get());

	}
}

5、 ... and 、 summary

        Future Express Asynchronous computation （ Threads ） Result . The method provided is used to check whether the calculation is completed 、 Wait for it to complete and retrieve the results of the calculation .

        FutureTask Class is Future Implementation class of interface