Java中的Runnable，Callable，Future，FutureTask的比较

Java中的Runnable，Callable，Future，FutureTask的比较

Java中存在Runnable、Callable、Future、FutureTask这几个与线程相关的类或者接口，在Java中也是比较重要的几个概念，我们通过下面的简单示例来了解一下它们的作用于区别。

Runnable

其中Runnable应该是我们最熟悉的接口，它只有一个run()函数，用于将耗时操作写在其中， 该函数没有返回值 。然后使用某个线程去执行该runnable即可实现多线程，Thread类在调用start()函数后就是执行的是Runnable的run()函数。Runnable的声明如下 :

@FunctionalInterface
public interface Runnable {
 /**
  * When an object implementing interface <code>Runnable</code> is used
  * to create a thread, starting the thread causes the object's
  * <code>run</code> method to be called in that separately executing
  * thread.
  * <p>
  * The general contract of the method <code>run</code> is that it may
  * take any action whatsoever.
  *
  * @see   java.lang.Thread#run()
  */
 public abstract void run();
}

Callable

Callable与Runnable的功能大致相似，Callable中有一个call()函数，但是 call()函数有返回值 ，而Runnable的run()函数不能将结果返回给客户程序。Callable的声明如下 :

@FunctionalInterface
public interface Callable<V> {
 /**
  * Computes a result, or throws an exception if unable to do so.
  *
  * @return computed result
  * @throws Exception if unable to compute a result
  */
 V call() throws Exception;
}

可以看到，这是一个泛型接口，call()函数返回的类型就是客户程序传递进来的V类型。

Future

Executor就是Runnable和Callable的调度容器，Future就是对于具体的Runnable或者Callable任务的执行结果进行取消、查询是否完成、获取结果、设置结果操作。get方法会阻塞，直到任务返回结果(Future简介)。Future声明如下:

* @see FutureTask
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this Future's {@code get} method
*/
public interface Future<V> {

/**
  * Attempts to cancel execution of this task. This attempt will
  * fail if the task has already completed, has already been cancelled,
  * or could not be cancelled for some other reason. If successful,
  * and this task has not started when {@code cancel} is called,
  * this task should never run. If the task has already started,
  * then the {@code mayInterruptIfRunning} parameter determines
  * whether the thread executing this task should be interrupted in
  * an attempt to stop the task.
  *
  * <p>After this method returns, subsequent calls to {@link #isDone} will
  * always return {@code true}. Subsequent calls to {@link #isCancelled}
  * will always return {@code true} if this method returned {@code true}.
  *
  * @param mayInterruptIfRunning {@code true} if the thread executing this
  * task should be interrupted; otherwise, in-progress tasks are allowed
  * to complete
  * @return {@code false} if the task could not be cancelled,
  * typically because it has already completed normally;
  * {@code true} otherwise
  */
 boolean cancel(boolean mayInterruptIfRunning);

/**
  * Returns {@code true} if this task was cancelled before it completed
  * normally.
  *
  * @return {@code true} if this task was cancelled before it completed
  */
 boolean isCancelled();

/**
  * Returns {@code true} if this task completed.
  *
  * Completion may be due to normal termination, an exception, or
  * cancellation -- in all of these cases, this method will return
  * {@code true}.
  *
  * @return {@code true} if this task completed
  */
 boolean isDone();

/**
  * Waits if necessary for the computation to complete, and then
  * retrieves its result.
  *
  * @return the computed result
  * @throws CancellationException if the computation was cancelled
  * @throws ExecutionException if the computation threw an
  * exception
  * @throws InterruptedException if the current thread was interrupted
  * while waiting
  */
 V get() throws InterruptedException, ExecutionException;

/**
  * Waits if necessary for at most the given time for the computation
  * to complete, and then retrieves its result, if available.
  *
  * @param timeout the maximum time to wait
  * @param unit the time unit of the timeout argument
  * @return the computed result
  * @throws CancellationException if the computation was cancelled
  * @throws ExecutionException if the computation threw an
  * exception
  * @throws InterruptedException if the current thread was interrupted
  * while waiting
  * @throws TimeoutException if the wait timed out
  */
 V get(long timeout, TimeUnit unit)
   throws InterruptedException, ExecutionException, TimeoutException;
}

FutureTask

FutureTask则是一个RunnableFuture< V>，而RunnableFuture实现了Runnbale又实现了Futrue< V>这两个接口：

public class FutureTask<V> implements RunnableFuture<V> {
......
}

RunnableFuture

/**
* A {@link Future} that is {@link Runnable}. Successful execution of
* the {@code run} method causes completion of the {@code Future}
* and allows access to its results.
* @see FutureTask
* @see Executor
* @since 1.6
* @author Doug Lea
* @param <V> The result type returned by this Future's {@code get} method
*/
public interface RunnableFuture<V> extends Runnable, Future<V> {
 /**
  * Sets this Future to the result of its computation
  * unless it has been cancelled.
  */
 void run();
}

另外FutureTask还可以包装Runnable和Callable< V>， 由构造函数注入依赖。

/**
  * Creates a {@code FutureTask} that will, upon running, execute the
  * given {@code Callable}.
  *
  * @param callable the callable task
  * @throws NullPointerException if the callable is null
  */
 public FutureTask(Callable<V> callable) {
   if (callable == null)
     throw new NullPointerException();
   this.callable = callable;
   this.state = NEW;    // ensure visibility of callable
 }

/**
  * Creates a {@code FutureTask} that will, upon running, execute the
  * given {@code Runnable}, and arrange that {@code get} will return the
  * given result on successful completion.
  *
  * @param runnable the runnable task
  * @param result the result to return on successful completion. If
  * you don't need a particular result, consider using
  * constructions of the form:
  * {@code Future<?> f = new FutureTask<Void>(runnable, null)}
  * @throws NullPointerException if the runnable is null
  */
 public FutureTask(Runnable runnable, V result) {
   this.callable = Executors.callable(runnable, result);
   this.state = NEW;    // ensure visibility of callable
 }

可以看到，Runnable注入会被Executors.callable()函数转换为Callable类型，即FutureTask最终都是执行Callable类型的任务。该适配函数的实现如下 ：

/**
  * Returns a {@link Callable} object that, when
  * called, runs the given task and returns the given result. This
  * can be useful when applying methods requiring a
  * {@code Callable} to an otherwise resultless action.
  * @param task the task to run
  * @param result the result to return
  * @param <T> the type of the result
  * @return a callable object
  * @throws NullPointerException if task null
  */
 public static <T> Callable<T> callable(Runnable task, T result) {
   if (task == null)
     throw new NullPointerException();
   return new RunnableAdapter<T>(task, result);
 }

RunnableAdapter适配器

/**
  * A callable that runs given task and returns given result
  */
 static final class RunnableAdapter<T> implements Callable<T> {
   final Runnable task;
   final T result;
   RunnableAdapter(Runnable task, T result) {
     this.task = task;
     this.result = result;
   }
   public T call() {
     task.run();
     return result;
   }
 }

由于FutureTask实现了Runnable，因此它既可以通过Thread包装来直接执行，也可以提交给ExecuteService来执行。并且还可以直接通过get()函数获取执行结果，该函数会阻塞，直到结果返回。

因此FutureTask既是Future、Runnable，又是包装了Callable(如果是Runnable最终也会被转换为Callable )， 它是这两者的合体。

完整示例：

package com.stay4it.rx;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;

public class FutureTest {

public static class Task implements Runnable {

@Override
   public void run() {
     // TODO Auto-generated method stub
     System.out.println("run");
   }

}
 public static class Task2 implements Callable<Integer> {

@Override
   public Integer call() throws Exception {
     System.out.println("call");
     return fibc(30);
   }

}

/**
  * runnable, 无返回值
  */
 public static void testRunnable(){
   ExecutorService executorService = Executors.newCachedThreadPool();

Future<String> future = (Future<String>) executorService.submit(new Task());
   try {
     System.out.println(future.get());
   } catch (InterruptedException e) {
     // TODO Auto-generated catch block
     e.printStackTrace();
   } catch (ExecutionException e) {
     // TODO Auto-generated catch block
     e.printStackTrace();
   }

executorService.shutdown();
 }

/**
  * Callable, 有返回值
  */
 public static void testCallable(){
   ExecutorService executorService = Executors.newCachedThreadPool();

Future<Integer> future = (Future<Integer>) executorService.submit(new Task2());
   try {
     System.out.println(future.get());
   } catch (InterruptedException e) {
     // TODO Auto-generated catch block
     e.printStackTrace();
   } catch (ExecutionException e) {
     // TODO Auto-generated catch block
     e.printStackTrace();
   }

executorService.shutdown();
 }

/**
  * FutureTask则是一个RunnableFuture<V>，即实现了Runnbale又实现了Futrue<V>这两个接口，
  * 另外它还可以包装Runnable(实际上会转换为Callable)和Callable
  * <V>，所以一般来讲是一个符合体了，它可以通过Thread包装来直接执行，也可以提交给ExecuteService来执行
  * ，并且还可以通过v get()返回执行结果，在线程体没有执行完成的时候，主线程一直阻塞等待，执行完则直接返回结果。
  */
 public static void testFutureTask(){
   ExecutorService executorService = Executors.newCachedThreadPool();
   FutureTask<Integer> futureTask = new FutureTask<Integer>(new Task2());

executorService.submit(futureTask);
   try {
     System.out.println(futureTask.get());
   } catch (InterruptedException e) {
     // TODO Auto-generated catch block
     e.printStackTrace();
   } catch (ExecutionException e) {
     // TODO Auto-generated catch block
     e.printStackTrace();
   }

executorService.shutdown();
 }

/**
  * FutureTask则是一个RunnableFuture<V>，即实现了Runnbale又实现了Futrue<V>这两个接口，
  * 另外它还可以包装Runnable(实际上会转换为Callable)和Callable
  * <V>，所以一般来讲是一个符合体了，它可以通过Thread包装来直接执行，也可以提交给ExecuteService来执行
  * ，并且还可以通过v get()返回执行结果，在线程体没有执行完成的时候，主线程一直阻塞等待，执行完则直接返回结果。
  */
 public static void testFutureTask2(){
   ExecutorService executorService = Executors.newCachedThreadPool();
   FutureTask<Integer> futureTask = new FutureTask<Integer>(new Runnable() {

@Override
     public void run() {
       // TODO Auto-generated method stub
       System.out.println("testFutureTask2 run");
     }
   },fibc(30));

executorService.submit(futureTask);
   try {
     System.out.println(futureTask.get());
   } catch (InterruptedException e) {
     // TODO Auto-generated catch block
     e.printStackTrace();
   } catch (ExecutionException e) {
     // TODO Auto-generated catch block
     e.printStackTrace();
   }

executorService.shutdown();
 }

public static void main(String[] args) {

testCallable();

}

/**
  * 效率低下的斐波那契数列, 耗时的操作
  *
  * @param num
  * @return
  */
 static int fibc(int num) {
   if (num == 0) {
     return 0;
   }
   if (num == 1) {
     return 1;
   }
   return fibc(num - 1) + fibc(num - 2);
 }

}

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