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,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。

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