详解Java中Callable和Future的区别

目录

• Java中为什么需要Callable
• Callable和Runnable的区别
• Future和RunnableFuture
• 不使用Callable和Future，仅使用Runnable实现相同功能

Java中为什么需要Callable

在java中有两种创建线程的方法：

一种是继承Thread类，重写run方法：

public class TestMain {
    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        t1.start();
    }
}
class MyThread extends Thread {
    public void run() {
        System.out.println("MyThread running...");
    }
}

第二种是使用Runnable创建一个线程：

public class TestMain {
    public static void main(String[] args) {
        Runnable r1 = new Runnable() {
            @Override
            public void run() {
                System.out.println("Thread created with runnable running...");
            }
        };
        Thread t1 = new Thread(r1);
        t1.start();
    }
}

其实这两种方式，底层都是执行Thread类的run方法：

无论使用这里的哪种方式创建线程，都无法在线程结束时return一个返回值。但是在非常多的场景下，我们都需要在线程执行结束时，将执行的结果封装为一个返回值返回给主线程(或者调用者线程)。因此java在1.5版本时，在java.util.concurrent包引入了Callable接口，用于线程执行完时return一个返回值。

Callable和Runnable的区别

Runnable和Callable都是接口，分别定义如下：

package java.lang;
​
/**
 * The <code>Runnable</code> interface should be implemented by any
 * class whose instances are intended to be executed by a thread. The
 * class must define a method of no arguments called <code>run</code>.
 * <p>
 * @since   JDK1.0
 */
@FunctionalInterface
public interface Runnable {
    public abstract void run();
}

package java.util.concurrent;
​
/**
 * A task that returns a result and may throw an exception.
 * Implementors define a single method with no arguments called
 * {@code call}.
 *
 * <p>The {@code Callable} interface is similar to {@link
 * java.lang.Runnable}, in that both are designed for classes whose
 * instances are potentially executed by another thread.  A
 * {@code Runnable}, however, does not return a result and cannot
 * throw a checked exception.
 *
 * <p>The {@link Executors} class contains utility methods to
 * convert from other common forms to {@code Callable} classes.
 *
 * @see Executor
 * @since 1.5
 * @author Doug Lea
 * @param <V> the result type of method {@code call}
 */
@FunctionalInterface
public interface Callable<V> {
    /**
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

可以看出，Callable和Runnable主要有两点区别：

• 有返回值；
• 可以抛出异常（这里抛出的异常，会在future.get()时可以通过ExectionException捕获）；

因此可以看出，Callable更加实用。这里举个Callable使用的例子：

Callable callable = new Callable<Integer>() {
  @Override
  public Integer call() throws Exception {
    int i = new Random().nextInt(5);
    try {
      Thread.sleep(i * 1000);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    return i;
  }
};

虽然Callable接口的call方法可以返回执行结果，但是有两个问题需要解决：

• 线程的创建只能通过Runnable，通过Callable又如何创建线程？
• 如何获取执行结果？

答案是Future和RunnableFuture。

Future和RunnableFuture

Future是一个接口，看下定义：

package java.util.concurrent;
​
/**
 * A {@code Future} represents the result of an asynchronous
 * computation.  Methods are provided to check if the computation is
 * complete, to wait for its completion, and to retrieve the result of
 * the computation.  The result can only be retrieved using method
 * {@code get} when the computation has completed, blocking if
 * necessary until it is ready.  Cancellation is performed by the
 * {@code cancel} method.  Additional methods are provided to
 * determine if the task completed normally or was cancelled. Once a
 * computation has completed, the computation cannot be cancelled.
 * If you would like to use a {@code Future} for the sake
 * of cancellability but not provide a usable result, you can
 * declare types of the form {@code Future<?>} and
 * return {@code null} as a result of the underlying task.
 *
 * @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;
}

可以看出，Future可以用来表示线程的未来执行结果：一个容器，这个容器内将来存放的是线程的执行结果，线程执行完之前该容器内没有值，但是线程一旦执行成功（Callable的call方法返回之后），就会将结果存入该容器。从Future的接口定义可看出，Future不仅支持阻塞获取执行结果，还支持取消任务的执行，判断任务是否执行完成等。因此通过Future，主线程(或者调用者线程)可以跟进子现场的执行情况。

Callable其实和Runnable很像，都会执行一个任务，只不过Callable可以返回执行的结果。一般将执行结果封装到Future，调用者线程即可以通过Future获取Callable的执行结果了。因此，一般Callable会和Future搭配使用。

但是问题来了：java创建线程，需要Runnable，获取执行结果又需要Future。因此RunnableFuture来了：

可以看出，通过RunnableFuture，既可以创建线程，又可以获取线程的执行结果，当然RunnableFuture也是一个接口，我们一般情况下会使用它的具体实现类FutureTask。

那可能又有人要问了，Callable又是如何建立联系的呢？看下FutureTask的使用方式就明白了：

public class TestMain {
    public static void main(String[] args) {
        Callable callable = new Callable<Integer>() {
            @Override
            public Integer call() throws Exception {
                int i = new Random().nextInt(5);
                try {
                    Thread.sleep(i * 1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                return i;
            }
        };
​
        /**
         * callable创建futureTask
         * FutureTask实现了RunnableFuture接口，因此即是Runnable又是Future
         * 作为Runnable可以传入Thread创建线程并执行
         * 作为Future，可以用来获取执行的结果。
         * 这里创建出来的futureTask对象有人称为"具柄"或者"存根"，大家可以理解为用来获取线程执行结果的一个"引用"即可。
         */
        FutureTask<Integer> futureTask = new FutureTask<Integer>(callable);
​
        // 作为Runnable使用
        Thread thread = new Thread(futureTask);
        thread.start();
​
        try {
            // 作为Future使用
            Integer integer = futureTask.get();
            System.out.println(integer);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

因此FutureTask是Callable到Runnable的桥梁。

不使用Callable和Future，仅使用Runnable实现相同功能

下面我们看下，如果不使用Callable和Future，仅使用Runnable如何实现返回值。

public class TestMain {
    public static void main(String[] args) {
        MyRunnable myRunnable = new MyRunnable();
        Thread t1 = new Thread(myRunnable);
        t1.start();
        Object o = myRunnable.get();
        System.out.println(o);
    }
}
​
class MyRunnable implements Runnable {
    // 存储执行结果
    private Object outCome = null;
​
    @Override
    public void run() {
        int i = new Random().nextInt(5);
        try {
            Thread.sleep(i * 1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        // 存储执行结果
        outCome = i;
        // 产出结果后唤醒等待的get方法
        synchronized (this) {
            notifyAll();
        }
    }
​
    public synchronized Object get() {
        while(outCome == null) {
            try {
                // 等待产出结果
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        return outCome;
    }
}

可以看出，通过Runnable实现更加麻烦，因此这也体现出了Callable+Future的优势。

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