获取线程的执行结果

一：Runnable和Callable的区别

最本质的区别在于，Runnable没有返回结果，Callable会有一个返回结果，返回结果是泛型，可以自己定义。举例子说明：

public class ThreadRunnable {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        MyRunnable runnable = new MyRunnable();
        new Thread(runnable).start();

        MyCallable callable = new MyCallable();
        FutureTask task = new FutureTask(callable);
        new Thread(task).start();
        System.out.println("callable: " + task.get());
    }
}

class MyRunnable implements Runnable {

    @Override
    public void run() {
        return;
    }
}

class MyCallable implements Callable<String> {

    @Override
    public String call() throws Exception {
        return "hello";
    }
}

 ​

      上述例子中可以看到，callable可以定义一个返回结果，通过FutureTask的get方法可以获得线程执行后的结果（阻塞等待结果）。

源码查看：

/**
     * Allocates a new {@code Thread} object. This constructor has the same
     * effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}
     * {@code (null, target, gname)}, where {@code gname} is a newly generated
     * name. Automatically generated names are of the form
     * {@code "Thread-"+}<i>n</i>, where <i>n</i> is an integer.
     *
     * @param  target
     *         the object whose {@code run} method is invoked when this thread
     *         is started. If {@code null}, this classes {@code run} method does
     *         nothing.
     */
    public Thread(Runnable target) {
        init(null, target, "Thread-" + nextThreadNum(), 0);
    }

可以看到，Thread里可以放Runnable，但是不可以放callable，继续查看FutureTask：

public class FutureTask<V> implements RunnableFuture<V> {
    /*
     * Revision notes: This differs from previous versions of this
     * class that relied on AbstractQueuedSynchronizer, mainly to
     * avoid surprising users about retaining interrupt status during
     * cancellation races. Sync control in the current design relies
     * on a "state" field updated via CAS to track completion, along
     * with a simple Treiber stack to hold waiting threads.
     *
     * Style note: As usual, we bypass overhead of using
     * AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
     */

    /**
     * The run state of this task, initially NEW.  The run state
     * transitions to a terminal state only in methods set,
     * setException, and cancel.  During completion, state may take on
     * transient values of COMPLETING (while outcome is being set) or
     * INTERRUPTING (only while interrupting the runner to satisfy a
     * cancel(true)). Transitions from these intermediate to final
     * states use cheaper ordered/lazy writes because values are unique
     * and cannot be further modified.
     *
     * Possible state transitions:
     * NEW -> COMPLETING -> NORMAL
     * NEW -> COMPLETING -> EXCEPTIONAL
     * NEW -> CANCELLED
     * NEW -> INTERRUPTING -> INTERRUPTED
     */
    private volatile int state;
    private static final int NEW          = 0;
    private static final int COMPLETING   = 1;
    private static final int NORMAL       = 2;
    private static final int EXCEPTIONAL  = 3;
    private static final int CANCELLED    = 4;
    private static final int INTERRUPTING = 5;
    private static final int INTERRUPTED  = 6;

    /** The underlying callable; nulled out after running */
    private Callable<V> callable;
    /** The result to return or exception to throw from get() */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;
}
点击并拖拽以移动
/**
 * 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();
}

上面的代码看到了：最终FutureTask也是实现了Runnable。

二：自定义一个FutureTask类

FutureTask的get可以阻塞等待结果，那么在写之前思考下，需要哪些步骤？

​

步骤如下：

1：需要去重写继承的Runnable接口的run方法；

2：需要一个callable变量，用来执行call()方法；

3：需要一个泛型的变量，用来存放结果；

4：还需要一个队列，用来存放那些阻塞还未返回结果的线程；

5：需要一个标志，判断线程是否执行完，可以返回结果；

6：最后，需要一个获取结果的方法。

代码实现如下，有注释可以方便查看：

public class ThreadFutureTask<T> implements Runnable {

    //判断线程有没有执行完，可以返回结果
    public static volatile boolean isEnding = false;

    //callable对象
    public Callable<T> callable;

    //线程执行后的结果
    public volatile T result;

    //队列，用于存放未执行完的线程，即阻塞的线程，放到队列中
    public LinkedBlockingDeque<Thread> waiter = new LinkedBlockingDeque<>();

    //构造函数
    public ThreadFutureTask(Callable<T> callable) {
        this.callable = callable;
    }

    //实现Runnable的run方法
    @Override
    public void run() {
        try {
            //获取callable执行的结果
            result = callable.call();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            //最后，不管有没有执行完，都结束获取结果
            isEnding = true;
        }

        //把队列中阻塞的线程，唤醒，否则获取不到结果
        while (true) {
            Thread thread = waiter.poll();
            //要是队列中没有线程在挂起，就直接返回
            if (thread == null) {
                break;
            }
            //唤醒挂起的线程
            LockSupport.unpark(thread);
        }

    }

    //获取结果的方法
    public T get() {
        //如果线程执行完了，就获取结果，否则就挂起
        if (!isEnding) {
            waiter.offer(Thread.currentThread());
        }
        //让线程挂起
        while (!isEnding) {
            LockSupport.park();
        }
        //返回结果
        return result;
    }
}

然后再去上面的runnable和callable区别的测试类里测试一下：

 public static void main(String[] args) throws ExecutionException, InterruptedException {
//        MyRunnable runnable = new MyRunnable();
//        new Thread(runnable).start();

        MyCallable callable = new MyCallable();

        ThreadFutureTask task = new ThreadFutureTask(callable);
//        FutureTask task = new FutureTask(callable);
        new Thread(task).start();
        System.out.println("callable: " + task.get());
}

返回结果如下：

​

可以看到，返回结果和FutureTask一样，这样就实现了FutureTask的功能。

 三：CountDownLatch

      CountDownLatch是一个同步工具类，它是让一个或者多个线程等待，直到其他线程执行完再执行；CountDownLatch是通过倒计数器来实现的，它的初始值就是线程的数量，每当一个线程执行完毕，计数器就减一，直到减到0，输出所有线程执行的结果，等待的线程就可以恢复继续执行。

public static AtomicLong num = new AtomicLong(0);

    public static void main(String[] args) throws InterruptedException {

        //倒计时计数器，
        CountDownLatch latch = new CountDownLatch(10);
        for (int i=0; i<10; i++) {
            new Thread(() -> {
                for (int j=0; j<100000; j++) {
                    num.getAndIncrement();
                }
                //计数器减1
                latch.countDown();
            }).start();
        }
        //打开开关
        latch.await();
        System.out.println(num.get());
}

四：Semaphore

Semaphore是一个计数信号量，可以控制访问资源的线程数量，也就是说，它是一个可以控制并发的共享锁，也就是限流。

  public static void main(String[] args) {

        //初始值就是可以通过的数量
        Semaphore semaphore = new Semaphore(2);

        for (int i=0; i<100; i++) {
            new Thread(() -> {
                try {
                    //拿到通道
                    semaphore.acquire();
                    System.out.println("开始通过桥梁。。。。");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                test("222");
                //执行完释放通道
                semaphore.release();
            }).start();
        }
    }

    public static void test(String name) {
        System.out.println(name + "成功通过");
        LockSupport.parkNanos(1000 * 1000 * 1000 * 3000);
    }

五：CyclicBarrier

CyclicBarrier循环栅栏，可以循环利用的屏障，它的作用就是让线程都等待完成后才会再去执行。

比如：去游乐园做摩天轮，假设只能上齐四个人才允许启动，那么再没有上齐之前，其他人就在等待。

public static void main(String[] args) {

        //初始值就是每次可以处理的数量
        CyclicBarrier barrier = new CyclicBarrier(2);

        for (int i=0; i<100; i++) {
            new Thread(() -> {
                try {
                    //等待
                    barrier.await();
                    System.out.println("摩天轮启动了。。。。");
                } catch (InterruptedException | BrokenBarrierException e) {
                    e.printStackTrace();
                }
            }).start();
        }

}

好了，到此，本篇文章就结束了。