Exchanger学习一

一、定义
Exchanger是一个用于线程间数据交换的工具类，它提供一个公共点，在这个公共点，两个线程可以交换彼此的数据。
当一个线程调用exchange方法后将进入等待状态，直到另外一个线程调用exchange方法，双方完成数据交换后继续执行。
Exchanger 是 JDK 1.5 开始提供的一个用于两个工作线程之间交换数据的封装工具类，简单说就是一个线程在完成一定的事务后想与另一个线程交换数据，则第一个先拿出数据的线程会一直等待第二个线程，直到第二个线程拿着数据到来时才能彼此交换对应数据。
从定义来看Exchanger是用于线程间进行通信、数据交换。Exchanger提供了一个同步点exchange方法，两个线程调用exchange方法时，无论调用时间先后，两个线程会互相等到线程到达exchange方法调用点，此时两个线程可以交换数据，将本线程产出数据传递给对方。

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * the current thread is {@linkplain Thread#interrupt interrupted}),
     * and then transfers the given object to it, receiving its object
     * in return.
     *
     * <p>If another thread is already waiting at the exchange point then
     * it is resumed for thread scheduling purposes and receives the object
     * passed in by the current thread.  The current thread returns immediately,
     * receiving the object passed to the exchange by that other thread.
     *
     * <p>If no other thread is already waiting at the exchange then the
     * current thread is disabled for thread scheduling purposes and lies
     * dormant until one of two things happens:
     * <ul>
     * <li>Some other thread enters the exchange; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread.
     * </ul>
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * for the exchange,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * @param x the object to exchange
     * @return the object provided by the other thread
     * @throws InterruptedException if the current thread was
     *         interrupted while waiting
     */
    @SuppressWarnings("unchecked")
    public V exchange(V x) throws InterruptedException {
        Object v;
        Object item = (x == null) ? NULL_ITEM : x; // translate null args
        if ((arena != null ||
             (v = slotExchange(item, false, 0L)) == null) &&
            ((Thread.interrupted() || // disambiguates null return
              (v = arenaExchange(item, false, 0L)) == null)))
            throw new InterruptedException();
        return (v == NULL_ITEM) ? null : (V)v;
    }

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * the current thread is {@linkplain Thread#interrupt interrupted} or
     * the specified waiting time elapses), and then transfers the given
     * object to it, receiving its object in return.
     *
     * <p>If another thread is already waiting at the exchange point then
     * it is resumed for thread scheduling purposes and receives the object
     * passed in by the current thread.  The current thread returns immediately,
     * receiving the object passed to the exchange by that other thread.
     *
     * <p>If no other thread is already waiting at the exchange then the
     * current thread is disabled for thread scheduling purposes and lies
     * dormant until one of three things happens:
     * <ul>
     * <li>Some other thread enters the exchange; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>The specified waiting time elapses.
     * </ul>
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * for the exchange,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then {@link
     * TimeoutException} is thrown.  If the time is less than or equal
     * to zero, the method will not wait at all.
     *
     * @param x the object to exchange
     * @param timeout the maximum time to wait
     * @param unit the time unit of the {@code timeout} argument
     * @return the object provided by the other thread
     * @throws InterruptedException if the current thread was
     *         interrupted while waiting
     * @throws TimeoutException if the specified waiting time elapses
     *         before another thread enters the exchange
     */
    @SuppressWarnings("unchecked")
    public V exchange(V x, long timeout, TimeUnit unit)
        throws InterruptedException, TimeoutException {
        Object v;
        Object item = (x == null) ? NULL_ITEM : x;
        long ns = unit.toNanos(timeout);
        if ((arena != null ||
             (v = slotExchange(item, true, ns)) == null) &&
            ((Thread.interrupted() ||
              (v = arenaExchange(item, true, ns)) == null)))
            throw new InterruptedException();
        if (v == TIMED_OUT)
            throw new TimeoutException();
        return (v == NULL_ITEM) ? null : (V)v;
    }

V exchange(V v)：等待另一个线程到达此交换点（除非当前线程被中断），然后将给定的对象传送给该线程，并接收该线程的对象。
V exchange(V v, long timeout, TimeUnit unit)：等待另一个线程到达此交换点，或者当前线程被中断——抛出中断异常；又或者是等候超时——抛出超时异常，然后将给定的对象传送给该线程，并接收该线程的对象。