Netty中的ChannelFuture和ChannelPromise-优快云博客

本文链接：https://blog.youkuaiyun.com/Z_ChenChen/article/details/90746281

在Netty使用ChannelFuture和ChannelPromise进行异步操作的处理

这是官方给出的ChannelFutur描述

 *                                      | Completed successfully    |
 *                                      +---------------------------+
 *                                 +---->      isDone() = true      |
 * +--------------------------+    |    |   isSuccess() = true      |
 * |        Uncompleted       |    |    +===========================+
 * +--------------------------+    |    | Completed with failure    |
 * |      isDone() = false    |    |    +---------------------------+
 * |   isSuccess() = false    |----+---->      isDone() = true      |
 * | isCancelled() = false    |    |    |       cause() = non-null  |
 * |       cause() = null     |    |    +===========================+
 * +--------------------------+    |    | Completed by cancellation |
 *                                 |    +---------------------------+
 *                                 +---->      isDone() = true      |
 *                                      | isCancelled() = true      |
 *                                      +---------------------------+

由图可以知道ChannelFutur有四种状态：Uncompleted、Completed successfully、Completed with failure、Completed by cancellation，这几种状态是由isDone、isSuccess、isCancelled、cause这四种方法的返回值决定的。

ChannelFutur接口的定义如下：

public interface ChannelFuture extends Future<Void> {
    Channel channel();

    ChannelFuture addListener(GenericFutureListener<? extends Future<? super Void>> var1);

    ChannelFuture addListeners(GenericFutureListener... var1);

    ChannelFuture removeListener(GenericFutureListener<? extends Future<? super Void>> var1);

    ChannelFuture removeListeners(GenericFutureListener... var1);

    ChannelFuture sync() throws InterruptedException;

    ChannelFuture syncUninterruptibly();

    ChannelFuture await() throws InterruptedException;

    ChannelFuture awaitUninterruptibly();

    boolean isVoid();
}

继承自Netty的Future：

public interface Future<V> extends java.util.concurrent.Future<V> {
    boolean isSuccess();

    boolean isCancellable();

    Throwable cause();

    Future<V> addListener(GenericFutureListener<? extends Future<? super V>> var1);

    Future<V> addListeners(GenericFutureListener... var1);

    Future<V> removeListener(GenericFutureListener<? extends Future<? super V>> var1);

    Future<V> removeListeners(GenericFutureListener... var1);

    Future<V> sync() throws InterruptedException;

    Future<V> syncUninterruptibly();

    Future<V> await() throws InterruptedException;

    Future<V> awaitUninterruptibly();

    boolean await(long var1, TimeUnit var3) throws InterruptedException;

    boolean await(long var1) throws InterruptedException;

    boolean awaitUninterruptibly(long var1, TimeUnit var3);

    boolean awaitUninterruptibly(long var1);

    V getNow();

    boolean cancel(boolean var1);
}

Netty的Future又继承自JDK的Future：

public interface Future<V> {

    boolean cancel(boolean mayInterruptIfRunning);
    
    boolean isCancelled();

    boolean isDone();
    
    V get() throws InterruptedException, ExecutionException;

    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

ChannelPromise继承了ChannelFuture：

public interface ChannelPromise extends ChannelFuture, Promise<Void> {
    Channel channel();

    ChannelPromise setSuccess(Void var1);

    ChannelPromise setSuccess();

    boolean trySuccess();

    ChannelPromise setFailure(Throwable var1);

    ChannelPromise addListener(GenericFutureListener<? extends Future<? super Void>> var1);

    ChannelPromise addListeners(GenericFutureListener... var1);

    ChannelPromise removeListener(GenericFutureListener<? extends Future<? super Void>> var1);

    ChannelPromise removeListeners(GenericFutureListener... var1);

    ChannelPromise sync() throws InterruptedException;

    ChannelPromise syncUninterruptibly();

    ChannelPromise await() throws InterruptedException;

    ChannelPromise awaitUninterruptibly();

    ChannelPromise unvoid();
}

其中Promise接口定义如下：

public interface Promise<V> extends Future<V> {
    Promise<V> setSuccess(V var1);

    boolean trySuccess(V var1);

    Promise<V> setFailure(Throwable var1);

    boolean tryFailure(Throwable var1);

    boolean setUncancellable();

    Promise<V> addListener(GenericFutureListener<? extends Future<? super V>> var1);

    Promise<V> addListeners(GenericFutureListener... var1);

    Promise<V> removeListener(GenericFutureListener<? extends Future<? super V>> var1);

    Promise<V> removeListeners(GenericFutureListener... var1);

    Promise<V> await() throws InterruptedException;

    Promise<V> awaitUninterruptibly();

    Promise<V> sync() throws InterruptedException;

    Promise<V> syncUninterruptibly();
}

在Netty中，无论是服务端还是客户端，在Channel注册时都会为其绑定一个ChannelPromise，默认实现是DefaultChannelPromise

DefaultChannelPromise定义如下：

public class DefaultChannelPromise extends DefaultPromise<Void> implements ChannelPromise, FlushCheckpoint {

    private final Channel channel;
    private long checkpoint;

    public DefaultChannelPromise(Channel channel) {
        this.channel = checkNotNull(channel, "channel");
    }
    
    public DefaultChannelPromise(Channel channel, EventExecutor executor) {
        super(executor);
        this.channel = checkNotNull(channel, "channel");
    }

    @Override
    protected EventExecutor executor() {
        EventExecutor e = super.executor();
        if (e == null) {
            return channel().eventLoop();
        } else {
            return e;
        }
    }

    @Override
    public Channel channel() {
        return channel;
    }

    @Override
    public ChannelPromise setSuccess() {
        return setSuccess(null);
    }

    @Override
    public ChannelPromise setSuccess(Void result) {
        super.setSuccess(result);
        return this;
    }

    @Override
    public boolean trySuccess() {
        return trySuccess(null);
    }

    @Override
    public ChannelPromise setFailure(Throwable cause) {
        super.setFailure(cause);
        return this;
    }

    @Override
    public ChannelPromise addListener(GenericFutureListener<? extends Future<? super Void>> listener) {
        super.addListener(listener);
        return this;
    }

    @Override
    public ChannelPromise addListeners(GenericFutureListener<? extends Future<? super Void>>... listeners) {
        super.addListeners(listeners);
        return this;
    }

    @Override
    public ChannelPromise removeListener(GenericFutureListener<? extends Future<? super Void>> listener) {
        super.removeListener(listener);
        return this;
    }

    @Override
    public ChannelPromise removeListeners(GenericFutureListener<? extends Future<? super Void>>... listeners) {
        super.removeListeners(listeners);
        return this;
    }

    @Override
    public ChannelPromise sync() throws InterruptedException {
        super.sync();
        return this;
    }

    @Override
    public ChannelPromise syncUninterruptibly() {
        super.syncUninterruptibly();
        return this;
    }

    @Override
    public ChannelPromise await() throws InterruptedException {
        super.await();
        return this;
    }

    @Override
    public ChannelPromise awaitUninterruptibly() {
        super.awaitUninterruptibly();
        return this;
    }

    @Override
    public long flushCheckpoint() {
        return checkpoint;
    }

    @Override
    public void flushCheckpoint(long checkpoint) {
        this.checkpoint = checkpoint;
    }

    @Override
    public ChannelPromise promise() {
        return this;
    }

    @Override
    protected void checkDeadLock() {
        if (channel().isRegistered()) {
            super.checkDeadLock();
        }
    }

    @Override
    public ChannelPromise unvoid() {
        return this;
    }

    @Override
    public boolean isVoid() {
        return false;
    }
}

可以看到这个DefaultChannelPromise仅仅是将Channel封装了，而且其基本上所有方法的实现都依赖于父类DefaultPromise

DefaultPromise中的实现是整个ChannelFuture和ChannelPromise的核心所在：

DefaultPromise中有如下几个状态量：

private static final int MAX_LISTENER_STACK_DEPTH = Math.min(8,
            SystemPropertyUtil.getInt("io.netty.defaultPromise.maxListenerStackDepth", 8));
private static final Object SUCCESS = new Object();
private static final Object UNCANCELLABLE = new Object();
private static final CauseHolder CANCELLATION_CAUSE_HOLDER = new CauseHolder(ThrowableUtil.unknownStackTrace(
            new CancellationException(), DefaultPromise.class, "cancel(...)"));
private static final AtomicReferenceFieldUpdater<DefaultPromise, Object> RESULT_UPDATER =
            AtomicReferenceFieldUpdater.newUpdater(DefaultPromise.class, Object.class, "result");

MAX_LISTENER_STACK_DEPTH：表示最多可执行listeners的数量，默认是8
SUCCESS ：表示异步操作正常完成
UNCANCELLABLE：表示异步操作不可取消，并且尚未完成
CANCELLATION_CAUSE_HOLDER：表示异步操作取消，用于cancel操作，
而CauseHolder 的实例对象是用来表示异步操作异常结束，同时保存异常信息：

private static final class CauseHolder {
    final Throwable cause;
    CauseHolder(Throwable cause) {
        this.cause = cause;
    }
}

RESULT_UPDATER：是一个原子更新器，通过CAS操作，原子化更新 DefaultPromise对象的名为result的成员，这个result成员是其异步操作判断的关键所在

DefaultPromise的成员及构造方法定义：

public class DefaultPromise<V> extends AbstractFuture<V> implements Promise<V> {
	private volatile Object result;
	private final EventExecutor executor;
	private Object listeners;
	private short waiters;
	private boolean notifyingListeners;
	
	public DefaultPromise(EventExecutor executor) {
		this.executor = checkNotNull(executor, "executor");
	}
}

result：就是前面说的，判断异步操作状态的关键
result的取值有：SUCCESS 、UNCANCELLABLE、CauseHolder以及null (其实还可以是泛型V类型的任意对象，这里暂不考虑)
executor：就是Channel绑定的NioEventLoop，在我之前的博客说过，Channel的异步操作都是在NioEventLoop的线程中完成的（Netty中NioEventLoopGroup的创建源码分析）
listeners：通过一个Object保存所有对异步操作的监听，用于异步操作的回调
waiters：记录阻塞中的listeners的数量
notifyingListeners：是否需要唤醒的标志

首先来看isDone方法，通过之前的图可以知道，
isDone为false对应了Uncompleted状态，即异步操作尚未完成；
isDone为true则代表了异步操作完成，但是还是有三种完成情况，需要结合别的判断方法才能具体知道是哪种情况；

isDone方法：

@Override
public boolean isDone() {
	return isDone0(result);
}

调用isDone0：

private static boolean isDone0(Object result) {
    return result != null && result != UNCANCELLABLE;
}

有如下几种情况：
result等于null，result没有赋值，表示异步操作尚未完成（从这里就能想到异步操作完成，需要调用某个set方法来改变result的状态）
result是UNCANCELLABLE状态，表示执行中的异步操作不可取消，当然也就是异步操作尚未完成
result不等于null，且不等于UNCANCELLABLE，就表示异步操作完成（包括正常完成，以及异常结束，需要由cause方法进一步判断）

isSuccess方法：

@Override
public boolean isSuccess() {
    Object result = this.result;
    return result != null && result != UNCANCELLABLE && !(result instanceof CauseHolder);
}

由这里可以知道当且仅当result 为SUCCESS状态时，才返回true（其余除UNCANCELLABLE和null的值其实也可以，这里暂不考虑）

isCancelled方法：

@Override
public boolean isCancelled() {
    return isCancelled0(result);
}

调用isCancelled0方法：

private static boolean isCancelled0(Object result) {
    return result instanceof CauseHolder && ((CauseHolder) result).cause instanceof CancellationException;
}

只有当result是CancellationException的实例时即CANCELLATION_CAUSE_HOLDER状态，表示取消异步操作

接着来看cause方法：

@Override
public Throwable cause() {
    Object result = this.result;
    return (result instanceof CauseHolder) ? ((CauseHolder) result).cause : null;
}

和上面同理，通过判别resul是否是CauseHolder的实现类，若是，将CauseHolder保存的异常返回。

几种状态的判别说完了，下面看一下如何设置这几种状态的：
setSuccess方法：

@Override
public Promise<V> setSuccess(V result) {
    if (setSuccess0(result)) {
        notifyListeners();
        return this;
    }
    throw new IllegalStateException("complete already: " + this);
}

首先调用setSuccess0方法，其中result的泛型通过DefaultChannelPromise可知是Void，在DefaultChannelPromise中所有的set和try操作参数都是null，这里的result也就不去考虑：

private boolean setSuccess0(V result) {
    return setValue0(result == null ? SUCCESS : result);
}

继续调用setValue0方法：

private boolean setValue0(Object objResult) {
    if (RESULT_UPDATER.compareAndSet(this, null, objResult) ||
        RESULT_UPDATER.compareAndSet(this, UNCANCELLABLE, objResult)) {
        checkNotifyWaiters();
        return true;
    }
    return false;
}

通过CAS操作，将result状态变为SUCCESS

其中checkNotifyWaiters方法：

private synchronized void checkNotifyWaiters() {
    if (waiters > 0) {
        notifyAll();
    }
}

检查waiters的个数，唤醒所有阻塞中的this，sync方法会引起阻塞

回到setSuccess方法中，setSuccess0通过CAS操作，将result状态更新为SUCCESS成功后，调用
notifyListeners方法，唤醒所有listener完成对异步操作的回调

listeners是通过addListener方法添加的，用来对异步操作进行侦听：
看到addListener方法：

@Override
public Promise<V> addListener(GenericFutureListener<? extends Future<? super V>> listener) {
	checkNotNull(listener, "listener");
	
	synchronized (this) {
	addListener0(listener);
	}
	
	if (isDone()) {
	notifyListeners();
	}
	
	return this;
}

@Override
public Promise<V> addListeners(GenericFutureListener<? extends Future<? super V>>... listeners) {
    checkNotNull(listeners, "listeners");

    synchronized (this) {
        for (GenericFutureListener<? extends Future<? super V>> listener : listeners) {
            if (listener == null) {
                break;
            }
            addListener0(listener);
        }
    }

    if (isDone()) {
        notifyListeners();
    }

    return this;
}

其中GenericFutureListener接口定义如下：

public interface GenericFutureListener<F extends Future<?>> extends EventListener {
 	/**
     * Invoked when the operation associated with the {@link Future} has been completed.
     *
     * @param future  the source {@link Future} which called this callback
     */
    void operationComplete(F future) throws Exception;
}

可以看到listener其实就是通过operationComplete方法，来完成回调，对Future对象进行处理，由注释可知operationComplete方法是在future操作完成时调用

addListeners方法的实现比较简单，实现核心是在addListener0中：

private void addListener0(GenericFutureListener<? extends Future<? super V>> listener) {
    if (listeners == null) {
        listeners = listener;
    } else if (listeners instanceof DefaultFutureListeners) {
        ((DefaultFutureListeners) listeners).add(listener);
    } else {
        listeners = new DefaultFutureListeners((GenericFutureListener<?>) listeners, listener);
    }
}

其中DefaultFutureListeners是将GenericFutureListener对象封装的一个数组：

final class DefaultFutureListeners {

    private GenericFutureListener<? extends Future<?>>[] listeners;
    private int size;
    private int progressiveSize;

    @SuppressWarnings("unchecked")
    DefaultFutureListeners(
            GenericFutureListener<? extends Future<?>> first, GenericFutureListener<? extends Future<?>> second) {
        listeners = new GenericFutureListener[2];
        listeners[0] = first;
        listeners[1] = second;
        size = 2;
        if (first instanceof GenericProgressiveFutureListener) {
            progressiveSize ++;
        }
        if (second instanceof GenericProgressiveFutureListener) {
            progressiveSize ++;
        }
    }

    public void add(GenericFutureListener<? extends Future<?>> l) {
        GenericFutureListener<? extends Future<?>>[] listeners = this.listeners;
        final int size = this.size;
        if (size == listeners.length) {
            this.listeners = listeners = Arrays.copyOf(listeners, size << 1);
        }
        listeners[size] = l;
        this.size = size + 1;

        if (l instanceof GenericProgressiveFutureListener) {
            progressiveSize ++;
        }
    }

    public void remove(GenericFutureListener<? extends Future<?>> l) {
        final GenericFutureListener<? extends Future<?>>[] listeners = this.listeners;
        int size = this.size;
        for (int i = 0; i < size; i ++) {
            if (listeners[i] == l) {
                int listenersToMove = size - i - 1;
                if (listenersToMove > 0) {
                    System.arraycopy(listeners, i + 1, listeners, i, listenersToMove);
                }
                listeners[-- size] = null;
                this.size = size;

                if (l instanceof GenericProgressiveFutureListener) {
                    progressiveSize --;
                }
                return;
            }
        }
    }

    public GenericFutureListener<? extends Future<?>>[] listeners() {
        return listeners;
    }

    public int size() {
        return size;
    }

    public int progressiveSize() {
        return progressiveSize;
    }
}

size：记录listeners的个数
progressiveSize：记录GenericProgressiveFutureListener类型的listeners的个数
DefaultFutureListeners 中对数组的操作比较简单，
add方法，当size达到数组长度时，进行二倍扩容，

其中GenericProgressiveFutureListener继承自GenericFutureListener：

public interface GenericProgressiveFutureListener<F extends ProgressiveFuture<?>> extends GenericFutureListener<F> {
    /**
     * Invoked when the operation has progressed.
     *
     * @param progress the progress of the operation so far (cumulative)
     * @param total the number that signifies the end of the operation when {@code progress} reaches at it.
     *              {@code -1} if the end of operation is unknown.
     */
    void operationProgressed(F future, long progress, long total) throws Exception;
}

由注释可知operationProgressed是在future操作进行时调用，这里不对GenericProgressiveFutureListener过多讨论

回到addListener0方法，由DefaultFutureListeners就可以知道，实际上通过DefaultFutureListeners管理的一维数组来保存listeners

在addListener方法完成对listener的添加后，还会调用isDone方法检查当前异步操作是否完成，若是完成需要调用notifyListeners，直接唤醒所有listeners完后对异步操作的回调

有add就有remove，removeListener方法：

@Override
public Promise<V> removeListener(final GenericFutureListener<? extends Future<? super V>> listener) {
    checkNotNull(listener, "listener");

    synchronized (this) {
        removeListener0(listener);
    }

    return this;
}

@Override
public Promise<V> removeListeners(final GenericFutureListener<? extends Future<? super V>>... listeners) {
    checkNotNull(listeners, "listeners");

    synchronized (this) {
        for (GenericFutureListener<? extends Future<? super V>> listener : listeners) {
            if (listener == null) {
                break;
            }
            removeListener0(listener);
        }
    }

    return this;
}

还是由removeListener0来实现：

private void removeListener0(GenericFutureListener<? extends Future<? super V>> listener) {
    if (listeners instanceof DefaultFutureListeners) {
        ((DefaultFutureListeners) listeners).remove(listener);
    } else if (listeners == listener) {
        listeners = null;
    }
}

看过之前的内容在看这里就比较简单了，通过DefaultFutureListeners去删除

notifyListeners方法：

private void notifyListeners() {
    EventExecutor executor = executor();
    if (executor.inEventLoop()) {
        final InternalThreadLocalMap threadLocals = InternalThreadLocalMap.get();
        final int stackDepth = threadLocals.futureListenerStackDepth();
        if (stackDepth < MAX_LISTENER_STACK_DEPTH) {
            threadLocals.setFutureListenerStackDepth(stackDepth + 1);
            try {
                notifyListenersNow();
            } finally {
                threadLocals.setFutureListenerStackDepth(stackDepth);
            }
            return;
        }
    }

    safeExecute(executor, new Runnable() {
        @Override
        public void run() {
            notifyListenersNow();
        }
    });
}

其中executor方法：

protected EventExecutor executor() {
    return executor;
}

用来获取executor轮询线程对象

判断executor是否处于轮询，否则需要通过safeExecute方法处理listeners的侦听，
safeExecute方法：

private static void safeExecute(EventExecutor executor, Runnable task) {
    try {
        executor.execute(task);
    } catch (Throwable t) {
        rejectedExecutionLogger.error("Failed to submit a listener notification task. Event loop shut down?", t);
    }
}

这里保证了listeners的侦听回调是异步执行

InternalThreadLocalMap在我之前的博客中说过，是Netty使用的ThreadLocal （Netty中FastThreadLocal源码分析）
去线程本地变量中找futureListenerStackDepth（默认为0），判断stackDepth是否小于MAX_LISTENER_STACK_DEPTH，否则也要通过safeExecute方法处理listeners的侦听
核心都是调用notifyListenersNow方法：

private void notifyListenersNow() {
    Object listeners;
    synchronized (this) {
        // Only proceed if there are listeners to notify and we are not already notifying listeners.
        if (notifyingListeners || this.listeners == null) {
            return;
        }
        notifyingListeners = true;
        listeners = this.listeners;
        this.listeners = null;
    }
    for (;;) {
        if (listeners instanceof DefaultFutureListeners) {
            notifyListeners0((DefaultFutureListeners) listeners);
        } else {
            notifyListener0(this, (GenericFutureListener<?>) listeners);
        }
        synchronized (this) {
            if (this.listeners == null) {
                // Nothing can throw from within this method, so setting notifyingListeners back to false does not
                // need to be in a finally block.
                notifyingListeners = false;
                return;
            }
            listeners = this.listeners;
            this.listeners = null;
        }
    }
}

先检查是否需要监听，满足条件后，判断listeners是否是DefaultFutureListeners，即包装后的数组
notifyListeners0方法：

private void notifyListeners0(DefaultFutureListeners listeners) {
   GenericFutureListener<?>[] a = listeners.listeners();
   int size = listeners.size();
   for (int i = 0; i < size; i ++) {
       notifyListener0(this, a[i]);
   }
}

遍历这个数组，实则调用notifyListener0方法：

private static void notifyListener0(Future future, GenericFutureListener l) {
    try {
        l.operationComplete(future);
    } catch (Throwable t) {
        if (logger.isWarnEnabled()) {
            logger.warn("An exception was thrown by " + l.getClass().getName() + ".operationComplete()", t);
        }
    }
}

这里就可以看到，完成了对operationComplete的回调，处理future

setSuccess结束，再来看trySuccess方法：

@Override
public boolean trySuccess(V result) {
    if (setSuccess0(result)) {
        notifyListeners();
        return true;
    }
    return false;
}

对比setSuccess来看，只有返回值不一样

setFailure方法：

@Override
public Promise<V> setFailure(Throwable cause) {
    if (setFailure0(cause)) {
        notifyListeners();
        return this;
    }
    throw new IllegalStateException("complete already: " + this, cause);
}

private boolean setFailure0(Throwable cause) {
    return setValue0(new CauseHolder(checkNotNull(cause, "cause")));
}

private boolean setValue0(Object objResult) {
    if (RESULT_UPDATER.compareAndSet(this, null, objResult) ||
        RESULT_UPDATER.compareAndSet(this, UNCANCELLABLE, objResult)) {
        checkNotifyWaiters();
        return true;
    }
    return false;
}

和setSuccess逻辑一样，只不过CAS操作将状态变为了CauseHolder对象，成功后唤醒listeners对异步操作的回调

tryFailure方法：

@Override
public boolean tryFailure(Throwable cause) {
    if (setFailure0(cause)) {
        notifyListeners();
        return true;
    }
    return false;
}

也都是一个逻辑

还有一个setUncancellable方法：

@Override
public boolean setUncancellable() {
    if (RESULT_UPDATER.compareAndSet(this, null, UNCANCELLABLE)) {
        return true;
    }
    Object result = this.result;
    return !isDone0(result) || !isCancelled0(result);
}

若是result状态为null，异步操作尚未结束，直接通过CAS操作将状态变为UNCANCELLABLE
否则若是根据状态来判断

下来看到cancel方法：

/**
 * {@inheritDoc}
 *
 * @param mayInterruptIfRunning this value has no effect in this implementation.
 */
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
    if (RESULT_UPDATER.compareAndSet(this, null, CANCELLATION_CAUSE_HOLDER)) {
        checkNotifyWaiters();
        notifyListeners();
        return true;
    }
    return false;
}

mayInterruptIfRunning正如注释中所说，在这里没有什么作用
还是通过CAS操作，将状态变为CANCELLATION_CAUSE_HOLDER，调用checkNotifyWaiters唤醒因sync阻塞的线程，notifyListeners方法回调listeners的侦听

最后看到sync方法：

@Override
public Promise<V> sync() throws InterruptedException {
    await();
    rethrowIfFailed();
    return this;
}

先调用await方法：

@Override
public Promise<V> await() throws InterruptedException {
    if (isDone()) {
        return this;
    }

    if (Thread.interrupted()) {
        throw new InterruptedException(toString());
    }

    checkDeadLock();

    synchronized (this) {
        while (!isDone()) {
            incWaiters();
            try {
                wait();
            } finally {
                decWaiters();
            }
        }
    }
    return this;
}

先判断能否执行（异步操作尚未结束，当前线程没有被中断），然后调用checkDeadLock方法：

protected void checkDeadLock() {
    EventExecutor e = executor();
    if (e != null && e.inEventLoop()) {
        throw new BlockingOperationException(toString());
    }
}

检查轮询线程是否在工作

在synchronized块中以自身为锁，自旋等待异步操作的完成，若是没完成，调用incWaiters方法：

private void incWaiters() {
    if (waiters == Short.MAX_VALUE) {
        throw new IllegalStateException("too many waiters: " + this);
    }
    ++waiters;
}

在小于Short.MAX_VALUE的情况下，对waiters自增，
然后使用wait将自身阻塞，等待被唤醒
所以在之前setValue0时，checkNotifyWaiters操作会notifyAll，
由此可以知道sync方法的作用：在某一线程中调用sync方法会使得当前线程被阻塞，只有当异步操作执完毕，通过上面的set方法改变状态后，才会调用checkNotifyWaiters方法唤醒当前线程。

当从阻塞中被唤醒后调用decWaiters方法：

private void decWaiters() {
	--waiters;
}

使得waiters自减
通过这样一种自旋方式，一直等到isDone成立，结束自旋，进而结束await方法，然后调用rethrowIfFailed方法：

private void rethrowIfFailed() {
    Throwable cause = cause();
    if (cause == null) {
        return;
    }

    PlatformDependent.throwException(cause);
}