Netty5源码分析（七） -- 异步执行Future和Promise

java.util.concurrent.Future是Java提供的接口，表示异步执行的状态，Future的get方法会判断任务是否执行完成，如果完成就返回结果，否则阻塞线程，直到任务完成。

// Java FutureTask.get()

public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

Netty扩展了Java的Future，最主要的改进就是增加了监听器Listener接口，通过监听器可以让异步执行更加有效率，不需要通过get来等待异步执行结束，而是通过监听器回调来精确地控制异步执行结束的时间点。

public interface Future<V> extends java.util.concurrent.Future<V> {

    boolean isSuccess();

    boolean isCancellable();

    Throwable cause();

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

    Future<V> addListeners(GenericFutureListener<? extends Future<? super V>>... listeners);

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

    Future<V> removeListeners(GenericFutureListener<? extends Future<? super V>>... listeners);

    Future<V> sync() throws InterruptedException;

    Future<V> syncUninterruptibly();

    Future<V> await() throws InterruptedException;

    Future<V> awaitUninterruptibly();

    boolean await(long timeout, TimeUnit unit) throws InterruptedException;

    boolean await(long timeoutMillis) throws InterruptedException;

    boolean awaitUninterruptibly(long timeout, TimeUnit unit);

    boolean awaitUninterruptibly(long timeoutMillis);

    V getNow();

    boolean cancel(boolean mayInterruptIfRunning);
}

ChannelFuture接口扩展了Netty的Future接口，表示一种没有返回值的异步调用，同时关联了Channel，跟一个Channel绑定

public interface ChannelFuture extends Future<Void> {

    Channel channel();

    @Override
    ChannelFuture addListener(GenericFutureListener<? extends Future<? super Void>> listener);

    @Override
    ChannelFuture addListeners(GenericFutureListener<? extends Future<? super Void>>... listeners);

    @Override
    ChannelFuture removeListener(GenericFutureListener<? extends Future<? super Void>> listener);

    @Override
    ChannelFuture removeListeners(GenericFutureListener<? extends Future<? super Void>>... listeners);

    @Override
    ChannelFuture sync() throws InterruptedException;

    @Override
    ChannelFuture syncUninterruptibly();

    @Override
    ChannelFuture await() throws InterruptedException;

    @Override
    ChannelFuture awaitUninterruptibly();
}

Promise接口也扩展了Future接口，它表示一种可写的Future，就是可以设置异步执行的结果

public interface Promise<V> extends Future<V> {

    Promise<V> setSuccess(V result);

    boolean trySuccess(V result);

    Promise<V> setFailure(Throwable cause);

    boolean tryFailure(Throwable cause);
} 

ChannelPromise接口扩展了Promise和ChannelFuture，绑定了Channel，又可写异步执行结构，又具备了监听者的功能，是Netty实际编程使用的表示异步执行的接口

public interface ChannelPromise extends ChannelFuture, Promise<Void> {

    @Override
    Channel channel();

    @Override
    ChannelPromise setSuccess(Void result);

    ChannelPromise setSuccess();

    boolean trySuccess();

    @Override
    ChannelPromise setFailure(Throwable cause);

    @Override
    ChannelPromise addListener(GenericFutureListener<? extends Future<? super Void>> listener);

    @Override
    ChannelPromise addListeners(GenericFutureListener<? extends Future<? super Void>>... listeners);

    @Override
    ChannelPromise removeListener(GenericFutureListener<? extends Future<? super Void>> listener);

    @Override
    ChannelPromise removeListeners(GenericFutureListener<? extends Future<? super Void>>... listeners);

    @Override
    ChannelPromise sync() throws InterruptedException;

    @Override
    ChannelPromise syncUninterruptibly();

    @Override
    ChannelPromise await() throws InterruptedException;

    @Override
    ChannelPromise awaitUninterruptibly();
}

DefaultChannelPromise是ChannelPromise的实现类，它是实际运行时的Promoise实例。Channel接口提供了newPromise接口，表示Channel要创建一个异步执行的动作

public interface Channel extends AttributeMap, Comparable<Channel> {
    ChannelPromise newPromise();
}

public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {
    public ChannelPromise newPromise() {
        return new DefaultChannelPromise(this);
    }
}

Netty推荐使用addListener的方式来回调异步执行的结果，这种方式优于Future.get，能够更精确地把握异步执行结束的时间。

看一下DefaultPromise的addListener方法，它判断异步任务执行的状态，如果执行完成，就理解通知监听者，否则加入到监听者队列

通知监听者就是找一个线程来执行调用监听的回调函数。

// DefaultPromise.addListener
public Promise<V> addListener(GenericFutureListener<? extends Future<? super V>> listener) {
        if (listener == null) {
            throw new NullPointerException("listener");
        }

        if (isDone()) {
            notifyListener(executor(), this, listener);
            return this;
        }

        synchronized (this) {
            if (!isDone()) {
                if (listeners == null) {
                    listeners = listener;
                } else {
                    if (listeners instanceof DefaultFutureListeners) {
                        ((DefaultFutureListeners) listeners).add(listener);
                    } else {
                        @SuppressWarnings("unchecked")
                        final GenericFutureListener<? extends Future<V>> firstListener =
                                (GenericFutureListener<? extends Future<V>>) listeners;
                        listeners = new DefaultFutureListeners(firstListener, listener);
                    }
                }
                return this;
            }
        }

        notifyListener(executor(), this, listener);
        return this;
    }

protected static void notifyListener(
            final EventExecutor eventExecutor, final Future<?> future, final GenericFutureListener<?> l) {

        if (eventExecutor.inEventLoop()) {
            final Integer stackDepth = LISTENER_STACK_DEPTH.get();
            if (stackDepth < MAX_LISTENER_STACK_DEPTH) {
                LISTENER_STACK_DEPTH.set(stackDepth + 1);
                try {
                    notifyListener0(future, l);
                } finally {
                    LISTENER_STACK_DEPTH.set(stackDepth);
                }
                return;
            }
        }

        try {
            eventExecutor.execute(new Runnable() {
                @Override
                public void run() {
                    notifyListener0(future, l);
                }
            });
        } catch (Throwable t) {
            logger.error("Failed to notify a listener. Event loop shut down?", t);
        }
    }

    @SuppressWarnings({ "unchecked", "rawtypes" })
    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);
            }
        }
    }

再来看监听者的接口，就一个方法，即等异步任务执行完成后，拿到Future结果，执行回调的逻辑

public interface GenericFutureListener<F extends Future<?>> extends EventListener {

    void operationComplete(F future) throws Exception;
}

来看一个实例，在服务器绑定的过程中，会调用initAndRegister()来创建并注册Channel。

1. 这个过程中先通过Channel.newPromise创建了一个异步任务，然后把Promise实例传递给register方法。

2. register方法找一个线程来执行register0方法，第一步的方法在把任务交给线程后就返回了。这是一个异步执行的过程

3. 在新起的线程中，设置Promise的状态。可以看到Promise表示了可写的Future

4. 原线程在拿到initAndRegister返回的结果后，继续执行，这时候是由两个线程在执行

5. 原线程判断Promise的状态来判断是否注册完成，如果注册完成就执行后续的doBind0，如果没有完成，就通过添加回调的方法来进行异步执行

final ChannelFuture initAndRegister() {
        Channel channel;
        try {
            channel = createChannel();
        } catch (Throwable t) {
            return VoidChannel.INSTANCE.newFailedFuture(t);
        }

        try {
            init(channel);
        } catch (Throwable t) {
            channel.unsafe().closeForcibly();
            return channel.newFailedFuture(t);
        }

        ChannelPromise regFuture = channel.newPromise();
        channel.unsafe().register(regFuture);
        if (regFuture.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }
      because register(), bind(), and connect() are all bound to the same thread.

      return regFuture;
    }

<pre name="code" class="java"> public final void register(final ChannelPromise promise) {
            if (eventLoop.inEventLoop()) {
                register0(promise);
            } else {
                try {
                   <strong> eventLoop.execute(new Runnable() {
                        @Override
                        public void run() {
                            register0(promise);
                        }
                    });</strong>
                } catch (Throwable t) {
                    logger.warn(
                            "Force-closing a channel whose registration task was not accepted by an event loop: {}",
                            AbstractChannel.this, t);
                    closeForcibly();
                    closeFuture.setClosed();
                    promise.setFailure(t);
                }
            }
        }

        private void register0(ChannelPromise promise) {
            try {
                // check if the channel is still open as it could be closed in the mean time when the register
                // call was outside of the eventLoop
                if (!ensureOpen(promise)) {
                    return;
                }
                doRegister();
                registered = true;
                promise.setSuccess();
                pipeline.fireChannelRegistered();
                if (isActive()) {
                    pipeline.fireChannelActive();
                }
            } catch (Throwable t) {
                // Close the channel directly to avoid FD leak.
                closeForcibly();
                closeFuture.setClosed();
                if (!promise.tryFailure(t)) {
                    logger.warn(
                            "Tried to fail the registration promise, but it is complete already. " +
                                    "Swallowing the cause of the registration failure:", t);
                }
            }
        }

private ChannelFuture doBind(final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();
        final Channel channel = regFuture.channel();
        if (regFuture.cause() != null) {
            return regFuture;
        }

        final ChannelPromise promise;
        if (regFuture.isDone()) {
            promise = channel.newPromise();
            doBind0(regFuture, channel, localAddress, promise);
        } else {
            // Registration future is almost always fulfilled already, but just in case it's not.
            promise = new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE);
            regFuture.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    doBind0(regFuture, channel, localAddress, promise);
                }
            });
        }

        return promise;
    }