netty 源码阅读 及 future promise学习

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netty文档说明netty的网络操作都是async的， 在源码上大量使用了future, promise这种类，自己在js框架中也看到了很多future的使用，以前不太明白，这次好好学学。

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wiki里面写到 ?a future is a read-only placeholder view of a variable, while a promise is a writable

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在netty里面也是这样定义的，?

future接口定义了isSuccess(),isCancellable(),cause(),这些判断异步执行状态的方法。（read-only）

Promise接口在extneds future的基础上增加了setSuccess(), setFailure()这些方法。（writable）

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future接口就是用来封装异步操作的执行状态的，在执行异步操作时，可以立马返回一个future，future可以sync来等待执行结果，如果有些操作要等到future代表的异步操作完了才能执行，可以通过future.addListener()的方式来在之前的异步操作完成的时候执行新的操作。

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外界看到的是future，内部其实是promise，异步的时候一般是初始线程代码里封装一个runnable，new一个promise, ?把runnable放到其他线程池去执行，执行的时候把promise对象传进去（通过final关键字），在run()方法里结束时去改变promise的状态或设置result value。初始线程在设置异步操作时可以立马返回future（其实是promise），可以根据情况选择future.sync()来同步等待结果或者future.addListener()来设置异步操作。

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下面结合netty的源码来具体看一个列子

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	public void start() throws Exception {
		EventLoopGroup group = new NioEventLoopGroup(); // 3
		try {
			ServerBootstrap b = new ServerBootstrap();
			b.group(group) // 4
					.channel(NioServerSocketChannel.class) // 5
					.localAddress(new InetSocketAddress(port)) // 6
					.childHandler(new ChannelInitializer<SocketChannel>() {// 7
								@Override
								public void initChannel(SocketChannel ch)
										throws Exception {
									ch.pipeline().addLast(
											new EchoServerHandler());
								}
							});
			ChannelFuture f = b.bind().sync(); // 8
			f.channel().closeFuture().sync(); // 9
		} finally {
			group.shutdownGracefully().sync(); // 10
		}
	}

	这段代码用过netty的应该很熟悉了，服务器启动的代码， 其中注释8的地方返回了一个future对象。来看看里面发生了什么：
	几番跳转来到了AbstractBootstrap的doBind方法。


    private ChannelFuture doBind(final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();  //1
        final Channel channel = regFuture.channel();
        if (regFuture.cause() != null) {         //2
            return regFuture;
        }

        if (regFuture.isDone()) {         //3
            // At this point we know that the registration was complete and successful.
            ChannelPromise promise = channel.newPromise();  //5
            doBind0(regFuture, channel, localAddress, promise);
            return promise;
        } else {
            // Registration future is almost always fulfilled already, but just in case it‘s not.
            final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);  //5
            regFuture.addListener(new ChannelFutureListener() {    //4
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    Throwable cause = future.cause();
                    if (cause != null) {
                        // Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an
                        // IllegalStateException once we try to access the EventLoop of the Channel.
                        promise.setFailure(cause);
                    } else {
                        // Registration was successful, so set the correct executor to use.
                        // See https://github.com/netty/netty/issues/2586
                        promise.executor = channel.eventLoop();
                    }
                    doBind0(regFuture, channel, localAddress, promise);  //1
                }
            });
            return promise;
        }
    }

    上面这段代码里
    1.开始会去注册initAndRegister()， 后面才会去bind  doBind0()。
    2.最开始拿到注册操作的future时就做了一次失败的判断，因为假如注册失败后面就没必要做了。
    3.这里检查了下future有没有做完，假如做完了这里就可以顺序执行，不用异步了。
    4.在没有做完，必须异步的情况下，因为之前的future就是异步的，没做完的，这时接下来的bind操作也只能是异步的，采用Listener的方式，最后也是返回个future（promise）
    5.同步执行和异步执行的情况下都返回的是promise,  promise只是个result的placeholder，所以同步的情况下可以用。


    final ChannelFuture initAndRegister() {
        final Channel channel = channelFactory().newChannel();
        try {
            init(channel);
        } catch (Throwable t) {
            channel.unsafe().closeForcibly();
            // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
            return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
        }

        ChannelFuture regFuture = group().register(channel);
        if (regFuture.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }

        // If we are here and the promise is not failed, it‘s one of the following cases:
        // 1) If we attempted registration from the event loop, the registration has been completed at this point.
        //    i.e. It‘s safe to attempt bind() or connect() now because the channel has been registered.
        // 2) If we attempted registration from the other thread, the registration request has been successfully
        //    added to the event loop‘s task queue for later execution.
        //    i.e. It‘s safe to attempt bind() or connect() now:
        //         because bind() or connect() will be executed *after* the scheduled registration task is executed
        //         because register(), bind(), and connect() are all bound to the same thread.

        return regFuture;
    }


    public final void register(EventLoop eventLoop, final ChannelPromise promise) {
        if (eventLoop == null) {
            throw new NullPointerException("eventLoop");
        }
        if (isRegistered()) {
            promise.setFailure(new IllegalStateException("registered to an event loop already"));
            return;
        }
        if (!isCompatible(eventLoop)) {
            promise.setFailure(
                    new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));
            return;
        }

        AbstractChannel.this.eventLoop = eventLoop;

        if (eventLoop.inEventLoop()) {  //1
            register0(promise);
        } else {
            try {
                eventLoop.execute(new OneTimeTask() {
                    @Override
                    public void run() {
                        register0(promise);
                    }
                });
            } 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();
                safeSetFailure(promise, t);
            }
        }
    }

    上面这段
    1. 判断eventloop是不是当前运行线程，是就直接做。不是就放到队列里异步做。



    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 (!promise.setUncancellable() || !ensureOpen(promise)) {
                return;
            }
            boolean firstRegistration = neverRegistered;
            doRegister();
            neverRegistered = false;
            registered = true;
            safeSetSuccess(promise);
            pipeline.fireChannelRegistered();
            // Only fire a channelActive if the channel has never been registered. This prevents firing
            // multiple channel actives if the channel is deregistered and re-registered.
            if (firstRegistration && isActive()) {
                pipeline.fireChannelActive();
            }
        } catch (Throwable t) {
            // Close the channel directly to avoid FD leak.
            closeForcibly();
            closeFuture.setClosed();
            safeSetFailure(promise, t);
        }
    }

    上面的代码就是真正做Register操作的代码了，这里全是同步的代码，做完了后设置promise的状态(safeSetSuccess(promise))。

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