Netty的基本组件之Channel源码剖析(下)

5.2 AbstractNioChannel源码分析

 AbstractNioChannel从名字可以看出是对NIO的抽象，首先看下这个类的NioUnsafe接口：

/**
     * Special {@link Unsafe} sub-type which allows to access the underlying {@link SelectableChannel}
     */
    public interface NioUnsafe extends Unsafe {
        /**
         * Return underlying {@link SelectableChannel}
         */
        SelectableChannel ch(); // 对应NIO中的JDK实现的Channel

        /**
         * Finish connect
         */
        void finishConnect(); // 连接完成

        /**
         * Read from underlying {@link SelectableChannel}
         */
        void read(); // 从JDK的Channel中读取数据

        void forceFlush();
    }

 回忆NIO的三大概念：Channel、Buffer、Selector，Netty的Channel包装了JDK的Channel从而实现更为复杂的功能。Unsafe中可以使用ch()方法，NioChannel中可以使用javaChannel()方法获得JDK的Channel。接口中定义了finishConnect()方法是因为SelectableChannel设置为非阻塞模式时，connect()方法会立即返回，此时连接操作可能没有完成，如果没有完成，则需要调用JDK的finishConnect()方法完成连接操作。也许你已经注意到，AbstractUnsafe中并没有connect事件框架，这是因为并不是所有连接都有标准的connect过程，比如Netty的LocalChannel和EmbeddedChannel。但是NIO中的连接操作则有较为标准的流程，在介绍Connect事件框架前，先介绍一下其中使用到的相关字段，这些字段定义在AbstractNioChannel中：

/**
     * The future of the current connection attempt.  If not null, subsequent
     * connection attempts will fail.
     */
    private ChannelPromise connectPromise; // 连接异步结果
    private ScheduledFuture connectTimeoutFuture; // 连接超时检测任务异步结果
    private SocketAddress requestedRemoteAddress; // 连接的远端地址

Connect事件框架：

@Override
        public final void connect(
                final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
            if (!promise.setUncancellable() || !ensureOpen(promise)) {
                return;   // Channel已被关闭
            }

            try {
                if (connectPromise != null) {
                    // Already a connect in process.
                    throw new ConnectionPendingException(); // 已有连接操作正在进行
                }

                boolean wasActive = isActive();
                // 模板方法，细节子类完成
                if (doConnect(remoteAddress, localAddress)) {
                    fulfillConnectPromise(promise, wasActive); // 连接操作已完成
                } else {
                    // 连接操作尚未完成
                    connectPromise = promise;
                    requestedRemoteAddress = remoteAddress;

                    // 这部分代码为Netty的连接超时机制
                    // Schedule connect timeout.
                    int connectTimeoutMillis = config().getConnectTimeoutMillis();
                    if (connectTimeoutMillis > 0) {
                        connectTimeoutFuture = eventLoop().schedule(new Runnable() {
                            @Override
                            public void run() {
                                ChannelPromise connectPromise = AbstractNioChannel.this.connectPromise;
                                ConnectTimeoutException cause =
                                        new ConnectTimeoutException("connection timed out: " + remoteAddress);
                                if (connectPromise != null && connectPromise.tryFailure(cause)) {
                                    close(voidPromise());
                                }
                            }
                        }, connectTimeoutMillis, TimeUnit.MILLISECONDS);
                    }

                    promise.addListener(new ChannelFutureListener() {
                        @Override
                        public void operationComplete(ChannelFuture future) throws Exception {
                            // 连接操作取消则连接超时检测任务取消
                            if (future.isCancelled()) {
                                if (connectTimeoutFuture != null) {
                                    connectTimeoutFuture.cancel(false);
                                }
                                connectPromise = null;
                                close(voidPromise());
                            }
                        }
                    });
                }
            } catch (Throwable t) {
                promise.tryFailure(annotateConnectException(t, remoteAddress));
                closeIfClosed();
            }
        }

 Connect事件框架中包含了Netty的连接超时检测机制：向EventLoop提交一个调度任务，设定的超时时间已到则向连接操作的异步结果设置失败然后关闭连接。fulfillConnectPromise()设置异步结果为成功并触发Channel的Active事件：

private void fulfillConnectPromise(ChannelPromise promise, boolean wasActive) {
            if (promise == null) {
                // Closed via cancellation and the promise has been notified already.
                return; // 操作已取消或Promise已被通知
            }

            // Get the state as trySuccess() may trigger an ChannelFutureListener that will close the Channel.
            // We still need to ensure we call fireChannelActive() in this case.
            boolean active = isActive();

            // trySuccess() will return false if a user cancelled the connection attempt.
            boolean promiseSet = promise.trySuccess();

            // Regardless if the connection attempt was cancelled, channelActive() event should be triggered,
            // because what happened is what happened.
            if (!wasActive && active) {
                pipeline().fireChannelActive();
            }

            // If a user cancelled the connection attempt, close the channel, which is followed by channelInactive().
            if (!promiseSet) {
                close(voidPromise());
            }
        }

FinishConnect事件框架：

@Override
        public final void finishConnect() {
            // Note this method is invoked by the event loop only if the connection attempt was
            // neither cancelled nor timed out.

            assert eventLoop().inEventLoop();

            try {
                boolean wasActive = isActive();
                doFinishConnect();
                fulfillConnectPromise(connectPromise, wasActive); // 首次Active触发Active事件
            } catch (Throwable t) {
                fulfillConnectPromise(connectPromise, annotateConnectException(t, requestedRemoteAddress));
            } finally {
                // Check for null as the connectTimeoutFuture is only created if a connectTimeoutMillis > 0 is used
                // See https://github.com/netty/netty/issues/1770
                if (connectTimeoutFuture != null) {
                    connectTimeoutFuture.cancel(false);  // 连接完成，取消超时检测任务
                }
                connectPromise = null;
            }
        }

finishConnect()只由EventLoop处理就绪selectionKey的OP_CONNECT事件时调用，从而完成连接操作。注意：连接操作被取消或者超时不会使该方法被调用。
Flush事件细节：

@Override
        protected final void flush0() {
            // Flush immediately only when there's no pending flush.
            // If there's a pending flush operation, event loop will call forceFlush() later,
            // and thus there's no need to call it now.
            if (!isFlushPending()) {
                super.flush0(); // 调用父类方法，在父类判断是否已经又调用；
            }
        }

        @Override
        public final void forceFlush() {
            // directly call super.flush0() to force a flush now
            super.flush0();
        }

        private boolean isFlushPending() {
            SelectionKey selectionKey = selectionKey();
            return selectionKey.isValid() && (selectionKey.interestOps() & SelectionKey.OP_WRITE) != 0;
        }

 forceFlush()方法由EventLoop处理就绪selectionKey的OP_WRITE事件时调用，将缓冲区中的数据写入Channel。isFlushPending()方法容易导致困惑：为什么selectionKey关心OP_WRITE事件表示正在Flush呢？OP_WRITE表示通道可写，而一般情况下通道都可写，如果selectionKey一直关心OP_WRITE事件，那么将不断从select()方法返回从而导致死循环。Netty使用一个写缓冲区，write操作将数据放入缓冲区中，flush时设置selectionKey关心OP_WRITE事件，完成后取消关心OP_WRITE事件。所以，如果selectionKey关心OP_WRITE事件表示此时正在Flush数据。
 AbstractNioUnsafe还有最后一个方法removeReadOp()：

protected final void removeReadOp() {
            SelectionKey key = selectionKey();
            // Check first if the key is still valid as it may be canceled as part of the deregistration
            // from the EventLoop
            // See https://github.com/netty/netty/issues/2104
            if (!key.isValid()) {
                return; // selectionKey已被取消
            }
            int interestOps = key.interestOps();
            if ((interestOps & readInterestOp) != 0) {
                // only remove readInterestOp if needed
                key.interestOps(interestOps & ~readInterestOp); //设置为不再感兴趣
            }
        }

 Netty中将服务端的OP_ACCEPT和客户端的Read统一抽象为Read事件，在NIO底层I/O事件使用bitmap表示，一个二进制位对应一个I/O事件。当一个二进制位为1时表示关心该事件，readInterestOp的二进制表示只有1位为1，所以体会interestOps & ~readInterestOp的含义，可知removeReadOp()的功能是设置SelectionKey不再关心Read事件。类似的，还有setReadOp()、removeWriteOp()、setWriteOp()等等。
 分析完AbstractNioUnsafe，我们再分析AbstractNioChannel，首先看其中还没讲解的字段：

private final SelectableChannel ch; // 包装的JDK Channel
    protected final int readInterestOp; // Read事件，服务端OP_ACCEPT，其他OP_READ
    volatile SelectionKey selectionKey; // JDK Channel对应的选择键
    boolean readPending;  // 底层读事件进行标记

再看一下构造方法：

/**
     * Create a new instance
     *
     * @param parent            the parent {@link Channel} by which this instance was created. May be {@code null}
     * @param ch                the underlying {@link SelectableChannel} on which it operates
     * @param readInterestOp    the ops to set to receive data from the {@link SelectableChannel}
     */
    protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent);
        this.ch = ch;
        this.readInterestOp = readInterestOp;
        try {
            ch.configureBlocking(false); // 设置为非阻塞模式
        } catch (IOException e) {
            try {
                ch.close();
            } catch (IOException e2) {
                if (logger.isWarnEnabled()) {
                    logger.warn(
                            "Failed to close a partially initialized socket.", e2);
                }
            }

            throw new ChannelException("Failed to enter non-blocking mode.", e);
        }
    }

 其中的ch.configureBlocking(false)方法设置Channel为非阻塞模式，从而为Netty提供非阻塞处理I/O事件的能力。
 对于AbstractNioChannel的方法，我们主要分析它实现I/O事件框架细节部分的doXXX()方法。

    @Override
    protected void doRegister() throws Exception {
        boolean selected = false;
        for (;;) {
            try {
                selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
                return;
            } catch (CancelledKeyException e) {
                if (!selected) {
                    // Force the Selector to select now as the "canceled" SelectionKey may still be
                    // cached and not removed because no Select.select(..) operation was called yet.
                    // 选择键取消重新selectNow()，清除因取消操作而缓存的选择键
                    eventLoop().selectNow();
                    selected = true;
                } else {
                    // We forced a select operation on the selector before but the SelectionKey is still cached
                    // for whatever reason. JDK bug ?
                    throw e;
                }
            }
        }
    }

    @Override
    protected void doDeregister() throws Exception {
        eventLoop().cancel(selectionKey());
    }

 对于Register事件，当Channel属于NIO时，已经可以确定注册操作的全部细节：将Channel注册到给定NioEventLoop的selector上即可。注意，其中第二个参数0表示注册时不关心任何事件，第三个参数为Netty的NioChannel对象本身。对于Deregister事件，选择键执行cancle()操作，选择键表示JDK Channel和selector的关系，调用cancle()终结这种关系，从而实现从NioEventLoop中Deregister。需要注意的是：cancle操作调用后，注册关系不会立即生效，而会将cancle的key移入selector的一个取消键集合，当下次调用select相关方法或一个正在进行的select调用结束时，会从取消键集合中移除该选择键，此时注销才真正完成。一个Cancel的选择键为无效键，调用它相关的方法会抛出CancelledKeyException。

@Override
    protected void doBeginRead() throws Exception {
        // Channel.read() or ChannelHandlerContext.read() was called
        final SelectionKey selectionKey = this.selectionKey;
        if (!selectionKey.isValid()) {
            return; // 选择键被取消而不再有效
        }

        readPending = true; // 设置底层读事件正在进行

        final int interestOps = selectionKey.interestOps();
        if ((interestOps & readInterestOp) == 0) {
            // 选择键关心Read事件
            selectionKey.interestOps(interestOps | readInterestOp);
        }
    }

 对于NioChannel的beginRead事件，只需将Read事件设置为选择键所关心的事件，则之后的select()调用如果Channel对应的Read事件就绪，便会触发Netty的read()操作。

@Override
    protected void doClose() throws Exception {
        ChannelPromise promise = connectPromise;
        if (promise != null) {
            // Use tryFailure() instead of setFailure() to avoid the race against cancel().
            // 连接操作还在进行，但用户调用close操作
            promise.tryFailure(DO_CLOSE_CLOSED_CHANNEL_EXCEPTION);
            connectPromise = null;
        }

        ScheduledFuture future = connectTimeoutFuture;
        if (future != null) { // 如果有连接超时检测任务，则取消
            future.cancel(false);
            connectTimeoutFuture = null;
        }
    }

 此处的doClose操作主要处理了连接操作相关的后续处理。并没有实际关闭Channel，所以需要子类继续增加细节实现。AbstractNioChannel中还有关于创建DirectBuffer的方法，将在以后必要时进行分析。其他的方法则较为简单，不在列出。最后提一下isCompatible()方法，说明NioChannel只在NioEventLoop中可用。

@Override
    protected boolean isCompatible(EventLoop loop) {
        return loop instanceof NioEventLoop;
    }

 AbstractNioChannel的子类实现分为服务端AbstractNioMessageChannel和客户端AbstractNioByteChannel，我们将首先分析服务端AbstractNioMessageChannel。

5.3 AbstractNioMessageChannel源码分析

 AbstractNioMessageChannel是底层数据为消息的NioChannel。在Netty中，服务端Accept的一个Channel被认为是一条消息，UDP数据报也是一条消息。该类主要完善flush事件框架的doWrite细节和实现read事件框架（在内部类NioMessageUnsafe完成）。首先看read事件框架：

@Override
        public void read() {
            assert eventLoop().inEventLoop();
            final ChannelConfig config = config();
            final ChannelPipeline pipeline = pipeline();
            final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
            allocHandle.reset(config);

            boolean closed = false;
            Throwable exception = null;
            try {
                try {
                    do {
                        int localRead = doReadMessages(readBuf); // 模板方法，读取消息
                        if (localRead == 0) { // 没有数据可读
                            break;
                        }
                        if (localRead < 0) { // 读取出错
                            closed = true;
                            break;
                        }

                        allocHandle.incMessagesRead(localRead);
                    } while (allocHandle.continueReading());
                } catch (Throwable t) {
                    exception = t;
                }

                int size = readBuf.size();
                for (int i = 0; i < size; i ++) {
                    readPending = false; // 已没有底层读事件
                    pipeline.fireChannelRead(readBuf.get(i)); //触发ChannelRead事件，用户处理
                }
                readBuf.clear();
                allocHandle.readComplete();
                // ChannelReadComplete事件中如果配置autoRead则会调用beginRead，从而不断进行读操作
                pipeline.fireChannelReadComplete(); // 触发ChannelReadComplete事件，用户处理

                if (exception != null) {
                    closed = closeOnReadError(exception);

                    pipeline.fireExceptionCaught(exception);
                }

                if (closed) {
                    inputShutdown = true;
                    if (isOpen()) {
                        close(voidPromise()); // 非serverChannel且打开则关闭
                    }
                }
            } finally {
                // Check if there is a readPending which was not processed yet.
                // This could be for two reasons:
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
                //
                // See https://github.com/netty/netty/issues/2254
                if (!readPending && !config.isAutoRead()) {
                    // 既没有配置autoRead也没有底层读事件进行
                    removeReadOp(); // 清除read事件，不再关心
                }
            }
        }
    }

 read事件框架的流程已在代码中注明，需要注意的是读取消息的细节doReadMessages(readBuf)方法由子类实现。
 我们主要分析NioServerSocketChannel，它不支持doWrite()操作，所以我们不再分析本类的flush事件框架的doWrite细节方法，直接转向下一个目标：NioServerSocketChannel。

5.4 NioServerSocketChannel源码分析

 你肯定已经使用过NioServerSocketChannel，Netty的example中大量使用了此类，作为处于Channel最底层的子类，NioServerSocketChannel会实现I/O事件框架的底层细节。首先需要注意的是：NioServerSocketChannel只支持bind、read和close操作。

    @Override
    protected void doBind(SocketAddress localAddress) throws Exception {
        if (PlatformDependent.javaVersion() >= 7) { // JDK版本1.7以上
            javaChannel().bind(localAddress, config.getBacklog());
        } else {
            javaChannel().socket().bind(localAddress, config.getBacklog());
        }
    }

    @Override
    protected void doClose() throws Exception {
        javaChannel().close();
    }

    @Override
    protected int doReadMessages(List