io.netty.channel.Channel是Netty网络操作抽象类，它聚合了一组功能，包括但不限于网路的读，写，客户端发起连接，主动关闭连接，链路关闭，获取通信双方的网络地址等。它也包含了Netty框架相关的一些功能，包括获取该Channel的EventLoop，获取缓冲分配器ByteBufAllocator和pipeline等。

Unsafe是个内部接口，聚合在Channel中协助进行网络读写相关的操作，因为它的设计初衷就是Channel的内部辅助类，不应该被Netty框架的上层使用者调用，所以被命名为Unsafe。这里不能仅从字面理解认为它是不安全的操作，而要从这个架构的设计层面体会它的设计初衷和职责。

1、Channel功能说明

1.1、netty自定义Channel的原因

• JDK的SocketChannel和ServerSocketChannel没有统一的Channel接口供业务开发者使用，对于用户而言，没有统一的操作视图，使用起来不方便。
• JDK的SocketChannel和ServerSocketChannel的主要职责就是网络I/O操作，由于它们是SPI类接口，由具体的虚拟机厂家来提供，所以通过继承SPI功能类来扩展其功能的难度很大；直接实现ServerSocketChannel和SocketChannel抽象类，其工作量和重新开发一个新的Channel功能类是差不多的。
• Netty的Channel需要能够跟Netty的整体架构融合在一起，例如I/O模型，基于ChannelPipeline的定制模型，以及基于元数据描述配置化的TCP参数等，这些JDK的SocketChannel和ServerSocketChanel都没有提供，需要重新封装。
• 自定义的Channel，功能实现更加灵活。

1.2、netty自定义Channel的设计理念

• 在Channel接口层，采用Facade模式进行统一封装，将网络I/O操作，网络I/O相关的其他操作封装起来，统一对外提供。
• Channel接口的定义尽量大而全，为SocketChannel和ServerSocketChannel提供统一的视图，由不同子类实现不同的功能，公共功能在抽象父类中实现，最大程度地实现功能和接口的重用。
• 具体实现采用聚合而非包含的方式，将相关的功能类聚合在Channel中，由Channel统一分配和调度，功能实现更加灵活。

1.3、Channel主要API

接口名	描述
EventLoop eventLoop()	Channel需要注册到EventLoop的多路复用器上，用于处理I/O事件，通过eventLoop()方法可以获取到Channel注册的EventLoop。EventLoop本质上就是处理网络读写事件的Reactor线程。在Netty中，它不仅仅用来处理网络事件，也可以用来执行定时任务和用户自定义NioTask等任务。
ChannelPipeline pipeline()	返回channel分配的ChannelPipeline
boolean isActive()	判断channel是否激活。激活的意义取决于底层的传输类型。例如，一个Socket传输一旦连接到了远程节点便是活动的，而一个Datagram传输一旦被打开便是活动的
boolean isOpen()	判断Channel是否已经打开
boolean isRegistered()	判断Channel是否已经在对应的EventLoop中注册
SocketAddress localAddress()	返回本地的socket地址
SocketAddress remoteAddress()	返回远程的socket地址
Channel flush()	将之前已写的数据冲刷到底层Channel上去
boolean isWritable()	当且仅当I/O线程可以立即处理写请求时，返回true；当本方法返回false时，任何写操作将进行入队，直到i/o线程准备好处理队列中的写请求
ChannelMetadata metadata()	熟悉TCP协议的读者可能知道，当创建Socket的时候需要指定TCP参数，例如接收和发送的TCP缓冲区大小，TCP的超时时间。是否重用地址等。在Netty中，每个Channel对应一个物理链接，每个连接都有自己的TCP参数配置。所以，Channel会聚合一个ChannelMetadata用来对TCP参数提供元数据描述信息，通过metadata()方法就可以获取当前Channel的TCP参数配置。
Channel read()	从Channel中读取数据到第一个inBound缓冲区，当读取完毕，触发Handler的channelRead()事件，同时触发Handler的channelReadComplete()事件，以让Handler决定是否继续进行数据读取。如果有正在读取的操作，则此方法不做任何操作。
ChannelFuture closeFuture()	当Channel关闭时，通知对应的ChannelFuture。此方法总是返回同一个实例
Unsafe unsafe()	提供一个内部使用的类，此类实现了Unsafae相关接口
Channel parent()	对于服务端Channel而言，它的父Channel为空；对于客户端Channel，它的父Channel就是创建它的ServerSocketChannel。
ChannelId id()	返回ChannelId对象，ChannelId是Channel的唯一标识。
ChannelConfig config()	获取当前Channel的配置信息，例如CONNECT_TIMEOUT_MILLS。
long bytesBeforeUnwritable()	isWritable()返回false时，其返回可写的字节数；否则返回0
long bytesBeforeWritable()	isWritable()返回true时，其返回底层缓存未写的的字节数；否则返回0
ByteBufAllocator alloc()	返回内存分配器

2、Channel类继承图

Channel类继承图如下：

Channel类继承图.png

从类继承图可以看出：

（1）Channel是所有通用类的基础接口，ServerChannel是所有服务端Channel的通用接口；
（2）AbstractChannel为Channel的基础抽象类，其对Channel的一些通用功能做了简单实现；
（3）从AbstractChannel继承出基于不同协议及I/O类型的Channel实现类；

• NioSocketChannel：异步I/O的客户端 TCP Socket 实现
• NioServerSocketChannel：异步I/O的服务端 TCP Socket 实现
• NioDatagramChannel：异步I/O的 UDP Socket 实现
• NioSctpChannel：异步I/O的客户端 Sctp Socket 实现
• NioSctpServerChannel：异步I/O的服务端 Sctp Socket 实现
• EpollSocketChannel：基于linux的Epoll实现的事件驱动的客户端TCP Socket实现；
• EpollServerSocketChannel：基于linux的Epoll实现事件驱动的服务端TCP Socket实现；
• OioSocketChannel：同步I/O的客户端 TCP Socket 实现
• OioServerSocketChannel：同步I/O的服务端 TCP Socket 实现
• OioDatagramChannel：同步I/O的 UDP Socket 实现
• OioSctpChannel：同步I/O的客户端 Sctp Socket 实现
• OioSctpServerChannel：同步I/O的服务端 Sctp Socket 实现

3、AbstractChannel源码分析

AbstractChannel 是Channel的部分实现，维护了一个通道相关的资源，如channel id, pipeline等；而且实现了对该套接字的IO操作，以及设置interestOps；这里还没有牵扯到底层的细节，只是这个框架的结构。

3.1、成员变量

private static final ClosedChannelException FLUSH0_CLOSED_CHANNEL_EXCEPTION = ThrowableUtil.unknownStackTrace(
            new ClosedChannelException(), AbstractUnsafe.class, "flush0()");
    private static final ClosedChannelException ENSURE_OPEN_CLOSED_CHANNEL_EXCEPTION = ThrowableUtil.unknownStackTrace(
            new ClosedChannelException(), AbstractUnsafe.class, "ensureOpen(...)");
    private static final ClosedChannelException CLOSE_CLOSED_CHANNEL_EXCEPTION = ThrowableUtil.unknownStackTrace(
            new ClosedChannelException(), AbstractUnsafe.class, "close(...)");
    private static final ClosedChannelException WRITE_CLOSED_CHANNEL_EXCEPTION = ThrowableUtil.unknownStackTrace(
            new ClosedChannelException(), AbstractUnsafe.class, "write(...)");
    private static final NotYetConnectedException FLUSH0_NOT_YET_CONNECTED_EXCEPTION = ThrowableUtil.unknownStackTrace(
            new NotYetConnectedException(), AbstractUnsafe.class, "flush0()");

    private final Channel parent;
    private final ChannelId id;
    private final Unsafe unsafe;
    private final DefaultChannelPipeline pipeline;
    private final VoidChannelPromise unsafeVoidPromise = new VoidChannelPromise(this, false);
    private final CloseFuture closeFuture = new CloseFuture(this);

    private volatile SocketAddress localAddress;
    private volatile SocketAddress remoteAddress;
    private volatile EventLoop eventLoop;
    private volatile boolean registered;
    private boolean closeInitiated;

    /** Cache for the string representation of this channel */
    private boolean strValActive;
    private String strVal;

• FLUSH0_CLOSED_CHANNEL_EXCEPTION：当Channel已关闭时调用AbstractUnsafe的flush0()，设置此异常；
• ENSURE_OPEN_CLOSED_CHANNEL_EXCEPTION ：当Channel已关闭时调用AbstractUnsafe的ensureOpen()，设置此异常；
• CLOSE_CLOSED_CHANNEL_EXCEPTION ：当Channel已关闭时调用AbstractUnsafe的close()，设置此异常；
• WRITE_CLOSED_CHANNEL_EXCEPTION：当Channel已关闭时调用AbstractUnsafe的write()，设置此异常；
• FLUSH0_NOT_YET_CONNECTED_EXCEPTION ：当Channel未连接时调用AbstractUnsafe的flush0()，设置此异常；
• parent：父Channel；
• id：Channel对应的全局唯一ID；
• unsafe：Unsafe实例；
• pipeline：当前Channel对应的DefaultChannelPipeline；
• unsafeVoidPromise ：异常通知，默认不使用。当我们不需要异常在pipeline中传播时，无需设置；
• closeFuture：Channel关闭通知；
• localAddress：本地地址；
• remoteAddress：远程地址；
• eventLoop：当前Channel注册的EventLoop；
• registered：是否已经注册到EventLoop；
• closeInitiated：关闭时的基础参数是否已设置；
• strValActive：是否已经缓存Channel变成active之后的toString()值；
• strVal：Channel的toString()值的缓存；

3.2、构造函数

Channel构造函数如下：

protected AbstractChannel(Channel parent) {
        this.parent = parent;
        id = newId();
        unsafe = newUnsafe();
        pipeline = newChannelPipeline();
}

• id：通过newId()接口实现，默认实现方式为DefaultChannelId的newInstance()实现，DefaultChannelId的ID生成规则为：机器ID+处理器ID+序列号+时间戳+随机码；newId()为protected类型接口，可由子类重载；
• unsafe：通过newUnsafe()接口实现，此方法为抽象方法，具体实现由子类实现；
• pipeline：通过newChannelPipeline()接口实现，默认实现类为DefaultChannelPipeline，其设定了pipeline的头处理器（head）和尾处理器（tail）等基本信息；newChannelPipeline()接口未protected类型，可由子类重载；

3.3、核心方法

AbstractChannel的接口实现都比较简单，其具体实现基本都交由pipeline和unsafe进行处理。AbstractUnsafe是AbstractChannel的对应Unsafe接口实现。此处对AbstractUnsafe的核心接口实现进行分析。

3.3.1、register事件框架

register()实现源码：

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()) {
                register0(promise);
            } else {
                try {
                    eventLoop.execute(new Runnable() {
                        @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);
                }
            }
        }

此处对一些基本信息进行检查，如是否已经注册，EventLoop是否兼容等；同时判断当前线程是否与EventLoop在同一线程中，如果是则进行注册（register0()），否则将以任务方式将注册事件放入EventLoop的执行队列中，以防止多线程多线程并发情况。具体注册处理交由register0()。

register0()实现源码：

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;

                // Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the
                // user may already fire events through the pipeline in the ChannelFutureListener.
                pipeline.invokeHandlerAddedIfNeeded();

                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 (isActive()) {
                    if (firstRegistration) {
                        pipeline.fireChannelActive();
                    } else if (config().isAutoRead()) {
                        // This channel was registered before and autoRead() is set. This means we need to begin read
                        // again so that we process inbound data.
                        //
                        // See https://github.com/netty/netty/issues/4805
                        beginRead();
                    }
                }
            } catch (Throwable t) {
                // Close the channel directly to avoid FD leak.
                closeForcibly();
                closeFuture.setClosed();
                safeSetFailure(promise, t);
            }
        }

注册流程如下：

• 设置不可取消表示，并确保Channel已经打开；
• 具体注册细节由子类的doRegister()实现；
• 确保用户的handler已经添加到pipeline中；
• 异步设置注册成功通知，并调用fireChannelRegistered()方法异步通知register事件；
• 对于服务端接受的客户端连接，如果首次注册，触发Channel的Active事件，如果已设置autoRead，则调用beginRead()开始读取数据。

beginRead()实现源码:

public final void beginRead() {
            assertEventLoop();

            if (!isActive()) {
                return;
            }

            try {
                doBeginRead();
            } catch (final Exception e) {
                invokeLater(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.fireExceptionCaught(e);
                    }
                });
                close(voidPromise());
            }
        }

对EventLoop及Channel是否为active进行检查，调用doBeginRead()模板方法执行具体的处理；若处理异常，则异步调用fireExceptionCaught()方法，进行异常通知。

3.3.2、bind事件框架

bind()实现源码：

public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
            assertEventLoop();

            if (!promise.setUncancellable() || !ensureOpen(promise)) {
                return;
            }

            // See: https://github.com/netty/netty/issues/576
            if (Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) &&
                localAddress instanceof InetSocketAddress &&
                !((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress() &&
                !PlatformDependent.isWindows() && !PlatformDependent.maybeSuperUser()) {
                // Warn a user about the fact that a non-root user can't receive a
                // broadcast packet on *nix if the socket is bound on non-wildcard address.
                logger.warn(
                        "A non-root user can't receive a broadcast packet if the socket " +
                        "is not bound to a wildcard address; binding to a non-wildcard " +
                        "address (" + localAddress + ") anyway as requested.");
            }

            boolean wasActive = isActive();
            try {
                doBind(localAddress);
            } catch (Throwable t) {
                safeSetFailure(promise, t);
                closeIfClosed();
                return;
            }

            if (!wasActive && isActive()) {
                invokeLater(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.fireChannelActive();
                    }
                });
            }

            safeSetSuccess(promise);
        }

处理流程：

• 对基础的EventLoop、Channel是否打开等进行检查；
• 广播检测、地址检查、平台相关检查、权限检查等；
• 调用doBind()进行实际的绑定，具体有由子类实现；
• 如果是首次Active，则异步进行fireChannelActive()通知；

3.3.3、disconnect事件框架

disconnect()实现源码:

public final void disconnect(final ChannelPromise promise) {
            assertEventLoop();

            if (!promise.setUncancellable()) {
                return;
            }

            boolean wasActive = isActive();
            try {
                doDisconnect();
            } catch (Throwable t) {
                safeSetFailure(promise, t);
                closeIfClosed();
                return;
            }

            if (wasActive && !isActive()) {
                invokeLater(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.fireChannelInactive();
                    }
                });
            }

            safeSetSuccess(promise);
            closeIfClosed(); // doDisconnect() might have closed the channel
        }

处理流程：

• EventLoop检查，设置不可取消等；
• 调用doDisconnect()进行实际的断开连接处理；
• Channel从非Inactive变为Active时，异步调用fireChannelInactive()进行Inactive通知；

3.3.4、close事件框架

close()实现源码：

private void close(final ChannelPromise promise, final Throwable cause,
                           final ClosedChannelException closeCause, final boolean notify) {
            if (!promise.setUncancellable()) {
                return;
            }

            if (closeInitiated) {
                if (closeFuture.isDone()) {
                    // Closed already.
                    safeSetSuccess(promise);
                } else if (!(promise instanceof VoidChannelPromise)) { // Only needed if no VoidChannelPromise.
                    // This means close() was called before so we just register a listener and return
                    closeFuture.addListener(new ChannelFutureListener() {
                        @Override
                        public void operationComplete(ChannelFuture future) throws Exception {
                            promise.setSuccess();
                        }
                    });
                }
                return;
            }

            closeInitiated = true;

            final boolean wasActive = isActive();
            final ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
            this.outboundBuffer = null; // Disallow adding any messages and flushes to outboundBuffer.
            Executor closeExecutor = prepareToClose();
            if (closeExecutor != null) {
                closeExecutor.execute(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            // Execute the close.
                            doClose0(promise);
                        } finally {
                            // Call invokeLater so closeAndDeregister is executed in the EventLoop again!
                            invokeLater(new Runnable() {
                                @Override
                                public void run() {
                                    if (outboundBuffer != null) {
                                        // Fail all the queued messages
                                        outboundBuffer.failFlushed(cause, notify);
                                        outboundBuffer.close(closeCause);
                                    }
                                    fireChannelInactiveAndDeregister(wasActive);
                                }
                            });
                        }
                    }
                });
            } else {
                try {
                    // Close the channel and fail the queued messages in all cases.
                    doClose0(promise);
                } finally {
                    if (outboundBuffer != null) {
                        // Fail all the queued messages.
                        outboundBuffer.failFlushed(cause, notify);
                        outboundBuffer.close(closeCause);
                    }
                }
                if (inFlush0) {
                    invokeLater(new Runnable() {
                        @Override
                        public void run() {
                            fireChannelInactiveAndDeregister(wasActive);
                        }
                    });
                } else {
                    fireChannelInactiveAndDeregister(wasActive);
                }
            }
        }

处理流程：

• 判断关闭的初始化信息是否设置，若未设置，当Channel已经关闭时，设置关闭成功通知；否则添加异步关闭完成监听器；
• 获取输出缓冲区ChannelOutboundBuffer及关闭执行线程；
• 若执行线程不为空，则在执行线程中添加任务，进行实际的关闭处理，并处理输出缓冲区的冲刷数据失败处理及fireChannelInactiveAndDeregister()事件处理；
• 若执行线程为空，则直接在本线程中执行关闭Channel的处理，并处理输出缓冲区的冲刷数据失败处理;
• 当inFlush0为true，即当前输出缓冲区正在冲刷数据，则异步进行fireChannelInactiveAndDeregister()事件通知，否则直接进行通知；

doColse0()实现源码：

private void doClose0(ChannelPromise promise) {
            try {
                doClose();
                closeFuture.setClosed();
                safeSetSuccess(promise);
            } catch (Throwable t) {
                closeFuture.setClosed();
                safeSetFailure(promise, t);
            }
        }

doClose()由具体子类实现。

3.3.5、deregister事件框架

deregister()实现源码：

private void deregister(final ChannelPromise promise, final boolean fireChannelInactive) {
            if (!promise.setUncancellable()) {
                return;
            }

            if (!registered) {
                safeSetSuccess(promise);
                return;
            }

            // As a user may call deregister() from within any method while doing processing in the ChannelPipeline,
            // we need to ensure we do the actual deregister operation later. This is needed as for example,
            // we may be in the ByteToMessageDecoder.callDecode(...) method and so still try to do processing in
            // the old EventLoop while the user already registered the Channel to a new EventLoop. Without delay,
            // the deregister operation this could lead to have a handler invoked by different EventLoop and so
            // threads.
            //
            // See:
            // https://github.com/netty/netty/issues/4435
            invokeLater(new Runnable() {
                @Override
                public void run() {
                    try {
                        doDeregister();
                    } catch (Throwable t) {
                        logger.warn("Unexpected exception occurred while deregistering a channel.", t);
                    } finally {
                        if (fireChannelInactive) {
                            pipeline.fireChannelInactive();
                        }
                        // Some transports like local and AIO does not allow the deregistration of
                        // an open channel.  Their doDeregister() calls close(). Consequently,
                        // close() calls deregister() again - no need to fire channelUnregistered, so check
                        // if it was registered.
                        if (registered) {
                            registered = false;
                            pipeline.fireChannelUnregistered();
                        }
                        safeSetSuccess(promise);
                    }
                }
            });
        }

处理流程：

• 如果还未注册，则直接设置成功通知；否则异步进行注销处理；
• 异步调用doDeregister()进行注销处理，进行inactive事件通知处理，进行unregister的事件通知处理；

3.3.6、write事件框架

write()实现源码：

public final void write(Object msg, ChannelPromise promise) {
            assertEventLoop();

            ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
            if (outboundBuffer == null) {
                // If the outboundBuffer is null we know the channel was closed and so
                // need to fail the future right away. If it is not null the handling of the rest
                // will be done in flush0()
                // See https://github.com/netty/netty/issues/2362
                safeSetFailure(promise, WRITE_CLOSED_CHANNEL_EXCEPTION);
                // release message now to prevent resource-leak
                ReferenceCountUtil.release(msg);
                return;
            }

            int size;
            try {
                msg = filterOutboundMessage(msg);
                size = pipeline.estimatorHandle().size(msg);
                if (size < 0) {
                    size = 0;
                }
            } catch (Throwable t) {
                safeSetFailure(promise, t);
                ReferenceCountUtil.release(msg);
                return;
            }

            outboundBuffer.addMessage(msg, size, promise);
        }

如果outboundBuffer为空，表示Channel正在关闭，则不进行写处理，直接设置写失败，并释放msg；filterOutboundMessage()为消息过滤器，由子类实现；outboundBuffer.addMessage()将消息添加到输出缓冲区中；

3.3.7、flush事件框架

flush()实现源码：

 public final void flush() {
            assertEventLoop();

            ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
            if (outboundBuffer == null) {
                return;
            }

            outboundBuffer.addFlush();
            flush0();
        }

对输出缓冲区器进行检查，为空则不进行处理；冲刷unflushedEntry中的数据；冲刷输出缓冲区的数据；

flush0()实现源码：

protected void flush0() {
            if (inFlush0) {
                // Avoid re-entrance
                return;
            }

            final ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
            if (outboundBuffer == null || outboundBuffer.isEmpty()) {
                return;
            }

            inFlush0 = true;

            // Mark all pending write requests as failure if the channel is inactive.
            if (!isActive()) {
                try {
                    if (isOpen()) {
                        outboundBuffer.failFlushed(FLUSH0_NOT_YET_CONNECTED_EXCEPTION, true);
                    } else {
                        // Do not trigger channelWritabilityChanged because the channel is closed already.
                        outboundBuffer.failFlushed(FLUSH0_CLOSED_CHANNEL_EXCEPTION, false);
                    }
                } finally {
                    inFlush0 = false;
                }
                return;
            }

            try {
                doWrite(outboundBuffer);
            } catch (Throwable t) {
                if (t instanceof IOException && config().isAutoClose()) {
                    /**
                     * Just call {@link #close(ChannelPromise, Throwable, boolean)} here which will take care of
                     * failing all flushed messages and also ensure the actual close of the underlying transport
                     * will happen before the promises are notified.
                     *
                     * This is needed as otherwise {@link #isActive()} , {@link #isOpen()} and {@link #isWritable()}
                     * may still return {@code true} even if the channel should be closed as result of the exception.
                     */
                    close(voidPromise(), t, FLUSH0_CLOSED_CHANNEL_EXCEPTION, false);
                } else {
                    try {
                        shutdownOutput(voidPromise(), t);
                    } catch (Throwable t2) {
                        close(voidPromise(), t2, FLUSH0_CLOSED_CHANNEL_EXCEPTION, false);
                    }
                }
            } finally {
                inFlush0 = false;
            }
        }

处理流程：

• 若正在冲刷数据，则不进行处理；
• 若输出缓冲区为空，则不进行处理；
• 设置inFlush0为true，表示正在冲刷数据；
• 若Channel为Inactive状态，则设置冲刷数据失败；
• 执行doWrite()进行实际的写数据，若写异常，设置相应的写失败；
• 最后设置inFlush0为false，表示完成数据冲刷；

4、AbstractNioChannel源码分析

AbstractNioChannel类继承图：

AbstractNioChannel类继承图.png

4.1、成员变量

private static final ClosedChannelException DO_CLOSE_CLOSED_CHANNEL_EXCEPTION = ThrowableUtil.unknownStackTrace(
            new ClosedChannelException(), AbstractNioChannel.class, "doClose()");

    private final SelectableChannel ch;
    protected final int readInterestOp;
    volatile SelectionKey selectionKey;
    boolean readPending;
    private final Runnable clearReadPendingRunnable = new Runnable() {
        @Override
        public void run() {
            clearReadPending0();
        }
    };

    /**
     * The future of the current connection attempt.  If not null, subsequent
     * connection attempts will fail.
     */
    private ChannelPromise connectPromise;
    private ScheduledFuture<?> connectTimeoutFuture;
    private SocketAddress requestedRemoteAddress;

DO_CLOSE_CLOSED_CHANNEL_EXCEPTION：调用doClose()时错误的异常；
ch：NioSocketChannel和ServerSocketChannel的公共父类，用于设置参数和进行I/O操作；
readInterestOp：Read事件，服务端OP_ACCEPT，其他OP_READ
selectionKey：Channel注册到EventLoop后返回的选择键；
readPending：底层读事件标记
clearReadPendingRunnable：清除底层读事件标记任务
connectPromise：链接的异步结果
connectTimeoutFuture：连接超时检测任务异步结果
requestedRemoteAddress：连接的远端地址

4.2、核心方法

AbstractNioChannel的方法实现很简单，其主要是通过NioUnsafe接口进行实现的，而AbstractNioUnsafe为NioUnsafe的具体实现类，其接口继承图如下：

NioUnsafe类继承图.png

故只需要分析AbstractNioUnsafe的核心方法即可。

4.2.1、connect处理

connect()实现源码：

public final void connect(
                final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
            if (!promise.setUncancellable() || !ensureOpen(promise)) {
                return;
            }

            try {
                if (connectPromise != null) {
                    // Already a connect in process.
                    throw new ConnectionPendingException();
                }

                boolean wasActive = isActive();
                if (doConnect(remoteAddress, localAddress)) {
                    fulfillConnectPromise(promise, wasActive);
                } else {
                    connectPromise = promise;
                    requestedRemoteAddress = remoteAddress;

                    // 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 {
                            if (future.isCancelled()) {
                                if (connectTimeoutFuture != null) {
                                    connectTimeoutFuture.cancel(false);
                                }
                                connectPromise = null;
                                close(voidPromise());
                            }
                        }
                    });
                }
            } catch (Throwable t) {
                promise.tryFailure(annotateConnectException(t, remoteAddress));
                closeIfClosed();
            }
        }

处理流程:

• 设置不可取消标志，确保Channel已经打开，检查连接异步监听是否已存在等；
• 调用doConnect()进行实际连接处理，此方法为抽象方法，由子类实现；
• 如果连接成功，则设置连接成功的异步通知；
• 如果设置连接超时时间，则添加超时任务，进行连接超时的异步通知；
• 添加异步连接通知的监听器，若异步结果被取消，则取消连接超时任务，清除连接的异步结果；

fulfileConnectPromise()实现源码：

private void fulfillConnectPromise(ChannelPromise promise, boolean wasActive) {
            if (promise == null) {
                // Closed via cancellation and the promise has been notified already.
                return;
            }

            // 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());
            }
        }
private void fulfillConnectPromise(ChannelPromise promise, Throwable cause) {
            if (promise == null) {
                // Closed via cancellation and the promise has been notified already.
                return;
            }

            // Use tryFailure() instead of setFailure() to avoid the race against cancel().
            promise.tryFailure(cause);
            closeIfClosed();
        }

fulfileConnectPromise()主要设置异步结果为成功，并出发Channel的Active事件。

finishConnect()实现源码：

 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);
            } 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);
                }
                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。

4.2.2、doRegister处理

doRegister()实现源码：

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.
                    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，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。如果抛出CancelledKeyException 表示当前的Channel对应的SelectionKey已经被取消，此时立即对Selector进行select操作，其原因如下：

当select( )中的任意一种被调用时，如下步骤将被执行：

（1）已取消的键的集合将会被检查。如果它是非空的，每个已取消的键的集合中的键将从另外两个集合中移除，并且相关的通道将被注销。这个步骤结束后，已取消的键的集合将是空的。

（2）已注册的键的集合中的键的interest集合将被检查。在这个步骤中的检查执行过后，对interest集合的改动不会影响剩余的检查过程。

对于Deregister事件，选择键执行cancle()操作，选择键表示JDK Channel和selctor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Cancle的选择键为无效键，调用它相关的方法会抛出CancelledKeyException。

4.2.3、doRead处理

doRead()实现源码：

protected void doBeginRead() throws Exception {
        // Channel.read() or ChannelHandlerContext.read() was called
        final SelectionKey selectionKey = this.selectionKey;
        if (!selectionKey.isValid()) {
            return;
        }

        readPending = true;

        final int interestOps = selectionKey.interestOps();
        if ((interestOps & readInterestOp) == 0) {
            selectionKey.interestOps(interestOps | readInterestOp);
        }
    }

doRead()操作只是简单将read事件添加到SelectionKey中。

5、AbstractNioByteChannel源码分析

AbstractNioByteChannel类继承图如下：

[图片上传中...(NioByteUnsafe类继承图.png-376062-1542718590051-0)]

其对应的Unsafe类继承图如下：

NioByteUnsafe类继承图.png

AbstractNioByteChannel的实现很简单，本处主要分析其对应的NioByteUnsafe类。其是 NioSocketChannel 的父类，只有一个成员变量 flushTask，负责写半包消息。

5.1、核心方法

5.1.1、read处理

read()实现源码：

public final void read() {
            final ChannelConfig config = config();
            if (shouldBreakReadReady(config)) {
                clearReadPending();
                return;
            }
            final ChannelPipeline pipeline = pipeline();
            final ByteBufAllocator allocator = config.getAllocator();
            final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();
            allocHandle.reset(config);

            ByteBuf byteBuf = null;
            boolean close = false;
            try {
                do {
                    byteBuf = allocHandle.allocate(allocator);
                    allocHandle.lastBytesRead(doReadBytes(byteBuf));
                    if (allocHandle.lastBytesRead() <= 0) {
                        // nothing was read. release the buffer.
                        byteBuf.release();
                        byteBuf = null;
                        close = allocHandle.lastBytesRead() < 0;
                        if (close) {
                            // There is nothing left to read as we received an EOF.
                            readPending = false;
                        }
                        break;
                    }

                    allocHandle.incMessagesRead(1);
                    readPending = false;
                    pipeline.fireChannelRead(byteBuf);
                    byteBuf = null;
                } while (allocHandle.continueReading());

                allocHandle.readComplete();
                pipeline.fireChannelReadComplete();

                if (close) {
                    closeOnRead(pipeline);
                }
            } catch (Throwable t) {
                handleReadException(pipeline, byteBuf, t, close, allocHandle);
            } 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()) {
                    removeReadOp();
                }
            }
        }
    }

处理流程：

• 检查输入流是否已关闭或远端已经关闭，是则清除读处理；
• 调用doReadBytes()抽象方法读取数据；
• 若数据长度小于等于0，表示无数据可读，释放缓冲区；若读取的数据长度小于0表读出错，设置关闭相关标志；
• 数据读取完成后，pipeline进行read事件通知；
• 若需要关闭Channel，则调用closeOnRead()进行关闭处理；
• 若读处理异常，则调用handleReadException()进行读异常处理；

closeOnRead()实现源码：

private void closeOnRead(ChannelPipeline pipeline) {
            if (!isInputShutdown0()) {
                if (isAllowHalfClosure(config())) {
                    shutdownInput();
                    pipeline.fireUserEventTriggered(ChannelInputShutdownEvent.INSTANCE);
                } else {
                    close(voidPromise());
                }
            } else {
                inputClosedSeenErrorOnRead = true;
                pipeline.fireUserEventTriggered(ChannelInputShutdownReadComplete.INSTANCE);
            }
        }

当Input流已未关闭，且Channel参数ALLOW_HALF_CLOSURE为True时，会触发用户事件ChannelInputShutdownEvent，否则，直接关闭该Channel；否则会触发用户事件ChannelInputShutdownReadComplete。

handleReadException()实现源码：

private void handleReadException(ChannelPipeline pipeline, ByteBuf byteBuf, Throwable cause, boolean close,
                RecvByteBufAllocator.Handle allocHandle) {
            if (byteBuf != null) {
                if (byteBuf.isReadable()) {
                    readPending = false;
                    pipeline.fireChannelRead(byteBuf);
                } else {
                    byteBuf.release();
                }
            }
            allocHandle.readComplete();
            pipeline.fireChannelReadComplete();
            pipeline.fireExceptionCaught(cause);
            if (close || cause instanceof IOException) {
                closeOnRead(pipeline);
            }
        }

当缓存区有数据时，会触发read事件，触发读完成时间，触发异常时间；

5.1.2、write处理

doWrite()实现源码：

protected void doWrite(ChannelOutboundBuffer in) throws Exception {
  int writeSpinCount = config().getWriteSpinCount();
  do {
      Object msg = in.current();
      //无数据需要写，则清除SelectionKey的写标志
      if (msg == null) {
        // Wrote all messages.
        clearOpWrite();
        // Directly return here so incompleteWrite(...) is not called.
         return;
    }
      //调用doWriteInternal做实际的写处理
      writeSpinCount -= doWriteInternal(in, msg);
  } while (writeSpinCount > 0);
      incompleteWrite(writeSpinCount < 0);
}

此处处理比较简单，如果无数据可写，则清除SelectionKeyd的写事件；否则调用doWriteInternal()进行数据的写入；之后调用incompleteWrite()对是否写完成进行处理。

doWriteInternal()实现源码：

private int doWriteInternal(ChannelOutboundBuffer in, Object msg) throws Exception {
  //msg类型为ByteBuf类型？
  if (msg instanceof ByteBuf) {
    ByteBuf buf = (ByteBuf) msg;
    if (!buf.isReadable()) {
      in.remove();
      return 0;
    }

    final int localFlushedAmount = doWriteBytes(buf);
    if (localFlushedAmount > 0) {
      in.progress(localFlushedAmount);
      if (!buf.isReadable()) {
      in.remove();
    }
      return 1;
    }

  //msg类型为FileRegion类型？表示写文件数据
  } else if (msg instanceof FileRegion) {
    FileRegion region = (FileRegion) msg;
    if (region.transferred() >= region.count()) {
    in.remove();
    return 0;
  }
    long localFlushedAmount = doWriteFileRegion(region);
    if (localFlushedAmount > 0) {
      in.progress(localFlushedAmount);
      if (region.transferred() >= region.count()) {
      in.remove();
    }
    return 1;
    }
  } else {
    // Should not reach here.
    throw new Error();
  }
  return WRITE_STATUS_SNDBUF_FULL;
}

此处主要对两种缓冲区进行了区分，ByteBuf和FileRegion；如果为ByteBuf，则最终调用doWriteBytes()进行数据的写入；如果为fileRegion则最终调用doWriteFileRegion()对文件进行写入。两个写入方法都为抽象方法，由子类实现。

incompleteWrite()实现源码：

protected final void incompleteWrite(boolean setOpWrite) {
  // Did not write completely.
  // setOpWrite 为 true表示数据未写完，设置 SelectionKey 写标志位
  if (setOpWrite) {
    setOpWrite();
  } else {
  // It is possible that we have set the write OP, woken up by NIO because the socket is writable, and then
  // use our write quantum. In this case we no longer want to set the write OP because the socket is still
  // writable (as far as we know). We will find out next time we attempt to write if the socket is writable
  // and set the write OP if necessary.
  // 清除写标志
    clearOpWrite();
  // Schedule flush again later so other tasks can be picked up in the meantime
  // 启动清除flushTask任务，继续写半包消息
    eventLoop().execute(flushTask);
  }
}

6、NioSocketChannel源码分析

NioSocketChannel实现源码:

NioSocketChannel类继承图.png

NioSocketChannel为Netty的NIO底层客户端的具体实现类，一些具体的底层处理在此处实现。

6.1、核心方法

6.1.1、doBind操作

doBind()实现源码：

    @Override
    protected void doBind(SocketAddress localAddress) throws Exception {
        doBind0(localAddress);
    }

    private void doBind0(SocketAddress localAddress) throws Exception {
        if (PlatformDependent.javaVersion() >= 7) {
            SocketUtils.bind(javaChannel(), localAddress);
        } else {
            SocketUtils.bind(javaChannel().socket(), localAddress);
        }
    }

此处调用底层JDK进行Channel的绑定操作。

6.1.2、doConnect操作

doConnect()实现源码：

protected boolean doConnect(SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
        if (localAddress != null) {
            doBind0(localAddress);
        }

        boolean success = false;
        try {
            boolean connected = SocketUtils.connect(javaChannel(), remoteAddress);
            if (!connected) {
                selectionKey().interestOps(SelectionKey.OP_CONNECT);
            }
            success = true;
            return connected;
        } finally {
            if (!success) {
                doClose();
            }
        }
    }

    @Override
    protected void doFinishConnect() throws Exception {
        if (!javaChannel().finishConnect()) {
            throw new Error();
        }
    }

JDK中的Channel在非阻塞模式下调用connect()方法时，会立即返回结果：成功建立连接返回True，操作还在进行时返回False。返回False时，需要在底层OP_CONNECT事件就绪时，调用finishConnect()方法完成连接操作。

6.1.3、doWrite操作

protected void doWrite(ChannelOutboundBuffer in) throws Exception {
        SocketChannel ch = javaChannel();
        int writeSpinCount = config().getWriteSpinCount();
        do {
            if (in.isEmpty()) {
                // All written so clear OP_WRITE
                clearOpWrite();
                // Directly return here so incompleteWrite(...) is not called.
                return;
            }

            // Ensure the pending writes are made of ByteBufs only.
            int maxBytesPerGatheringWrite = ((NioSocketChannelConfig) config).getMaxBytesPerGatheringWrite();
            ByteBuffer[] nioBuffers = in.nioBuffers(1024, maxBytesPerGatheringWrite);
            int nioBufferCnt = in.nioBufferCount();

            // Always us nioBuffers() to workaround data-corruption.
            // See https://github.com/netty/netty/issues/2761
            switch (nioBufferCnt) {
                case 0:
                    // We have something else beside ByteBuffers to write so fallback to normal writes.
                    writeSpinCount -= doWrite0(in);
                    break;
                case 1: {
                    // Only one ByteBuf so use non-gathering write
                    // Zero length buffers are not added to nioBuffers by ChannelOutboundBuffer, so there is no need
                    // to check if the total size of all the buffers is non-zero.
                    ByteBuffer buffer = nioBuffers[0];
                    int attemptedBytes = buffer.remaining();
                    final int localWrittenBytes = ch.write(buffer);
                    if (localWrittenBytes <= 0) {
                        incompleteWrite(true);
                        return;
                    }
                    adjustMaxBytesPerGatheringWrite(attemptedBytes, localWrittenBytes, maxBytesPerGatheringWrite);
                    in.removeBytes(localWrittenBytes);
                    --writeSpinCount;
                    break;
                }
                default: {
                    // Zero length buffers are not added to nioBuffers by ChannelOutboundBuffer, so there is no need
                    // to check if the total size of all the buffers is non-zero.
                    // We limit the max amount to int above so cast is safe
                    long attemptedBytes = in.nioBufferSize();
                    final long localWrittenBytes = ch.write(nioBuffers, 0, nioBufferCnt);
                    if (localWrittenBytes <= 0) {
                        incompleteWrite(true);
                        return;
                    }
                    // Casting to int is safe because we limit the total amount of data in the nioBuffers to int above.
                    adjustMaxBytesPerGatheringWrite((int) attemptedBytes, (int) localWrittenBytes,
                            maxBytesPerGatheringWrite);
                    in.removeBytes(localWrittenBytes);
                    --writeSpinCount;
                    break;
                }
            }
        } while (writeSpinCount > 0);

        incompleteWrite(writeSpinCount < 0);
    }

本段代码做的优化是：当输出缓冲区中有多个buffer时，采用Gathering Writes将数据从这些buffer写入到同一个channel。同时动态调整一次写的最大缓冲区大小。

7、AbstractNioMessageChannel源码分析

AbstractNioMessageChannel类继承图如下：

AbstractNioMessageChannel类继承图.png

7.1、核心方法

AbstractNioMessageChannel 是 NioServerSocketChannel、NioDatagramChannel 的父类。其主要方法也是 doWrite，功能和 AbstractNioByteChannel 的 doWrite 也类似，区别只是后者只处理 ByteBuf 和 FileRegion，前者无此限制，处理所有 Object。

7.1.1、doWrite

doWrite实现源码：

 protected void doWrite(ChannelOutboundBuffer in) throws Exception {
        final SelectionKey key = selectionKey();
        final int interestOps = key.interestOps();

        for (;;) {
            Object msg = in.current();
            if (msg == null) {
                // Wrote all messages.
              //无数据可写则清除SelectionKey的写事件标志
                if ((interestOps & SelectionKey.OP_WRITE) != 0) {
                    key.interestOps(interestOps & ~SelectionKey.OP_WRITE);
                }
                break;
            }
            try {
                boolean done = false;
                for (int i = config().getWriteSpinCount() - 1; i >= 0; i--) {
                    //doWriteMessage为抽象方法，由子类实现
                    if (doWriteMessage(msg, in)) {
                        done = true;
                        break;
                    }
                }

                if (done) {
                    in.remove();
                } else {
                    // Did not write all messages.
                    //未写完则设置SelectionKey的写标志位，等待底层可写时继续写入
                    if ((interestOps & SelectionKey.OP_WRITE) == 0) {
                        key.interestOps(interestOps | SelectionKey.OP_WRITE);
                    }
                    break;
                }
            } catch (Exception e) {
                if (continueOnWriteError()) {
                    in.remove(e);
                } else {
                    throw e;
                }
            }
        }
    }

doWriteMessage 方法在 NioServerSocketChannel 中实现如下所示，是因为 NioServerSocketChannel 只是用来监听端口，接收客户端请求，不负责传输实际数据。

NioServerSocketChannel中doWriteMessage()实现：

protected boolean doWriteMessage(Object msg, ChannelOutboundBuffer in) throws Exception {
    throw new UnsupportedOperationException();
}

7.1.2、read实现

read()实现源码：

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 {
                        //doReadMessages为抽象方法，由子类实现
                        int localRead = doReadMessages(readBuf);
                        //无数据可读？
                        if (localRead == 0) {
                            break;
                        }
                          //读取数据出错？
                        if (localRead < 0) {
                            closed = true;
                            break;
                        }
                        //增加消息数
                        allocHandle.incMessagesRead(localRead);
                    } while (allocHandle.continueReading());
                } catch (Throwable t) {
                    exception = t;
                }

                int size = readBuf.size();
                for (int i = 0; i < size; i ++) {
                    readPending = false;
                    //触发pipeline的read事件，将读取到的数据交由应用层handler进行处理
                    pipeline.fireChannelRead(readBuf.get(i));
                }
                readBuf.clear();
                allocHandle.readComplete();
                //触发pipeline的读取完成事件
                pipeline.fireChannelReadComplete();

                if (exception != null) {
                    closed = closeOnReadError(exception);
                    //触发pipeline的异常事件
                    pipeline.fireExceptionCaught(exception);
                }

                if (closed) {
                    inputShutdown = true;
                    if (isOpen()) {
                        close(voidPromise());
                    }
                }
            } 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()) {
                    removeReadOp();
                }
            }
        }
    }

8、NioServerSocketChannel源码分析

NioServerSocketChannel类继承图：

NioServerSocketChannel类继承图.png

NioServerSocketChannel 是服务端 Channel 的实现类，有一个用于配置 TCP 参数的 ServerSocketChannelConfig。

8.1、核心方法

NioServerSocketChannel为Netty的服务端NIO的实现，其只支持bind、read和close操作。

8.1.1、doBind实现

doBind()实现源码：

protected void doBind(SocketAddress localAddress) throws Exception {
        if (PlatformDependent.javaVersion() >= 7) {
            javaChannel().bind(localAddress, config.getBacklog());
        } else {
            javaChannel().socket().bind(localAddress, config.getBacklog());
        }
    }

bind()的实现都是基于底层JDK实现的。

8.1.2、doReadMessage实现

doReadMessage()实现源码：

protected int doReadMessages(List<Object> buf) throws Exception {
        SocketChannel ch = SocketUtils.accept(javaChannel());

        try {
            if (ch != null) {
                buf.add(new NioSocketChannel(this, ch));
                return 1;
            }
        } catch (Throwable t) {
            logger.warn("Failed to create a new channel from an accepted socket.", t);

            try {
                ch.close();
            } catch (Throwable t2) {
                logger.warn("Failed to close a socket.", t2);
            }
        }

        return 0;
    }

NioServerSocketChannel用于服务端的连接监听，此处的doReadMessages()方法每次最多返回一个消息（客户端连接），由此可知NioServerSocketChannel的read操作一次至多处理的连接数为config.getMaxMessagesPerRead()，也就是参数值MAX_MESSAGES_PER_READ。

8.1.3、doClose实现

doClose()实现源码：

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

调用底层JDK的Channel进行连接的关闭。

9、其他实现类

通过Channel的相关类继承图知道，Netty还有其他不同实现的Channel，除了NIO类型的，还有基于OIO的OioSocketChannel和OioServerSocketChannel，基于Epoll的EpollSocketChannel和EpollServerSocketChannel等，在此不多赘述。

相关阅读：
Netty源码愫读（一）ByteBuf相关源码学习 【https://www.jianshu.com/p/016daa404957】
Netty源码愫读（三）ChannelPipeline、ChannelHandlerContext相关源码学习【https://www.jianshu.com/p/be82d0fcdbcc】
Netty源码愫读（四）ChannelHandler相关源码学习【https://www.jianshu.com/p/6ee0a3b9d73a】
Netty源码愫读（五）EventLoop与EventLoopGroup相关源码学习【https://www.jianshu.com/p/05096995d296】
Netty源码愫读（六）ServerBootstrap相关源码学习【https://www.jianshu.com/p/a71a9a0291f3】

参考书籍：

《Netty权威指南》第二版

参考博客：

https://www.jianshu.com/p/9258af254e1d
https://blog.csdn.net/vonzhoufz/article/details/39159193
https://hk.saowen.com/a/6cb46b200931fa05dbb222736de625a1d31e664e336a139f1f4794ff83d038ad
https://blog.csdn.net/lz710117239/article/details/77651209
https://juejin.im/post/5bda9ed6f265da3913474110