背景

本文介绍Netty的通道组件NioServerSocketChannel和NioSocketChannel，从源码的角度介绍其实现原理。

1.NioServerSocketChannel

Netty本质是对NIO的封装和增强，因此Netty框架中必然包含了对于ServerSocketChannel的构建、配置以及向选择器注册，如下所示:

// 创建ServerSocketChannel对象
ServerSocketChannel serverSocketChannel = SelectorProvider.provider().openServerSocketChannel();// ServerSocketChannel通道设置为非阻塞
serverSocketChannel.configureBlocking(false);// 将ServerSocketChannel通道注册至选择器
serverSocketChannel.register(Selector, opts, attachment);// 接收客户端连接得到SocketChannel通道
SocketChannel socketChannel = serverSocketChannel.accept();

其中的构建和配置过程发生在NioServerSocketChannel的实例化过程。

1.1 NioServerSocketChannel构造函数

NioServerSocketChannel实例化过程包含了对serverSocketChannel的创建以及配置

Netty启动时，通过反射调用NioServerSocketChannel的无参构造函数创建NioServerSocketChannel对象.

private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();public NioServerSocketChannel() {this(newSocket(DEFAULT_SELECTOR_PROVIDER));
}public NioServerSocketChannel(ServerSocketChannel channel) {super(null, channel, SelectionKey.OP_ACCEPT);config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}

DEFAULT_SELECTOR_PROVIDER是Provider对象，用于创建通道和选择器，newSocket方法返回一个ServerSocketChannel对象，如下所示:

private static ServerSocketChannel newSocket(SelectorProvider provider) {try {return provider.openServerSocketChannel();} catch (IOException e) {throw new ChannelException("Failed to open a server socket.", e);}
}

NioServerSocketChannel中还维护了一个config对象用于储存该通道相关的配置，后续通过通道对象的config()方法获取该config对象。
继续调用父类的构造方法:

protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {super(parent);this.ch = ch;this.readInterestOp = readInterestOp;try {ch.configureBlocking(false);} catch (IOException e) {try {ch.close();} catch (IOException e2) {logger.warn("Failed to close a partially initialized socket.", e2);}throw new ChannelException("Failed to enter non-blocking mode.", e);}
}// super(parent)内容如下:
protected AbstractChannel(Channel parent) {this.parent = parent;id = newId();unsafe = newUnsafe();pipeline = newChannelPipeline();
}

因此NioServerSocketChannel中包含如下属性:
[1] SelectableChannel ch：实际为ServerSocketChannel类型，即NIO中的服务端通道类型，并将其配置为非阻塞类型，以便后续向选择器注册；
[2] int readInterestOp: 值固定为SelectionKey.OP_ACCEPT，表示仅处理连接事件;
[3] pipeline: Netty的Pipeline组件，每个channel都有一个属于自己的Pipeline对象;
[4] unsafe: 对底层IO进行了封装，实际的读写操作在该类中进行处理;
[5] 其他: id唯一ID标识，parent固定为空。

1.2 NioServerSocketChannel注册

NioServerSocketChannel包含了ServerSocketChannel对象，向选择器注册NioServerSocketChannel本质是将ServerSocketChannel注册到选择器

在Netty启动流程流程中，依次构造ServerSocketChannel, 并注册到选择器上，具体逻辑为:

// NioServerSocketChannel的父类AbstractNioChannel中
// 删除try-catch异常逻辑
protected void doRegister() throws Exception {boolean selected = false;for (;;) {selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);return;}
}

其中: javaChannel()获取NioServerSocketChannel对象的ServerSocketChannel属性；eventLoop().unwrappedSelector()为NioEventLoop这个线程绑定的选择器；此处的this表明将ServerSocketChannel注册到选择器上时，将当前的NioServerSocketChannel对象作为attachment保存到SelectionKey中，并使用volatile SelectionKey selectionKey;属性保存了注册结果。

说明：后续选择器会执行select而阻塞，当该选择器被IO事件唤醒时，可通过SelectionKey的attachment获取NioServerSocketChannel对象，从而可以获取包括ServerSocketChannel、Pipeline、Config等其他所有相关信息。

1.3 NioServerSocketChannel处理连接

章节1.1中提到了NioServerSocketChannel的unsafe属性，unsafe用于封装底层具体的IO行为，具体的实现类为NioMessageUnsafe.

当有连接请求到达NioServerSocketChannel后，进入NioMessageUnsafe的read()方法中(详细的调用流程和线程处理关系在后续Netty的消息处理流程中介绍, 这里仅对read方法实现逻辑进行说明)，read方法省去内存分配优化策略以及异常处理逻辑后的主线逻辑如下:

private final class NioMessageUnsafe extends AbstractNioUnsafe {private final List<Object> readBuf = new ArrayList<Object>();@Overridepublic void read() {// ...final ChannelPipeline pipeline = pipeline();do {// ...doReadMessages(readBuf);} while (allocHandle.continueReading());int size = readBuf.size();for (int i = 0; i < size; i ++) {readPending = false;pipeline.fireChannelRead(readBuf.get(i));}readBuf.clear();pipeline.fireChannelReadComplete();}
}

readBuf是一个列表类型，用于存放解析后的消息对象，解析完成后，依次遍历readBuf，并调用pipeline.fireChannelRead将消息对象发送至Netty的Pipeline组件(后面单独介绍)。

解析逻辑在doReadMessages方法中:

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;
}// SocketUtils.accept(javaChannel())代码逻辑:
public static SocketChannel accept(final ServerSocketChannel serverSocketChannel) throws IOException {// 删除try-catch异常逻辑return AccessController.doPrivileged(new PrivilegedExceptionAction<SocketChannel>() {@Overridepublic SocketChannel run() throws IOException {return serverSocketChannel.accept();}});
}

javaChannel()得到ServerSocketChannel对象，serverSocketChannel.accept()得到客户端通道对象SocketChannel。将当前服务端通道NioServerSocketChannel对象和得到的客户端通道对象SocketChannel作为参数构造NioSocketChannel对象。

2.NioSocketChannel

与NioServerSocketChannel相似，NioSocketChannel也是Netty对NIO中ServerSocketChannel的封装和增强。本章节内容将包含SocketChannel的构建、配置、向选择器注册以及读取数据，如下所示:

// 得到SocketChannel对象
SocketChannel socketChannel = serverSocketChannel.accept();// SocketChannel通道设置为非阻塞
socketChannel.configureBlocking(false);// 将SocketChannel通道注册至选择器
socketChannel.register(Selector, opts, attachment);// 从SocketChannel通道读取数据值缓冲区
socketChannel.read(ByteBuffer)

2.1 NioSocketChannel构造函数

每个客户端连接对应一个通道，即一个NioSocketChannel对象。

Netty收到客户端连接时，会调用NioSocketChannel构造函数创建通道对象，如下所示:

public NioSocketChannel(Channel parent, SocketChannel socket) {super(parent, socket);config = new NioSocketChannelConfig(this, socket.socket());
}

parent为NioServerSocketChannel对象，socket为NIO中SocketChannel对象。NioSocketChannel与NioServerSocketChannel相似，维持了一个config配置类用于存放和读取通道的配置信息。
继续沿着super调用父类的构造方法:

protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {super(parent, ch, SelectionKey.OP_READ);
}protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {super(parent);this.ch = ch;this.readInterestOp = readInterestOp;try {ch.configureBlocking(false);} catch (IOException e) {try {ch.close();} catch (IOException e2) {logger.warn("Failed to close a partially initialized socket.", e2);}throw new ChannelException("Failed to enter non-blocking mode.", e);}
}protected AbstractChannel(Channel parent) {this.parent = parent;id = newId();unsafe = newUnsafe();pipeline = newChannelPipeline();
}

上述构造过程逻辑较为简单，为NioSocketChannel创建一个Unsafe对象和Pipeline对象；以及将ch属性即SocketChannel设置为非阻塞。

2.2 注册选择器

NioServerSocketChannel接收客户端连接构造出NioSocketChannel对象，并通过Pipeline.fireChannelRead触发Inbound读事件后，通过Pipiline进入ServerBootstrapAcceptor处理器的channelRead方法:

public void channelRead(ChannelHandlerContext ctx, Object msg) {final Channel child = (Channel) msg;// ...childGroup.register(child).addListener(new ChannelFutureListener() {//...});
}

由章节1可知msg消息为NioSocketChannel，childGroup为线程池NioEventLoopGroup对象(workgroup)。
childGroup.register(child)表示将NioSocketChannel注册到workgroup的一个线程中，经过Unsafe对象最终会进入NioSocketChannel的doRegister方法:

@Override
protected void doRegister() throws Exception {// ...selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);// ...
}

javaChannel()为NioSocketChannel的ch属性，即SocketChannel通道对象；eventLoop().unwrappedSelector()为选择器；this为NioSocketChannel对象本身；返回的SelectionKey也作为属性保存在NioSocketChannel类中。
说明：后续选择器会执行select而阻塞，当有可读消息到达时被唤醒。可通过SelectionKey得到NioSocketChannel对象，从而得到相关的SocketChannel、Pipeline、Config等其他所有相关信息。

2.3 读取消息

当有可读时间到达时，NioEvetLoop会从阻塞中被唤醒，从而执行processSelectedKeys处理IO事件：

private void processSelectedKeys() {// ...processSelectedKeysOptimized();// ...
}private void processSelectedKeysOptimized() {for (int i = 0; i < selectedKeys.size; ++i) {final SelectionKey k = selectedKeys.keys[i];selectedKeys.keys[i] = null;final Object a = k.attachment();processSelectedKey(k, (AbstractNioChannel) a);}
}

遍历已就绪的IO事件，调用processSelectedKey方法处理，此时k为NIO的SelectionKey对象，而attachment为NioSocketChannel对象。

private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();int readyOps = k.readyOps();//...if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {unsafe.read();}// ...
}

根据SelectionKey和NioSocketChannel对象的readyOps确定此时IO事件为可读消息，进入unsafe.read()：

@Override
public final void read() {final ChannelConfig config = config();final ChannelPipeline pipeline = pipeline();final ByteBufAllocator allocator = config.getAllocator();ByteBuf byteBuf = null;boolean close = false;// ...do {// ...// 1.分配ButeBuf缓冲对象byteBuf = allocHandle.allocate(allocator);// 2.将数据读取到ButeBuf缓冲对象allocHandle.lastBytesRead(doReadBytes(byteBuf));if (allocHandle.lastBytesRead() <= 0) {byteBuf.release();byteBuf = null;break;}readPending = false;// 3.向Pipeline传递可读消息pipeline.fireChannelRead(byteBuf);byteBuf = null;// 直到读取完所有消息内容} while (allocHandle.continueReading());// ...// 触发消息读取完成事件pipeline.fireChannelReadComplete();// ...
}

代码较为清晰，重点包含3个步骤：创建ByteBuf缓冲对象(Netty自定义的，而非NIO的ByteBuffer); 将消息读取到ButeBuf对象，向Pipeline触发可读事件(在Pipeline的Handler中传递并处理消息)；其中，核心逻辑在于doReadBytes(byteBuf):

@Override
protected int doReadBytes(ByteBuf byteBuf) throws Exception {// ...return byteBuf.writeBytes(javaChannel(), allocHandle.attemptedBytesRead());
}

javaChannel()是NIO的SocketChannel对象，继续跟进ByteBuf的writeBytes方法进入:

@Override
public int writeBytes(ScatteringByteChannel in, int length) throws IOException {//...int writtenBytes = setBytes(writerIndex, in, length);//...return writtenBytes;
}@Override
public final int setBytes(int index, ScatteringByteChannel in, int length) throws IOException {try {return in.read(internalNioBuffer(index, length));} catch (ClosedChannelException ignored) {return -1;}
}

可以看到底层逻辑在于in.read(internalNioBuffer(index, length)), 返回一个ByteBuffer对象，in此时为SocketChannel, 即本质是调用NIO通道的API将数据读取至缓冲区: SocketChannel.read(ByteBuffer).

2.3 响应消息

Netty中Pipeline的任何一个Handler中都可以发送响应消息，响应消息也会沿着Pipeline的流水线传递，并经过网卡传递出去:

@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {ctx.writeAndFlush("hello");
}

注意：需要在此Handler前添加StringEncoder编码器，将String类型转为ByteBuf类型，否则会抛出异常。因为NioSocketChannel的Unsafe对象也维持在了Pipeline的HeadContext对象中，所有的消息最终会经过Unsafe的write方法，而Unsafe只会处理ByteBuf类型消息，其他类型会抛出异常。

追踪ctx.writeAndFlush("hello")进入invokeWriteAndFlush方法:

void invokeWriteAndFlush(Object msg, ChannelPromise promise) {// ...invokeWrite0(msg, promise);invokeFlush0();// ...
}

依次调用invokeWrite0和invokeFlush0实现写操作和刷盘操作, 分别进入Unsafe对象的write和flush方法:

public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) {unsafe.write(msg, promise);
}public void flush(ChannelHandlerContext ctx) {unsafe.flush();
}

unsafe最终调用doWrite方法实现IO功能:

protected void doWrite(ChannelOutboundBuffer in) throws Exception {SocketChannel ch = javaChannel();int writeSpinCount = config().getWriteSpinCount();do {// ...			ByteBuffer buffer = nioBuffers[0];int attemptedBytes = buffer.remaining();final int localWrittenBytes = ch.write(buffer);--writeSpinCount;// ...					} while (writeSpinCount > 0);incompleteWrite(writeSpinCount < 0);
}

核心逻辑在与ch.write(buffer)，其中ch和buffer分别是NIO的SocketChannel和ByteBuffer,
即Netty向客户端发送消息底层仍是借助NIO的API.