本章分析Nio Channel的数据读取功能的实现。
Channel读取数据需要Channel和ChannelHandler配合使用，netty设计数据读取功能包括三个要素：Channel, EventLoop和ChannelHandler。Channel有个read方法，这个方法不会直接读取数据，它的作用是通知持有当前channel的eventLoop可以从这个这个channel读取数据了，这个方法被调用之后eventLoop会在channel有数据可读的时候从channel读出数据然后把数据放在channelRead事件中交给ChannelInboundHandler的channelRead方法处理，当eventLoop发现channel中暂时没时间可读会触发一个channelReadComplete事件。
read: Nio Channel通知eventLoop开始读数据
channel read方法的调用栈:
1 io.netty.channel.AbstractChannel#read2 io.netty.channel.DefaultChannelPipeline#read3 io.netty.channel.AbstractChannelHandlerContext#read4 io.netty.channel.AbstractChannelHandlerContext#invokeRead5 io.netty.channel.DefaultChannelPipeline.HeadContext#read6 io.netty.channel.AbstractChannel.AbstractUnsafe#beginRead7 io.netty.channel.nio.AbstractNioChannel#doBeginRead
调用channel的read的方法，会触发read事件，通过pipeline调用AbstractChannel unsafe的beginRead方法，这个方法的语义是通知eventLoop可以从channel读数据了，但他没有实现具体功能，把具体功能留给doBeginRead实现。doBeginRead在AbstractChannel中定义，它是一个抽象方法。AbstractNioChannel实现了这个方法:
1 @Override 2 protected void doBeginRead() throws Exception { 3 ????// Channel.read() or ChannelHandlerContext.read() was called 4 ????if (inputShutdown) { 5 ????????return; 6 ????} 7 ?8 ????final SelectionKey selectionKey = this.selectionKey; 9 ????if (!selectionKey.isValid()) {10 ????????return;11 ????}12 13 ????readPending = true;14 15 ????final int interestOps = selectionKey.interestOps();16 ????if ((interestOps & readInterestOp) == 0) {17 ????????selectionKey.interestOps(interestOps | readInterestOp);18 ????}19 }这里的doBeginRead实现，只有第17行是核心代码：把readInterestOps保存是的read操作标志添加到SelectableChannel的SelectionKey中。这里的readInterestOps是一个类的属性，在AbstractNioChannel中，它没有明确的定义，只有一个抽象的定义:NIO中的一个可以可以当成read操作的的标志。在NIO中可以当成read的有SelectionKey.OP_READ和SelectionKey.OP_ACCEPT。readInterestOps在AbstractNioChannel的构造方法中使用传入的参数初始化，子类就可以根据需要确定interestOps的具体含义。
设置好beginRead之后，NioEventLoop就可以使用Selector得到检测到channel上的read事件了，下面是NioEventLoop中处理read事件的代码:
1 //io.netty.channel.nio.NioEventLoop#processSelectedKey(java.nio.channels.SelectionKey, io.netty.channel.nio.AbstractNioChannel)2 if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {3 ????unsafe.read();4 }这里调用了unsafe的read的方法，在Channel的Unsafe中并没有定义这个方法，它在io.netty.channel.nio.AbstractNioChannel.NioUnsafe中定义，在io.netty.channel.nio.AbstractNioMessageChannel.NioMessageUnsafe和io.netty.channel.nio.AbstractNioByteChannel.NioByteUnsafe中有两个不同的实现。这两个实现的区别是：NioMessageUnsafe.read是把从channel中读出的数据转换成Object, NioByteUnsafe.read是从channel中读出byte数据流。下面来详解分析这两种实现。
AbstractNioChannel.NioUnsafe.read实现：从channel读取数据
netty在NIO Channel的设计上，把读数据设计成独立的抽象层。之所以这样设计有两个方面的原因:
- 在NIO中，三中不同类型的Channel读取的数据类型是不一样的，NioServerSocketChannel读出的是一个新建的NioSockeChannel, NioSocketChannel读出的byte数据流，NioDatagramChannel读出是数据报。
- NIO三种Channel都运行在非阻塞模式下，相比于阻塞模式，非阻塞模式下读数据要处理的问题要复杂的多。使用Selector和非阻塞模式被动地读取数据，需要处理连接断开和socket异常，由于Selector使用的是边缘触发模式，一次read调用务必要把已经在socket recvbuffer中的数据全部读出来，否则可以导致数据丢失或数据接收不及时。把read独立出来处理读取数据的复杂性，代码结构会比较清晰。
接下来开始详细分析NioUnsafe read方法的两种不同的实现。
io.netty.channel.nio.AbstractNioMessageChannel.NioMessageUnsafe.read实现: 从channel中读出Object
这个实现是主要功能是调用doReadMessages方法，从channel中读出Object消息，具体的类型这里没有限制，doReadMessages是一个抽象方法，留给子类实现, 下面是read方法的实现:
1 //io.netty.channel.nio.AbstractNioMessageChannel.NioMessageUnsafe 2 @Override 3 public void read() { 4 ????assert eventLoop().inEventLoop(); 5 ????final ChannelConfig config = config(); 6 ????if (!config.isAutoRead() && !isReadPending()) { 7 ????????// ChannelConfig.setAutoRead(false) was called in the meantime 8 ????????removeReadOp(); 9 ????????return;10 ????}11 12 ????final int maxMessagesPerRead = config.getMaxMessagesPerRead();13 ????final ChannelPipeline pipeline = pipeline();14 ????boolean closed = false;15 ????Throwable exception = null;16 ????try {17 ????????try {18 ????????????for (;;) {19 ????????????????int localRead = doReadMessages(readBuf);20 ????????????????if (localRead == 0) {21 ????????????????????break;22 ????????????????}23 ????????????????if (localRead < 0) {24 ????????????????????closed = true;25 ????????????????????break;26 ????????????????}27 28 ????????????????// stop reading and remove op29 ????????????????if (!config.isAutoRead()) {30 ????????????????????break;31 ????????????????}32 33 ????????????????if (readBuf.size() >= maxMessagesPerRead) {34 ????????????????????break;35 ????????????????}36 ????????????}37 ????????} catch (Throwable t) {38 ????????????exception = t;39 ????????}40 ????????setReadPending(false);41 ????????int size = readBuf.size();42 ????????for (int i = 0; i < size; i ++) {43 ????????????pipeline.fireChannelRead(readBuf.get(i));44 ????????}45 46 ????????readBuf.clear();47 ????????pipeline.fireChannelReadComplete();48 49 ????????if (exception != null) {50 ????????????closed = closeOnReadError(exception);51 52 ????????????pipeline.fireExceptionCaught(exception);53 ????????}54 55 ????????if (closed) {56 ????????????if (isOpen()) {57 ????????????????close(voidPromise());58 ????????????}59 ????????}60 ????} finally {61 ????????// Check if there is a readPending which was not processed yet.62 ????????// This could be for two reasons:63 ????????// * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method64 ????????// * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method65 ????????//66 ????????// See https://github.com/netty/netty/issues/225467 ????????if (!config.isAutoRead() && !isReadPending()) {68 ????????????removeReadOp();69 ????????}70 ????}71 }第12行，得到一次循环读取消息的最大数量maxMessagesPerRead，这个配置的默认值因不同的channel类型而不同，io.netty.channel.ChannelConfig提供了setMaxMessagesPerRead方法设置这个配置的值。调节这个值的大小可以影响I/O操作在eventLoop线程分配的执行时间，它的值越大，I/O操作站的时间越大。
18-36行，使用doReadMessages读取消息，并把消息放到readBuf中，readBuf是List<Object>类型。20,21行，没有可读的数据结束循环。23-25行，socket已经关闭。33，34行，readBuf中的消息数量已经超过限制，跳出循环。
41-47行，对readBuf中的每一个消息触发一次channelRead事件，然后清空readBuf, 触发channelReadComplete事件。
49-53行，处理异常。
55-59行，处理channel正常关闭。
doReadMessages方法有两个实现。一个是io.netty.channel.socket.nio.NioServerSocketChannel#doReadMessages，这个实现中读出的消息是NioSocketChannel。另一个是io.netty.channel.socket.nio.NioDatagramChannel#doReadMessages，这个实现中读出的消息时DatagramPacket。
io.netty.channel.socket.nio.NioServerSocketChannel#doReadMessages实现代码:
1 @Override 2 protected int doReadMessages(List<Object> buf) throws Exception { 3 ????SocketChannel ch = SocketUtils.accept(javaChannel()); 4 ?5 ????try { 6 ????????if (ch != null) { 7 ????????????buf.add(new NioSocketChannel(this, ch)); 8 ????????????return 1; 9 ????????}10 ????} catch (Throwable t) {11 ????????logger.warn("Failed to create a new channel from an accepted socket.", t);12 13 ????????try {14 ????????????ch.close();15 ????????} catch (Throwable t2) {16 ????????????logger.warn("Failed to close a socket.", t2);17 ????????}18 ????}19 20 ????return 0;21 }第3行, 使用accept方法得到一个新的SocketChannel。
7,8行，使用新的SocketChannel创建NioSocketChannel，并把它放到buf中。
11-20行，出现异常，关闭这个socket, 最后返回0.
io.netty.channel.socket.nio.NioDatagramChannel#doReadMessages实现代码：
1 @Override 2 protected int doReadMessages(List<Object> buf) throws Exception { 3 ????DatagramChannel ch = javaChannel(); 4 ????DatagramChannelConfig config = config(); 5 ????RecvByteBufAllocator.Handle allocHandle = this.allocHandle; 6 ????if (allocHandle == null) { 7 ????????this.allocHandle = allocHandle = config.getRecvByteBufAllocator().newHandle(); 8 ????} 9 ????ByteBuf data = allocHandle.allocate(config.getAllocator());10 ????boolean free = true;11 ????try {12 ????????ByteBuffer nioData = data.internalNioBuffer(data.writerIndex(), data.writableBytes());13 ????????int pos = nioData.position();14 ????????InetSocketAddress remoteAddress = (InetSocketAddress) ch.receive(nioData);15 ????????if (remoteAddress == null) {16 ????????????return 0;17 ????????}18 19 ????????int readBytes = nioData.position() - pos;20 ????????data.writerIndex(data.writerIndex() + readBytes);21 ????????allocHandle.record(readBytes);22 23 ????????buf.add(new DatagramPacket(data, localAddress(), remoteAddress));24 ????????free = false;25 ????????return 1;26 ????} catch (Throwable cause) {27 ????????PlatformDependent.throwException(cause);28 ????????return -1;29 ????} ?finally {30 ????????if (free) {31 ????????????data.release();32 ????????}33 ????}34 }4-12行，得到接收数据的缓冲区data。
13-21行，从socket收到一个数据包，这个数据报包含两部分: data中的二进制数据和发送端的地址remoteAddress(第14行)。然后设置data中的数据长度。
23-25行，把数据报转换成DatagramPacket类型放到buf中返回。
io.netty.channel.nio.AbstractNioByteChannel.NioByteUnsafe#read实现：从channel中读byte流
这个实现的主要功能是调用doReadBytes读取byte流。doReadBytes是一个抽象方法，留给子类实现。下面是这个read的实现。
1 @Override 2 public final void read() { 3 ????final ChannelConfig config = config(); 4 ????if (!config.isAutoRead() && !isReadPending()) { 5 ????????// ChannelConfig.setAutoRead(false) was called in the meantime 6 ????????removeReadOp(); 7 ????????return; 8 ????} 9 10 ????final ChannelPipeline pipeline = pipeline();11 ????final ByteBufAllocator allocator = config.getAllocator();12 ????final int maxMessagesPerRead = config.getMaxMessagesPerRead();13 ????RecvByteBufAllocator.Handle allocHandle = this.allocHandle;14 ????if (allocHandle == null) {15 ????????this.allocHandle = allocHandle = config.getRecvByteBufAllocator().newHandle();16 ????}17 18 ????ByteBuf byteBuf = null;19 ????int messages = 0;20 ????boolean close = false;21 ????try {22 ????????int totalReadAmount = 0;23 ????????boolean readPendingReset = false;24 ????????do {25 ????????????byteBuf = allocHandle.allocate(allocator);26 ????????????int writable = byteBuf.writableBytes();27 ????????????int localReadAmount = doReadBytes(byteBuf);28 ????????????if (localReadAmount <= 0) {29 ????????????????// not was read release the buffer30 ????????????????byteBuf.release();31 ????????????????byteBuf = null;32 ????????????????close = localReadAmount < 0;33 ????????????????if (close) {34 ????????????????????// There is nothing left to read as we received an EOF.35 ????????????????????setReadPending(false);36 ????????????????}37 ????????????????break;38 ????????????}39 ????????????if (!readPendingReset) {40 ????????????????readPendingReset = true;41 ????????????????setReadPending(false);42 ????????????}43 ????????????pipeline.fireChannelRead(byteBuf);44 ????????????byteBuf = null;45 46 ????????????if (totalReadAmount >= Integer.MAX_VALUE - localReadAmount) {47 ????????????????// Avoid overflow.48 ????????????????totalReadAmount = Integer.MAX_VALUE;49 ????????????????break;50 ????????????}51 52 ????????????totalReadAmount += localReadAmount;53 54 ????????????// stop reading55 ????????????if (!config.isAutoRead()) {56 ????????????????break;57 ????????????}58 59 ????????????if (localReadAmount < writable) {60 ????????????????// Read less than what the buffer can hold,61 ????????????????// which might mean we drained the recv buffer completely.62 ????????????????break;63 ????????????}64 ????????} while (++ messages < maxMessagesPerRead);65 66 ????????pipeline.fireChannelReadComplete();67 ????????allocHandle.record(totalReadAmount);68 69 ????????if (close) {70 ????????????closeOnRead(pipeline);71 ????????????close = false;72 ????????}73 ????} catch (Throwable t) {74 ????????handleReadException(pipeline, byteBuf, t, close);75 ????} finally {76 ????????// Check if there is a readPending which was not processed yet.77 ????????// This could be for two reasons:78 ????????// * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method79 ????????// * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method80 ????????//81 ????????// See https://github.com/netty/netty/issues/225482 ????????if (!config.isAutoRead() && !isReadPending()) {83 ????????????removeReadOp();84 ????????}85 ????}86 }10-16行，得到一个接受缓冲区的分配器和分配器的的专用handle。这两个东西的功能是高效的创建大量的接接收数据缓冲区，具体原理和实现会在后面buffer相关章节中详细分析，这里暂时略过。
24-64行，这是一个使用doReadBytes读取数据并触发channelRead事件的循环。25-27行，得到一个接受数据的缓冲区，然后从socket中读取数据。28-38行，没有数据可读了，或socket已经断开了。43行，正确收到了数据，触发channelRead事件。59-62行，读出的数据小于缓冲区的长度，表示没有socket中暂时没有数据可读了。 64行，读取次数大于上限配置，跳出。
66行，读循环完成，触发channelReadComplete事件。
69-72, 处理socket正常关闭。
74,83行，处理其他异常。
doReadBytes只有一个实现：
//io.netty.channel.socket.nio.NioSocketChannel#doWriteBytes@Overrideprotected int doWriteBytes(ByteBuf buf) throws Exception { ???final int expectedWrittenBytes = buf.readableBytes(); ???return buf.readBytes(javaChannel(), expectedWrittenBytes);}这个实现非常简单，使用ByteBuf的能力从SocketChannel中读取byte流。
netty源码解解析(4.0)-14 Channel NIO实现:读取数据
原文地址：https://www.cnblogs.com/brandonli/p/10278285.html