29.Netty源码之服务端启动:创建EventLoop&Selector流程

然而,然而2023-08-20 11:28

highlight: arduino-light

源码篇：从 Linux 出发深入剖析服务端启动流程

通过前几章课程的学习，我们已经对 Netty 的技术思想和基本原理有了初步的认识，从今天这节课开始我们将正式进入 Netty 核心源码学习的课程。希望能够通过源码解析的方式让你更加深入理解 Netty 的精髓，如 Netty 的设计思想、工程技巧等，为之后继续深入研究 Netty 打下坚实的基础。

在课程开始之前，我想分享一下关于源码学习的几点经验和建议。

第一，很多同学在开始学习源码时面临的第一个问题就是不知道从何下手，这个时候一定不能对着源码毫无意义地四处翻看。建议你可以通过 Hello World 或者 TestCase 作为源码学习的入口，然后再通过 Debug 断点的方式调试并跑通源码。

第二，阅读源码一定要有全局观。首先要把握源码的主流程，避免刚开始陷入代码细节的死胡同。

第三，源码一定要反复阅读，让自己每一次读都有不同的收获。我们可以通过画图、注释的方式帮助自己更容易理解源码的核心流程，方便后续的复习和回顾。

作为源码解析的第一节课，我们将深入分析 Netty 服务端的启动流程。启动服务的过程中我们可以了解到 Netty 各大核心组件的关系，这将是学习 Netty 源码一个非常好的切入点，让我们一起看看 Netty 的每个零件是如何运转起来的吧。

调试示例代码

位于netty-example工程下的handler工程的 io.netty.example.echo包下

java ​ package io.netty.example.echo; ​ import io.netty.bootstrap.Bootstrap; import io.netty.channel.ChannelFuture; import io.netty.channel.ChannelInitializer; import io.netty.channel.ChannelOption; import io.netty.channel.ChannelPipeline; import io.netty.channel.EventLoopGroup; import io.netty.channel.nio.NioEventLoopGroup; import io.netty.channel.socket.SocketChannel; import io.netty.channel.socket.nio.NioSocketChannel; import io.netty.handler.logging.LogLevel; import io.netty.handler.logging.LoggingHandler; import io.netty.handler.ssl.SslContext; import io.netty.handler.ssl.SslContextBuilder; import io.netty.handler.ssl.util.InsecureTrustManagerFactory; ​ public final class EchoClient { ​ static final boolean SSL = System.getProperty("ssl") != null; static final String HOST = System.getProperty("host", "127.0.0.1"); static final int PORT = Integer.parseInt(System.getProperty("port", "8007")); static final int SIZE = Integer.parseInt(System.getProperty("size", "256")); ​ public static void main(String[] args) throws Exception { // Configure SSL.git final SslContext sslCtx; if (SSL) { sslCtx = SslContextBuilder.forClient() .trustManager(InsecureTrustManagerFactory.INSTANCE).build(); } else { sslCtx = null; } ​ // Configure the client. EventLoopGroup group = new NioEventLoopGroup(); try { Bootstrap b = new Bootstrap(); b.group(group) .channel(NioSocketChannel.class) .option(ChannelOption.TCP_NODELAY, true) .handler(new ChannelInitializer<SocketChannel>() { @Override public void initChannel(SocketChannel ch) throws Exception { System.out.println("客户端bootstrap.handler()方法中指定的处理器被调用"); ChannelPipeline p = ch.pipeline(); if (sslCtx != null) { p.addLast(sslCtx.newHandler(ch.alloc(), HOST, PORT)); } // p.addLast(new LoggingHandler(LogLevel.INFO)); p.addLast(new EchoClientHandler()); } }); ​ // Start the client. ChannelFuture f = b.connect(HOST, PORT).sync(); ​ // Wait until the connection is closed. f.channel().closeFuture().sync(); } finally { // Shut down the event loop to terminate all threads. group.shutdownGracefully(); } } } ​

java ​ package io.netty.example.echo; ​ import io.netty.buffer.ByteBuf; import io.netty.buffer.Unpooled; import io.netty.channel.ChannelHandlerContext; import io.netty.channel.ChannelInboundHandlerAdapter; ​ /** * Handler implementation for the echo client. It initiates the ping-pong * traffic between the echo client and server by sending the first message to * the server. */ public class EchoClientHandler extends ChannelInboundHandlerAdapter { ​ private final ByteBuf firstMessage; ​ /** * Creates a client-side handler. */ public EchoClientHandler() { firstMessage = Unpooled.buffer(EchoClient.SIZE); for (int i = 0; i < firstMessage.capacity(); i ++) { firstMessage.writeByte((byte) i); } firstMessage.writeByte((byte) 98); } ​ @Override public void channelActive(ChannelHandlerContext ctx) { System.out.println("EchoClientHandler.channelActive"); ctx.writeAndFlush(firstMessage); } ​ @Override public void channelRead(ChannelHandlerContext ctx, Object msg) { System.out.println("客户端EchoClientHandler#channelRead被调用"); System.out.println("客户端收到数据" + msg); System.out.println("客户端发送数据" + msg); ctx.writeAndFlush(msg); } ​ @Override public void channelReadComplete(ChannelHandlerContext ctx) { ctx.flush(); } ​ @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { // Close the connection when an exception is raised. cause.printStackTrace(); ctx.close(); } } ​

java ​ package io.netty.example.echo; ​ import io.netty.bootstrap.ServerBootstrap; import io.netty.buffer.UnpooledByteBufAllocator; import io.netty.channel.ChannelFuture; import io.netty.channel.ChannelInitializer; import io.netty.channel.ChannelOption; import io.netty.channel.ChannelPipeline; import io.netty.channel.EventLoopGroup; import io.netty.channel.epoll.EpollEventLoopGroup; import io.netty.channel.nio.NioEventLoopGroup; import io.netty.channel.socket.SocketChannel; import io.netty.channel.socket.nio.NioChannelOption; import io.netty.channel.socket.nio.NioServerSocketChannel; import io.netty.handler.logging.ClientLoggingHandler; import io.netty.handler.logging.LogLevel; import io.netty.handler.logging.LoggingHandler; import io.netty.handler.logging.ServerLoggingHandler; import io.netty.handler.ssl.SslContext; import io.netty.handler.ssl.SslContextBuilder; import io.netty.handler.ssl.util.SelfSignedCertificate; ​ /** * Echoes back any received data from a client. */ public final class EchoServer { ​ static final boolean SSL = System.getProperty("ssl") != null; static final int PORT = Integer.parseInt(System.getProperty("port", "8007")); ​ public static void main(String[] args) throws Exception { // Configure SSL. final SslContext sslCtx; if (SSL) { SelfSignedCertificate ssc = new SelfSignedCertificate(); sslCtx = SslContextBuilder.forServer(ssc.certificate(), ssc.privateKey()).build(); } else { sslCtx = null; } ​ // Configure the server. EventLoopGroup bossGroup = new NioEventLoopGroup(1); EventLoopGroup workerGroup = new NioEventLoopGroup(); final EchoServerHandler serverHandler = new EchoServerHandler(); try { ServerBootstrap b = new ServerBootstrap(); b.group(bossGroup, workerGroup) .channel(NioServerSocketChannel.class) .option(ChannelOption.SO_BACKLOG, 100) .handler(new ServerLoggingHandler(LogLevel.INFO)) //两种设置keepalive风格 .childOption(ChannelOption.SO_KEEPALIVE, true) .childOption(NioChannelOption.SO_KEEPALIVE, true) ​ //切换到unpooled的方式之一 .childOption(ChannelOption.ALLOCATOR, UnpooledByteBufAllocator.DEFAULT) .childHandler(new ChannelInitializer<SocketChannel>() { @Override public void initChannel(SocketChannel ch) throws Exception { ChannelPipeline p = ch.pipeline(); p.addLast(new ClientLoggingHandler(LogLevel.INFO)); p.addLast(serverHandler); } }); ​ // Start the server. ChannelFuture f = b.bind(PORT).sync(); ​ // Wait until the server socket is closed. f.channel().closeFuture().sync(); } finally { // Shut down all event loops to terminate all threads. bossGroup.shutdownGracefully(); workerGroup.shutdownGracefully(); } } } ​

java ​ package io.netty.example.echo; ​ import io.netty.buffer.Unpooled; import io.netty.channel.ChannelHandler.Sharable; import io.netty.channel.ChannelHandlerContext; import io.netty.channel.ChannelInboundHandlerAdapter; import io.netty.util.CharsetUtil; ​ import java.util.Date; import java.util.concurrent.TimeUnit; ​ /** * Handler implementation for the echo server. */ @Sharable public class EchoServerHandler extends ChannelInboundHandlerAdapter { ​ @Override public void channelRead(final ChannelHandlerContext ctx, Object msg) { ctx.channel().eventLoop().schedule(new Runnable() { @Override public void run() { System.out.println(new Date().toString() + ":服务器收到消息"); try { Thread.sleep(2 * 1000); ctx.writeAndFlush(Unpooled.copiedBuffer("hello, 客户端~(>^ω^<)喵4", CharsetUtil.UTF_8)); System.out.println("channel code=" + ctx.channel().hashCode()); } catch (Exception ex) { System.out.println("发生异常" + ex.getMessage()); } } }, 5, TimeUnit.SECONDS); //ctx.write(msg); } ​ @Override public void channelReadComplete(ChannelHandlerContext ctx) { ctx.flush(); } ​ @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { // Close the connection when an exception is raised. cause.printStackTrace(); ctx.close(); } } ​

java ​ package io.netty.handler.logging; ​ import io.netty.buffer.ByteBuf; import io.netty.buffer.ByteBufHolder; import io.netty.channel.ChannelDuplexHandler; import io.netty.channel.ChannelHandler; import io.netty.channel.ChannelHandler.Sharable; import io.netty.channel.ChannelHandlerContext; import io.netty.channel.ChannelOutboundHandler; import io.netty.channel.ChannelPromise; import io.netty.util.internal.logging.InternalLogLevel; import io.netty.util.internal.logging.InternalLogger; import io.netty.util.internal.logging.InternalLoggerFactory; ​ import java.net.SocketAddress; ​ import static io.netty.buffer.ByteBufUtil.appendPrettyHexDump; import static io.netty.util.internal.StringUtil.NEWLINE; ​ /** * A {@link ChannelHandler} that logs all events using a logging framework. * By default, all events are logged at <tt>DEBUG</tt> level. */ @Sharable @SuppressWarnings({ "StringConcatenationInsideStringBufferAppend", "StringBufferReplaceableByString" }) public class ServerLoggingHandler extends ChannelDuplexHandler { ​ private static final LogLevel DEFAULT_LEVEL = LogLevel.DEBUG; ​ protected final InternalLogger logger; protected final InternalLogLevel internalLevel; ​ private final LogLevel level; ​ /** * Creates a new instance whose logger name is the fully qualified class * name of the instance with hex dump enabled. */ public ServerLoggingHandler() { this(DEFAULT_LEVEL); } ​ /** * Creates a new instance whose logger name is the fully qualified class * name of the instance. * * @param level the log level */ public ServerLoggingHandler(LogLevel level) { if (level == null) { throw new NullPointerException("level"); } ​ logger = InternalLoggerFactory.getInstance(getClass()); this.level = level; internalLevel = level.toInternalLevel(); } ​ /** * Creates a new instance with the specified logger name and with hex dump * enabled. * * @param clazz the class type to generate the logger for */ public ServerLoggingHandler(Class<?> clazz) { this(clazz, DEFAULT_LEVEL); } ​ /** * Creates a new instance with the specified logger name. * * @param clazz the class type to generate the logger for * @param level the log level */ public ServerLoggingHandler(Class<?> clazz, LogLevel level) { if (clazz == null) { throw new NullPointerException("clazz"); } if (level == null) { throw new NullPointerException("level"); } ​ logger = InternalLoggerFactory.getInstance(clazz); this.level = level; internalLevel = level.toInternalLevel(); } ​ /** * Creates a new instance with the specified logger name using the default log level. * * @param name the name of the class to use for the logger */ public ServerLoggingHandler(String name) { this(name, DEFAULT_LEVEL); } ​ /** * Creates a new instance with the specified logger name. * * @param name the name of the class to use for the logger * @param level the log level */ public ServerLoggingHandler(String name, LogLevel level) { if (name == null) { throw new NullPointerException("name"); } if (level == null) { throw new NullPointerException("level"); } ​ logger = InternalLoggerFactory.getInstance(name); this.level = level; internalLevel = level.toInternalLevel(); } ​ /** * Returns the {@link LogLevel} that this handler uses to log */ public LogLevel level() { return level; } ​ @Override public void channelRegistered(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "REGISTERED")); } ctx.fireChannelRegistered(); } ​ @Override public void channelUnregistered(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "UNREGISTERED")); } ctx.fireChannelUnregistered(); } ​ @Override public void channelActive(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "ACTIVE")); } ctx.fireChannelActive(); } ​ @Override public void channelInactive(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "INACTIVE")); } ctx.fireChannelInactive(); } ​ @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "EXCEPTION", cause), cause); } ctx.fireExceptionCaught(cause); } ​ @Override public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "USER_EVENT", evt)); } ctx.fireUserEventTriggered(evt); } ​ @Override public void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "BIND", localAddress)); } ctx.bind(localAddress, promise); } ​ @Override public void connect( ChannelHandlerContext ctx, SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "CONNECT", remoteAddress, localAddress)); } ctx.connect(remoteAddress, localAddress, promise); } ​ @Override public void disconnect(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "DISCONNECT")); } ctx.disconnect(promise); } ​ @Override public void close(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "CLOSE")); } ctx.close(promise); } ​ @Override public void deregister(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "DEREGISTER")); } ctx.deregister(promise); } ​ @Override public void channelReadComplete(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "READ COMPLETE")); } ctx.fireChannelReadComplete(); } ​ @Override public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { System.out.println("服务器端方法中指定的ServerLoggingHandler被调用"); if (logger.isEnabled(internalLevel)) { //这里被注释了 // logger.log(internalLevel, format(ctx, "READ", msg)); } ctx.fireChannelRead(msg); } ​ @Override public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "WRITE", msg)); } ctx.write(msg, promise); } ​ @Override public void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "WRITABILITY CHANGED")); } ctx.fireChannelWritabilityChanged(); } ​ @Override public void flush(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "FLUSH")); } ctx.flush(); } ​ /** * Formats an event and returns the formatted message. * * @param eventName the name of the event */ protected String format(ChannelHandlerContext ctx, String eventName) { String chStr = ctx.channel().toString(); return new StringBuilder(chStr.length() + 1 + eventName.length()) .append(chStr) .append(' ') .append(eventName) .toString(); } ​ /** * Formats an event and returns the formatted message. * * @param eventName the name of the event * @param arg the argument of the event */ protected String format(ChannelHandlerContext ctx, String eventName, Object arg) { if (arg instanceof ByteBuf) { return formatByteBuf(ctx, eventName, (ByteBuf) arg); } else if (arg instanceof ByteBufHolder) { return formatByteBufHolder(ctx, eventName, (ByteBufHolder) arg); } else { return formatSimple(ctx, eventName, arg); } } ​ /** * Formats an event and returns the formatted message. This method is currently only used for formatting * {@link ChannelOutboundHandler#connect(ChannelHandlerContext, SocketAddress, SocketAddress, ChannelPromise)}. * * @param eventName the name of the event * @param firstArg the first argument of the event * @param secondArg the second argument of the event */ protected String format(ChannelHandlerContext ctx, String eventName, Object firstArg, Object secondArg) { if (secondArg == null) { return formatSimple(ctx, eventName, firstArg); } ​ String chStr = ctx.channel().toString(); String arg1Str = String.valueOf(firstArg); String arg2Str = secondArg.toString(); StringBuilder buf = new StringBuilder( chStr.length() + 1 + eventName.length() + 2 + arg1Str.length() + 2 + arg2Str.length()); buf.append(chStr).append(' ').append(eventName).append(": ").append(arg1Str).append(", ").append(arg2Str); return buf.toString(); } ​ /** * Generates the default log message of the specified event whose argument is a {@link ByteBuf}. */ private static String formatByteBuf(ChannelHandlerContext ctx, String eventName, ByteBuf msg) { String chStr = ctx.channel().toString(); int length = msg.readableBytes(); if (length == 0) { StringBuilder buf = new StringBuilder(chStr.length() + 1 + eventName.length() + 4); buf.append(chStr).append(' ').append(eventName).append(": 0B"); return buf.toString(); } else { int rows = length / 16 + (length % 15 == 0? 0 : 1) + 4; StringBuilder buf = new StringBuilder(chStr.length() + 1 + eventName.length() + 2 + 10 + 1 + 2 + rows * 80); ​ buf.append(chStr).append(' ').append(eventName).append(": ").append(length).append('B').append(NEWLINE); appendPrettyHexDump(buf, msg); ​ return buf.toString(); } } ​ /** * Generates the default log message of the specified event whose argument is a {@link ByteBufHolder}. */ private static String formatByteBufHolder(ChannelHandlerContext ctx, String eventName, ByteBufHolder msg) { String chStr = ctx.channel().toString(); String msgStr = msg.toString(); ByteBuf content = msg.content(); int length = content.readableBytes(); if (length == 0) { StringBuilder buf = new StringBuilder(chStr.length() + 1 + eventName.length() + 2 + msgStr.length() + 4); buf.append(chStr).append(' ').append(eventName).append(", ").append(msgStr).append(", 0B"); return buf.toString(); } else { int rows = length / 16 + (length % 15 == 0? 0 : 1) + 4; StringBuilder buf = new StringBuilder( chStr.length() + 1 + eventName.length() + 2 + msgStr.length() + 2 + 10 + 1 + 2 + rows * 80); ​ buf.append(chStr).append(' ').append(eventName).append(": ") .append(msgStr).append(", ").append(length).append('B').append(NEWLINE); appendPrettyHexDump(buf, content); ​ return buf.toString(); } } ​ /** * Generates the default log message of the specified event whose argument is an arbitrary object. */ private static String formatSimple(ChannelHandlerContext ctx, String eventName, Object msg) { String chStr = ctx.channel().toString(); String msgStr = String.valueOf(msg); StringBuilder buf = new StringBuilder(chStr.length() + 1 + eventName.length() + 2 + msgStr.length()); return buf.append(chStr).append(' ').append(eventName).append(": ").append(msgStr).toString(); } } ​

java /* * Copyright 2012 The Netty Project * * The Netty Project licenses this file to you under the Apache License, * version 2.0 (the "License"); you may not use this file except in compliance * with the License. You may obtain a copy of the License at: * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the * License for the specific language governing permissions and limitations * under the License. */ package io.netty.handler.logging; ​ import io.netty.buffer.ByteBuf; import io.netty.buffer.ByteBufHolder; import io.netty.channel.ChannelDuplexHandler; import io.netty.channel.ChannelHandler; import io.netty.channel.ChannelHandler.Sharable; import io.netty.channel.ChannelHandlerContext; import io.netty.channel.ChannelOutboundHandler; import io.netty.channel.ChannelPromise; import io.netty.util.internal.logging.InternalLogLevel; import io.netty.util.internal.logging.InternalLogger; import io.netty.util.internal.logging.InternalLoggerFactory; ​ import java.net.SocketAddress; ​ import static io.netty.buffer.ByteBufUtil.appendPrettyHexDump; import static io.netty.util.internal.StringUtil.NEWLINE; ​ /** * A {@link ChannelHandler} that logs all events using a logging framework. * By default, all events are logged at <tt>DEBUG</tt> level. */ @Sharable @SuppressWarnings({ "StringConcatenationInsideStringBufferAppend", "StringBufferReplaceableByString" }) public class ClientLoggingHandler extends ChannelDuplexHandler { ​ private static final LogLevel DEFAULT_LEVEL = LogLevel.DEBUG; ​ protected final InternalLogger logger; protected final InternalLogLevel internalLevel; ​ private final LogLevel level; ​ /** * Creates a new instance whose logger name is the fully qualified class * name of the instance with hex dump enabled. */ public ClientLoggingHandler() { this(DEFAULT_LEVEL); } ​ /** * Creates a new instance whose logger name is the fully qualified class * name of the instance. * * @param level the log level */ public ClientLoggingHandler(LogLevel level) { if (level == null) { throw new NullPointerException("level"); } ​ logger = InternalLoggerFactory.getInstance(getClass()); this.level = level; internalLevel = level.toInternalLevel(); } ​ /** * Creates a new instance with the specified logger name and with hex dump * enabled. * * @param clazz the class type to generate the logger for */ public ClientLoggingHandler(Class<?> clazz) { this(clazz, DEFAULT_LEVEL); } ​ /** * Creates a new instance with the specified logger name. * * @param clazz the class type to generate the logger for * @param level the log level */ public ClientLoggingHandler(Class<?> clazz, LogLevel level) { if (clazz == null) { throw new NullPointerException("clazz"); } if (level == null) { throw new NullPointerException("level"); } ​ logger = InternalLoggerFactory.getInstance(clazz); this.level = level; internalLevel = level.toInternalLevel(); } ​ /** * Creates a new instance with the specified logger name using the default log level. * * @param name the name of the class to use for the logger */ public ClientLoggingHandler(String name) { this(name, DEFAULT_LEVEL); } ​ /** * Creates a new instance with the specified logger name. * * @param name the name of the class to use for the logger * @param level the log level */ public ClientLoggingHandler(String name, LogLevel level) { if (name == null) { throw new NullPointerException("name"); } if (level == null) { throw new NullPointerException("level"); } ​ logger = InternalLoggerFactory.getInstance(name); this.level = level; internalLevel = level.toInternalLevel(); } ​ /** * Returns the {@link LogLevel} that this handler uses to log */ public LogLevel level() { return level; } ​ @Override public void channelRegistered(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "REGISTERED")); } ctx.fireChannelRegistered(); } ​ @Override public void channelUnregistered(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "UNREGISTERED")); } ctx.fireChannelUnregistered(); } ​ @Override public void channelActive(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "ACTIVE")); } ctx.fireChannelActive(); } ​ @Override public void channelInactive(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "INACTIVE")); } ctx.fireChannelInactive(); } ​ @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "EXCEPTION", cause), cause); } ctx.fireExceptionCaught(cause); } ​ @Override public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "USER_EVENT", evt)); } ctx.fireUserEventTriggered(evt); } ​ @Override public void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "BIND", localAddress)); } ctx.bind(localAddress, promise); } ​ @Override public void connect( ChannelHandlerContext ctx, SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "CONNECT", remoteAddress, localAddress)); } ctx.connect(remoteAddress, localAddress, promise); } ​ @Override public void disconnect(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "DISCONNECT")); } ctx.disconnect(promise); } ​ @Override public void close(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "CLOSE")); } ctx.close(promise); } ​ @Override public void deregister(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "DEREGISTER")); } ctx.deregister(promise); } ​ @Override public void channelReadComplete(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "READ COMPLETE")); } ctx.fireChannelReadComplete(); } ​ @Override public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { System.out.println("服务器端方法中指定的ClientLoggingHandler被调用"); if (logger.isEnabled(internalLevel)) { //这里被注释了 // logger.log(internalLevel, format(ctx, "READ", msg)); } ctx.fireChannelRead(msg); } ​ @Override public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "WRITE", msg)); } ctx.write(msg, promise); } ​ @Override public void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "WRITABILITY CHANGED")); } ctx.fireChannelWritabilityChanged(); } ​ @Override public void flush(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "FLUSH")); } ctx.flush(); } ​ /** * Formats an event and returns the formatted message. * * @param eventName the name of the event */ protected String format(ChannelHandlerContext ctx, String eventName) { String chStr = ctx.channel().toString(); return new StringBuilder(chStr.length() + 1 + eventName.length()) .append(chStr) .append(' ') .append(eventName) .toString(); } ​ /** * Formats an event and returns the formatted message. * * @param eventName the name of the event * @param arg the argument of the event */ protected String format(ChannelHandlerContext ctx, String eventName, Object arg) { if (arg instanceof ByteBuf) { return formatByteBuf(ctx, eventName, (ByteBuf) arg); } else if (arg instanceof ByteBufHolder) { return formatByteBufHolder(ctx, eventName, (ByteBufHolder) arg); } else { return formatSimple(ctx, eventName, arg); } } ​ /** * Formats an event and returns the formatted message. This method is currently only used for formatting * {@link ChannelOutboundHandler#connect(ChannelHandlerContext, SocketAddress, SocketAddress, ChannelPromise)}. * * @param eventName the name of the event * @param firstArg the first argument of the event * @param secondArg the second argument of the event */ protected String format(ChannelHandlerContext ctx, String eventName, Object firstArg, Object secondArg) { if (secondArg == null) { return formatSimple(ctx, eventName, firstArg); } ​ String chStr = ctx.channel().toString(); String arg1Str = String.valueOf(firstArg); String arg2Str = secondArg.toString(); StringBuilder buf = new StringBuilder( chStr.length() + 1 + eventName.length() + 2 + arg1Str.length() + 2 + arg2Str.length()); buf.append(chStr).append(' ').append(eventName).append(": ").append(arg1Str).append(", ").append(arg2Str); return buf.toString(); } ​ /** * Generates the default log message of the specified event whose argument is a {@link ByteBuf}. */ private static String formatByteBuf(ChannelHandlerContext ctx, String eventName, ByteBuf msg) { String chStr = ctx.channel().toString(); int length = msg.readableBytes(); if (length == 0) { StringBuilder buf = new StringBuilder(chStr.length() + 1 + eventName.length() + 4); buf.append(chStr).append(' ').append(eventName).append(": 0B"); return buf.toString(); } else { int rows = length / 16 + (length % 15 == 0? 0 : 1) + 4; StringBuilder buf = new StringBuilder(chStr.length() + 1 + eventName.length() + 2 + 10 + 1 + 2 + rows * 80); ​ buf.append(chStr).append(' ').append(eventName).append(": ").append(length).append('B').append(NEWLINE); appendPrettyHexDump(buf, msg); ​ return buf.toString(); } } ​ /** * Generates the default log message of the specified event whose argument is a {@link ByteBufHolder}. */ private static String formatByteBufHolder(ChannelHandlerContext ctx, String eventName, ByteBufHolder msg) { String chStr = ctx.channel().toString(); String msgStr = msg.toString(); ByteBuf content = msg.content(); int length = content.readableBytes(); if (length == 0) { StringBuilder buf = new StringBuilder(chStr.length() + 1 + eventName.length() + 2 + msgStr.length() + 4); buf.append(chStr).append(' ').append(eventName).append(", ").append(msgStr).append(", 0B"); return buf.toString(); } else { int rows = length / 16 + (length % 15 == 0? 0 : 1) + 4; StringBuilder buf = new StringBuilder( chStr.length() + 1 + eventName.length() + 2 + msgStr.length() + 2 + 10 + 1 + 2 + rows * 80); ​ buf.append(chStr).append(' ').append(eventName).append(": ") .append(msgStr).append(", ").append(length).append('B').append(NEWLINE); appendPrettyHexDump(buf, content); ​ return buf.toString(); } } ​ /** * Generates the default log message of the specified event whose argument is an arbitrary object. */ private static String formatSimple(ChannelHandlerContext ctx, String eventName, Object msg) { String chStr = ctx.channel().toString(); String msgStr = String.valueOf(msg); StringBuilder buf = new StringBuilder(chStr.length() + 1 + eventName.length() + 2 + msgStr.length()); return buf.append(chStr).append(' ').append(eventName).append(": ").append(msgStr).toString(); } }

服务器调试示例结果

java 反射创建channelpublic io.netty.channel.socket.nio.NioServerSocketChannel() 创建pipeline 设置非阻塞模式 开始初始化channel 服务器端添加匿名处理器 do Register 注册事件 0 invokeHandlerAddedIfNeeded:添加处理器 invokeHandlerAddedIfNeeded->ChannelInitializer.initChannel(ChannelHandlerContext) 调用服务器端匿名处理器 服务器端添加serverBootstrap.handler()方法中指定的处理器 触发事件:fireChannelRegistered ServerLoggingHandler.channelRegistered 服务器端异步添加ServerBootstrapAcceptor ServerLoggingHandler.bind 触发事件:fireChannelActive ServerLoggingHandler.channelActive doBeginRead注册事件:16 ------------服务器channel初始化完成-------------- 创建pipeline 设置非阻塞模式 触发事件:fireChannelRead 服务器端方法中指定的ServerLoggingHandler被调用 服务器端ServerBootstrapAcceptor被调用 do Register 注册事件 0 invokeHandlerAddedIfNeeded:添加处理器 invokeHandlerAddedIfNeeded->ChannelInitializer.initChannel(ChannelHandlerContext) 触发事件:fireChannelRegistered 11:09:25.118 [nioEventLoopGroup-3-1] INFO i.n.h.logging.ClientLoggingHandler - [id: 0xb9e20c1b, L:/127.0.0.1:8007 - R:/127.0.0.1:56141] REGISTERED 触发事件:fireChannelActive 11:09:25.118 [nioEventLoopGroup-2-1] INFO i.n.h.logging.ServerLoggingHandler - [id: 0xd5ededb0, L:/0:0:0:0:0:0:0:0:8007] READ COMPLETE 11:09:25.127 [nioEventLoopGroup-3-1] INFO i.n.h.logging.ClientLoggingHandler - [id: 0xb9e20c1b, L:/127.0.0.1:8007 - R:/127.0.0.1:56141] ACTIVE doBeginRead注册事件:1 ------------客户端channel初始化完成-------------- 触发事件:fireChannelRead //服务器收到消息 ClientLoggingHandler#channelRead 11:09:25.187 [nioEventLoopGroup-3-1] INFO i.n.h.logging.ClientLoggingHandler - [id: 0xb9e20c1b, L:/127.0.0.1:8007 - R:/127.0.0.1:56141] READ COMPLETE ​ 11:09:25.187 [nioEventLoopGroup-3-1] INFO i.n.h.logging.ClientLoggingHandler - [id: 0xb9e20c1b, L:/127.0.0.1:8007 - R:/127.0.0.1:56141] FLUSH ​ Fri Sep 03 11:09:30 CST 2021:服务器收到消息 ​ 11:09:32.220 [nioEventLoopGroup-3-1] INFO i.n.h.logging.ClientLoggingHandler - [id: 0xb9e20c1b, L:/127.0.0.1:8007 - R:/127.0.0.1:56141] WRITE: 29B//服务器响应向客户端写消息 ​ +-------------------------------------------------+ | 0 1 2 3 4 5 6 7 8 9 a b c d e f | +--------+-------------------------------------------------+----------------+ |00000000| 68 65 6c 6c 6f 2c 20 e5 ae a2 e6 88 b7 e7 ab af |hello, .........| |00000010| 7e 28 3e 5e cf 89 5e 3c 29 e5 96 b5 34 |~(>^..^<)...4 | +--------+-------------------------------------------------+----------------+ ​ 11:09:32.235 [nioEventLoopGroup-3-1] INFO i.n.h.logging.ClientLoggingHandler - [id: 0xb9e20c1b, L:/127.0.0.1:8007 - R:/127.0.0.1:56141] FLUSH ​

客户端调试示例结果

java 反射创建channelpublic io.netty.channel.socket.nio.NioSocketChannel() 创建pipeline 设置非阻塞模式 开始初始化channel 客户端channel添加bootstrap.handler()方法指定的处理器 do Register 注册事件 0 invokeHandlerAddedIfNeeded:添加处理器 invokeHandlerAddedIfNeeded->ChannelInitializer.initChannel(ChannelHandlerContext) 客户端bootstrap.handler()方法中指定的处理器被调用 触发事件:fireChannelRegistered 完成连接 finishConnect 触发事件:fireChannelActive EchoClientHandler.channelActive//第一次向服务器发送消息 doBeginRead注册事件:1 ------------channel初始化完成-------------- 触发事件:fireChannelRead//客户端收到消息 客户端EchoClientHandler#channelRead被调用 客户端收到数据PooledUnsafeDirectByteBuf(ridx: 0, widx: 29, cap: 1024) 客户端发送数据PooledUnsafeDirectByteBuf(ridx: 0, widx: 29, cap: 1024)

addLast方法

java @Override public final ChannelPipeline addLast(ChannelHandler... handlers) { //handlers return addLast(null, handlers); }

``` @Override public final ChannelPipeline addLast(EventExecutorGroup executor, ChannelHandler... handlers) { if (handlers == null) { throw new NullPointerException("handlers"); }

for (ChannelHandler h: handlers) {
        if (h == null) {
            break;
        }
         //handlers
        addLast(executor, null, h);
    }

    return this;
}

```

```java @Override public final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) { final AbstractChannelHandlerContext newCtx; synchronized (this) { checkMultiplicity(handler);

newCtx = newContext(group, filterName(name, handler), handler);

        addLast0(newCtx);

        if (!registered) {
            newCtx.setAddPending();
            callHandlerCallbackLater(newCtx, true);
            return this;
        }

        EventExecutor executor = newCtx.executor();
        if (!executor.inEventLoop()) {
            callHandlerAddedInEventLoop(newCtx, executor);
            return this;
        }
    }
    //新添加的handler也会被执行initChannel方法
    callHandlerAdded0(newCtx);
    return this;
}

```

创建EventLoop&Selector流程

java EventLoopGroup workerGroup = new NioEventLoopGroup(); EventLoopGroup bossGroup = new NioEventLoopGroup();

```java public class NioEventLoopGroup extends MultithreadEventLoopGroup { //默认构造方法 public NioEventLoopGroup() { this(0); }

public NioEventLoopGroup(int nThreads) {
    this(nThreads, (Executor) null);
}

} ```

1.根据系统获取SelectorProvider

```java public NioEventLoopGroup(int nThreads, Executor executor) { //SelectorProvider.provider() //根据不同的系统创建不同的Selector 或者是说jdk不同 //Linux 下JOK 的下载和安装与Windows 下并没有太大的不同，只是对一些环境的设置稍有不同。 //在windows环境下的是 WindowsSelectorProvider this(nThreads, executor, SelectorProvider.provider()); }

//SelectorProvider.provider()
//1.读取配置根据配置的class获取provider 獲取不到的话分支走到第二步
//2.通过spi获取provider 获取不到到第三步
//3.DefaultSelectorProvider#create创建provider
//根据不同的系统创建不同的Selector 或者是说jdk不同
//Linux 下JOK 的下载和安装与Windows 下并没有太大的不同，只是对一些环境的设置稍有不同。
//在windows环境下的是 WindowsSelectorProvider

public static SelectorProvider provider() { synchronized (lock) { if (provider != null) return provider; return AccessController.doPrivileged( new PrivilegedAction () { public SelectorProvider run() { if (loadProviderFromProperty()) return provider; if (loadProviderAsService()) return provider;

provider = sun.nio.ch.DefaultSelectorProvider.create();
                        return provider;
                    }
                });
    }
}

// sun.nio.ch.DefaultSelectorProvider.create(); // 不同的系统根据jdk有不同的实现 public static SelectorProvider create() { return new WindowsSelectorProvider(); } ```

2.设置线程池数量,默认cpu数量*2

```java public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider) { this(nThreads, executor, selectorProvider, DefaultSelectStrategyFactory.INSTANCE); }

public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider,final SelectStrategyFactory selectStrategyFactory) { super(nThreads, executor, selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject()); }

//默认创建的线程数是 cpu核数 * 2

// DEFAULTEVENT LOOPTHREADS = Math.max(1, SystemPropertyUtil.getInt( // "io.netty.eventLoopThreads", NettyRuntime.availableProcessors() * 2)); //args是可变参 protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) { super(nThreads == 0 ? DEFAULT EVENTLOOPTHREADS : nThreads, executor, args); }

protected MultithreadEventExecutorGroup(int nThreads, Executor executor, Object... args) { this(nThreads, executor, DefaultEventExecutorChooserFactory.INSTANCE, args); } ```

3.循环创建NioEventLoop

```java

protected MultithreadEventExecutorGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory, Object... args) { if (nThreads <= 0) { throw new IllegalArgumentException (String.format("nThreads: %d (expected: > 0)", nThreads)); }

if (executor == null) {
        //一个task一个thread
        executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
    }
    //children是MultithreadEventExecutorGroup的属性
    //初始化数组 长度为线程数量
    children = new EventExecutor[nThreads];
    //遍历nThreads 循环创建EventExecutor
    //EventLoop继承自EventExecutor
    for (int i = 0; i < nThreads; i ++) {
        boolean success = false;
        try {
            //EventExecutor
            //其实是1个 NioEventLoop
            children[i] = newChild(executor, args);
            success = true;
        } catch (Exception e) {
            // TODO: Think about if this is a good exception type
            throw new IllegalStateException
                    ("failed to create a child event loop", e);
        } finally {
            if (!success) {
                for (int j = 0; j < i; j ++) {
                    children[j].shutdownGracefully();
                }

                for (int j = 0; j < i; j ++) {
                    EventExecutor e = children[j];
                    try {
                        while (!e.isTerminated()) {
                            e.awaitTermination
                                (Integer.MAX_VALUE, TimeUnit.SECONDS);
                        }
                    } catch (InterruptedException interrupted) {
                        // Let the caller handle the interruption.
                        Thread.currentThread().interrupt();
                        break;
                    }
                }
            }
        }
    }

    chooser = chooserFactory.newChooser(children);

   final FutureListener<Object> terminationListener = new FutureListener<Object>() {
        @Override
       public void operationComplete(Future<Object> future) throws Exception {
            if (terminatedChildren.incrementAndGet() == children.length) {
                terminationFuture.setSuccess(null);
            }
        }
    };

    for (EventExecutor e: children) {
        e.terminationFuture().addListener(terminationListener);
    }

    Set<EventExecutor> childrenSet = 
                    new LinkedHashSet<EventExecutor>(children.length);
    Collections.addAll(childrenSet, children);
    readonlyChildren = Collections.unmodifiableSet(childrenSet);
}

```

4.初始化NioEventLoop打开selector

```java

//返回的是EventLoop 实际是 NioEventLoop //io.netty.channel.nio.NioEventLoopGroup#newChild @Override protected EventLoop newChild(Executor executor, Object... args) throws Exception { EventLoopTaskQueueFactory queueFactory = args.length == 4 ? (EventLoopTaskQueueFactory) args[3] : null; return new NioEventLoop(this, executor, (SelectorProvider) args[0], ((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2], queueFactory); }

//构造函数 NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler, EventLoopTaskQueueFactory queueFactory) { super(parent, executor, false, newTaskQueue(queueFactory), newTaskQueue(queueFactory), rejectedExecutionHandler);

if (selectorProvider == null) {
        throw new NullPointerException("selectorProvider");
    }
    if (strategy == null) {
        throw new NullPointerException("selectStrategy");
    }
    provider = selectorProvider;
   //通过provider获取selector
   //selectorTuple是selector的包装类
   //获取到selectorTuple 
    final SelectorTuple selectorTuple = openSelector();
   //selectorTuple 获取 selector
    selector = selectorTuple.selector;
    unwrappedSelector = selectorTuple.unwrappedSelector;
    selectStrategy = strategy;
}

```

5.NioEventLoop的run方法

因为它继承自Excutor类 所以还要关注它的run方法。

在 Netty 中 EventLoop 可以理解为 Reactor 线程模型的事件处理引擎，每个 EventLoop 线程都维护一个 Selector 选择器和任务队列 taskQueue。它主要负责处理 I/O 事件、普通任务和定时任务。

Netty 中推荐使用 NioEventLoop 作为实现类，那么 Netty 是如何实现 NioEventLoop 的呢？首先我们来看 NioEventLoop 最核心的 run() 方法源码，本节课我们不会对源码做深入的分析，只是先了解 NioEventLoop 的实现结构。

首先，会根据 hasTasks() 的结果来决定是执行 selectNow() 还是 select(oldWakenUp)，这个应该好理解。如果有任务正在等待，那么应该使用无阻塞的 selectNow()，如果没有任务在等待，那么就可以使用带阻塞的 select 操作。 ioRatio 控制 IO 操作所占的时间比重： 如果设置为 100%，那么先执行 IO 操作，然后再执行任务队列中的任务。 如果不是 100%，那么先执行 IO 操作，然后执行 taskQueue 中的任务，但是需要控制执行任务的总时间。也就是说，非 IO 操作可以占用的时间，通过 ioRatio 以及这次 IO 操作耗时计算得出。 ​ 我们这里先不要去关心 select(oldWakenUp)、processSelectedKeys() 方法和 runAllTasks(...) 方法的细节，只要先理解它们分别做什么事情就可以了。

回过神来，我们前面在 register 的时候提交了 register 任务给 NioEventLoop，这是 NioEventLoop 接收到的第一个任务，所以这里会实例化 Thread 并且启动，然后进入到 NioEventLoop 中的 run 方法。 当然了，实际情况可能是，Channel 实例被 register 到一个已经启动线程的 NioEventLoop 实例中。

io.netty.channel.nio.NioEventLoop#run

```java @Override //死循环监听、处理事件 protected void run() { for (;;) { try { try { //hasTasks()判断是否有任务 tailTasks 和 taskQueue 是否为空 //如果有任务 返回的是io事件个数 那么直接进入default 什么也不做跳出switch //如果没有任务 返回的是SelectStrategy.SELECT //队列中有任务则调用selectNow返回当前已就绪IO事件的数量，否则继续select switch (selectStrategy.calculateStrategy (selectNowSupplier, hasTasks())) { case SelectStrategy.CONTINUE: continue;

case SelectStrategy.BUSY_WAIT:

                case SelectStrategy.SELECT:
                    // 轮询 I/O 事件
                    select(wakenUp.getAndSet(false));

                    if (wakenUp.get()) {
                        selector.wakeup();
                    }
                    // fall through
                default:
                }
            } catch (IOException e) {
                rebuildSelector0();
                handleLoopException(e);
                continue;
            }

            cancelledKeys = 0;
            needsToSelectAgain = false;
            //初始ioRatio是50
            final int ioRatio = this.ioRatio;
            //如果ioRatio是100 先处理就绪的io
            if (ioRatio == 100) {
                try {
                    //selector选择事件
                    //判断事件类型
                    //处理io事件
                    processSelectedKeys();
                } finally {
                    //确保我们总是可以执行任务
                    //执行任务
                    runAllTasks();
                }
            } else {
                final long ioStartTime = System.nanoTime();
                try {
                    // 处理 I/O 事件
                    processSelectedKeys();
                } finally {
                     //确保我们总是可以执行任务
                    //但是这次执行任务是带有超时时间的
                    final long ioTime = System.nanoTime() - ioStartTime;
                    runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                }
            }
        } catch (Throwable t) {
            handleLoopException(t);
        }
        // Always handle shutdown even if the loop processing threw an exception.
        try {
            if (isShuttingDown()) {
                closeAll();
                if (confirmShutdown()) {
                    return;
                }
            }
        } catch (Throwable t) {
            handleLoopException(t);
        }
    }
}

```

上述源码的结构比较清晰，NioEventLoop 每次循环的处理流程都包含事件轮询 select、事件处理 processSelectedKeys、任务处理 runAllTasks 几个步骤，是典型的 Reactor 线程模型的运行机制。而且 Netty 提供了一个参数 ioRatio，可以调整 I/O 事件处理和任务处理的时间比例。下面我们将着重从事件处理 和任务处理两个核心部分出发，详细介绍 Netty EventLoop 的实现原理。

结合 Netty 的整体架构，我们一起看下 EventLoop 的事件流转图，以便更好地理解 Netty EventLoop 的设计原理。NioEventLoop 的事件处理机制采用的是无锁串行化的设计思路。

  • BossEventLoopGroup 和 WorkerEventLoopGroup 包含一个或者多个 NioEventLoop。BossEventLoopGroup 负责监听客户端的 Accept 事件，当事件触发时，将事件注册至 WorkerEventLoopGroup 中的一个 NioEventLoop 上。每新建一个 Channel， 只选择一个 NioEventLoop 与其绑定。所以说 Channel 生命周期的所有事件处理都是线程独立的，不同的 NioEventLoop 线程之间不会发生任何交集。
  • NioEventLoop 完成数据读取后，会调用绑定的 ChannelPipeline 进行事件传播，ChannelPipeline 也是线程安全 的，数据会被传递到 ChannelPipeline 的第一个 ChannelHandler 中。数据处理完成后，将加工完成的数据再传递给下一个 ChannelHandler，整个过程是串行化执行，不会发生线程上下文切换的问题。

NioEventLoop 无锁串行化的设计不仅使系统吞吐量达到最大化，而且降低了用户开发业务逻辑的难度，不需要花太多精力关心线程安全问题。

虽然单线程执行避免了线程切换，但是它的缺陷就是不能执行时间过长的 I/O 操作，一旦某个 I/O 事件发生阻塞，那么后续的所有 I/O 事件都无法执行，甚至造成事件积压。

在使用 Netty 进行程序开发时，我们一定要对 ChannelHandler 的实现逻辑有充分的风险意识。

java @Override public int calculateStrategy(IntSupplier selectSupplier, boolean hasTasks) throws Exception { //判断是否有任务 //有任务唤醒 return hasTasks ? selectSupplier.get() : SelectStrategy.SELECT; }

``` /** * 核心思想：没有task要做时，select阻塞1s，如果有task,wakeup去做。 * @param oldWakenUp * @throws IOException */ private void select(boolean oldWakenUp) throws IOException { Selector selector = this.selector; try { int selectCnt = 0; long currentTimeNanos = System.nanoTime(); //按scheduled的task时间来计算select timeout时间。 long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos);

long normalizedDeadlineNanos = selectDeadLineNanos - initialNanoTime();
        if (nextWakeupTime != normalizedDeadlineNanos) {
            nextWakeupTime = normalizedDeadlineNanos;
        }

        for (;;) {
            long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
            if (timeoutMillis <= 0) { //已经有定时task需要执行了，或者超过最长等待时间了
                if (selectCnt == 0) {
                    //非阻塞，没有数据返回0
                    selector.selectNow();
                    selectCnt = 1;
                }
                break;
            }
            if (hasTasks() && wakenUp.compareAndSet(false, true)) {
                selector.selectNow();
                selectCnt = 1;
                break;
            }
            //下面select阻塞中，别人唤醒也可以可以的
            int selectedKeys = selector.select(timeoutMillis);
            selectCnt ++;

            if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {

                break;
            }
            if (Thread.interrupted()) {

                if (logger.isDebugEnabled()) {
                    logger.debug("Selector.select() returned prematurely because " +
                            "Thread.currentThread().interrupt() was called. Use " +
                            "NioEventLoop.shutdownGracefully() to shutdown the NioEventLoop.");
                }
                selectCnt = 1;
                break;
            }

            long time = System.nanoTime();
            if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
                // timeoutMillis elapsed without anything selected.
                selectCnt = 1;
            } else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
                    selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
                // The code exists in an extra method to ensure the method is not too big to inline as this
                // branch is not very likely to get hit very frequently.
                selector = selectRebuildSelector(selectCnt);
                selectCnt = 1;
                break;
            }

            currentTimeNanos = time;
        }

        if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS) {
            if (logger.isDebugEnabled()) {
                logger.debug("Selector.select() returned prematurely {} times in a row for Selector {}.",
                        selectCnt - 1, selector);
            }
        }
    } catch (CancelledKeyException e) {
        if (logger.isDebugEnabled()) {
            logger.debug(CancelledKeyException.class.getSimpleName() + " raised by a Selector {} - JDK bug?",
                    selector, e);
        }
        // Harmless exception - log anyway
    }
}

```

6.空轮询bug解决

NioEventLoop 线程的可靠性至关重要，一旦 NioEventLoop 发生阻塞或者陷入空轮询，就会导致整个系统不可用。

在 JDK 中， Epoll 的实现是存在漏洞的，即使 Selector 轮询的事件列表为空，NIO 线程一样可以被唤醒，导致 CPU 100% 占用。这就是臭名昭著的 JDK epoll 空轮询的 Bug。

Netty 作为一个高性能、高可靠的网络框架，需要保证 I/O 线程的安全性。那么它是如何解决 JDK epoll 空轮询的 Bug 呢？

实际上 Netty 并没有从根源上解决该问题，而是巧妙地规避了这个问题。

我们抛开其他细枝末节，直接定位到事件轮询 select() 方法中的最后一部分代码，一起看下 Netty 是如何解决 epoll 空轮询的 Bug。

Netty中的解决思路： 对Selector()方法中的阻塞定时 select(timeMIllinois)操作的 次数进行统计，每完成一次select操作进行一次计数，若在循环周期内 发生N次空轮询，如果N值大于BUG阈值(默认为512)，就进行空轮询BUG处理。 重建Selector，判断是否是其他线程发起的重建请求，若不是则将原SocketChannel从旧的Selector上去除注册，重新注册到新的 Selector上，并将原来的Selector关闭。 https://blog.csdn.net/qq_41884976/article/details/91913820

select方法分三个部分： //第一部分：超时处理逻辑 //第二部分：定时阻塞select(timeMillins) //第三部分： 解决空轮询 BUG long time = System.nanoTime(); //当前时间 - 循环开始时间 >= 定时select的时间timeoutMillis，说明已经执行过一次阻塞select() if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) { //说明发生过一次阻塞式轮询 重置次数 selectCnt = 1; } else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 && selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) { // 如果空轮询的次数大于空轮询次数阈值 SELECTOR_AUTO_REBUILD_THRESHOLD(512) //1.首先创建一个新的Selecor //2.将旧的Selector上面的键及其一系列的信息放到新的selector上面。 selector = selectRebuildSelector(selectCnt); selectCnt = 1; break; }

Netty 提供了一种检测机制判断线程是否可能陷入空轮询，具体的实现方式如下：

  1. 每次执行 Select 操作之前记录当前时间 currentTimeNanos。
  2. time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos，如果事件轮询的持续时间大于等于 timeoutMillis，那么说明是正常的，否则表明阻塞时间并未达到预期，可能触发了空轮询的 Bug。
  3. Netty 引入了计数变量 selectCnt。在正常情况下，selectCnt 会重置，否则会对 selectCnt 自增计数。当 selectCnt 达到 SELECTORAUTOREBUILD_THRESHOLD(默认512) 阈值时，会触发重建 Selector 对象。

Netty 采用这种方法巧妙地规避了 JDK Bug。异常的 Selector 中所有的 SelectionKey 会重新注册到新建的 Selector 上，重建完成之后异常的 Selector 就可以废弃了。

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