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

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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
// DEFAULTEVENTLOOPTHREADS = 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 ? DEFAULTEVENTLOOPTHREADS : 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：

每次执行 Select 操作之前记录当前时间 currentTimeNanos。
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。
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。文章来源地址https://www.toymoban.com/news/detail-655643.html

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