前言

提到 Netty 首当其冲被提起的肯定是支持它承受高并发的线程模型，说到线程模型就不得不提到 NioEventLoopGroup 这个线程池，接下来进入正题。

线程模型

首先来看一段 Netty 的使用示例

package com.coding.server;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;

public final class SimpleServer {

    public static void main(String[] args) throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .handler(new SimpleServerHandler())
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        public void initChannel(SocketChannel ch) throws Exception {
                        }
                    });

            ChannelFuture f = b.bind(8888).sync();

            f.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }

    private static class SimpleServerHandler extends ChannelInboundHandlerAdapter {
        @Override
        public void channelActive(ChannelHandlerContext ctx) throws Exception {
            System.out.println("channelActive");
        }

        @Override
        public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
            System.out.println("channelRegistered");
        }

        @Override
        public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
            System.out.println("handlerAdded");
        }
    }
}

下面将分析第一、二行代码，看下 NioEventLoopGroup 类的构造函数干了些什么。其余的部分将在其他博文中分析。

EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();

从代码中可以看到这里使用了两个线程池 bossGroup 和 workerGroup，那么为什么需要定义两个线程池呢？这就要说到 Netty 的线程模型了。

Netty 的线程模型被称为 Reactor 模型，具体如图所示，图上的 mainReactor 指的就是 bossGroup，这个线程池处理客户端的连接请求，并将 accept 的连接注册到 subReactor 的其中一个线程上；图上的 subReactor 当然指的就是 workerGroup，负责处理已建立的客户端通道上的数据读写；图上还有一块 ThreadPool 是具体的处理业务逻辑的线程池，一般情况下可以复用 subReactor，比我的项目中就是这种用法，但官方建议处理一些较为耗时的业务时还是要使用单独的 ThreadPool。

NioEventLoopGroup 构造函数

public NioEventLoopGroup() {
    this(0);
}

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

public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory) {
    this(nThreads, threadFactory, SelectorProvider.provider());
}

public NioEventLoopGroup(
        int nThreads, ThreadFactory threadFactory, final SelectorProvider selectorProvider) {
    super(nThreads, threadFactory, selectorProvider);
}

NioEventLoopGroup 类中的构造函数最终都是调用的父类 MultithreadEventLoopGroup 如下的构造函数：

protected MultithreadEventLoopGroup(int nThreads, ThreadFactory threadFactory, Object... args) {
    super(nThreads == 0? DEFAULT_EVENT_LOOP_THREADS : nThreads, threadFactory, args);
}

从上面的构造函数可以得到 如果使用EventLoopGroup workerGroup = new NioEventLoopGroup()来创建对象，即不指定线程个数，则 netty 给我们使用默认的线程个数，如果指定则用我们指定的线程个数。

默认线程个数相关的代码如下：

static {
    DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt(
            "io.netty.eventLoopThreads", Runtime.getRuntime().availableProcessors() * 2));

    if (logger.isDebugEnabled()) {
        logger.debug("-Dio.netty.eventLoopThreads: {}", DEFAULT_EVENT_LOOP_THREADS);
    }
}

而 SystemPropertyUtil.getInt 函数的功能为：得到系统属性中指定 key(这里：key ＝”io.netty.eventLoopThreads”)所对应的 value，如果获取不到获取失败则返回默认值，这里的默认值为：cpu 的核数的２倍。

结论：如果没有设置程序启动参数（或者说没有指定 key=”io.netty.eventLoopThreads”的属性值），那么默认情况下线程的个数为 cpu 的核数乘以 2。

继续看，由于 MultithreadEventLoopGroup 的构造函数是调用的是其父类 MultithreadEventExecutorGroup 的构造函数，因此，看下此类的构造函数

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

    if (threadFactory == null) {
        threadFactory = newDefaultThreadFactory();
    }

    children = new SingleThreadEventExecutor[nThreads];
    //根据线程个数是否为2的幂次方，采用不同策略初始化chooser
    if (isPowerOfTwo(children.length)) {
        chooser = new PowerOfTwoEventExecutorChooser();
    } else {
        chooser = new GenericEventExecutorChooser();
    }
        //产生nTreads个NioEventLoop对象保存在children数组中
    for (int i = 0; i < nThreads; i ++) {
        boolean success = false;
        try {
            children[i] = newChild(threadFactory, 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 {
                //如果newChild方法执行失败，则对前面执行new成功的几个NioEventLoop进行shutdown处理
            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) {
                        Thread.currentThread().interrupt();
                        break;
                    }
                }
            }
        }
    }
}

该构造函数干了如下三件事：

1. 产生了一个线程工场：threadFactory = newDefaultThreadFactory();

MultithreadEventExecutorGroup类

protected ThreadFactory newDefaultThreadFactory() {
    return new DefaultThreadFactory(getClass());//getClass()为：NioEventLoopGroup.class
}

DefaultThreadFactory类

public DefaultThreadFactory(Class<?> poolType) {
    this(poolType, false, Thread.NORM_PRIORITY);
}

2. 根据线程个数是否为 2 的幂次方，采用不同策略初始化 chooser

private static boolean isPowerOfTwo(int val) {
    return (val & -val) == val;
}

3. 产生 nTreads 个 NioEventLoop 对象保存在 children 数组中 ，线程都是通过调用 newChild 方法来产生的。

@Override
protected EventExecutor newChild(
        ThreadFactory threadFactory, Object... args) throws Exception {
    return new NioEventLoop(this, threadFactory, (SelectorProvider) args[0]);
}

这里传给 NioEventLoop 构造函数的参数为：NioEventLoopGroup、DefaultThreadFactory、SelectorProvider。

NioEventLoop 构造函数分析

既然上面提到来 new 一个 NioEventLoop 对象，下面我们就看下这个类以及其父类。

NioEventLoop(NioEventLoopGroup parent, ThreadFactory threadFactory, SelectorProvider selectorProvider) {
    super(parent, threadFactory, false);
    if (selectorProvider == null) {
        throw new NullPointerException("selectorProvider");
    }
    provider = selectorProvider;
    selector = openSelector();
}

继续看父类 SingleThreadEventLoop 的构造函数

protected SingleThreadEventLoop(EventLoopGroup parent, ThreadFactory threadFactory, boolean addTaskWakesUp) {
    super(parent, threadFactory, addTaskWakesUp);
}

又是直接调用来父类 SingleThreadEventExecutor 的构造函数，继续看

protected SingleThreadEventExecutor(
        EventExecutorGroup parent, ThreadFactory threadFactory, boolean addTaskWakesUp) {

    if (threadFactory == null) {
        throw new NullPointerException("threadFactory");
    }

    this.parent = parent;
    this.addTaskWakesUp = addTaskWakesUp;//false

    thread = threadFactory.newThread(new Runnable() {
        @Override
        public void run() {
            boolean success = false;
            updateLastExecutionTime();
            try {
            //调用NioEventLoop类的run方法
                SingleThreadEventExecutor.this.run();
                success = true;
            } catch (Throwable t) {
                logger.warn("Unexpected exception from an event executor: ", t);
            } finally {
                for (;;) {
                    int oldState = STATE_UPDATER.get(SingleThreadEventExecutor.this);
                    if (oldState >= ST_SHUTTING_DOWN || STATE_UPDATER.compareAndSet(
                            SingleThreadEventExecutor.this, oldState, ST_SHUTTING_DOWN)) {
                        break;
                    }
                }
                // Check if confirmShutdown() was called at the end of the loop.
                if (success && gracefulShutdownStartTime == 0) {
                    logger.error(
                            "Buggy " + EventExecutor.class.getSimpleName() + " implementation; " +
                            SingleThreadEventExecutor.class.getSimpleName() + ".confirmShutdown() must be called " +
                            "before run() implementation terminates.");
                }

                try {
                    // Run all remaining tasks and shutdown hooks.
                    for (;;) {
                        if (confirmShutdown()) {
                            break;
                        }
                    }
                } finally {
                    try {
                        cleanup();
                    } finally {
                        STATE_UPDATER.set(SingleThreadEventExecutor.this, ST_TERMINATED);
                        threadLock.release();
                        if (!taskQueue.isEmpty()) {
                            logger.warn(
                                    "An event executor terminated with " +
                                    "non-empty task queue (" + taskQueue.size() + ')');
                        }

                        terminationFuture.setSuccess(null);
                    }
                }
            }
        }
    });

    taskQueue = newTaskQueue();
}
protected Queue<Runnable> newTaskQueue() {
    return new LinkedBlockingQueue<Runnable>();
}

主要干如下两件事：

1. 利用 ThreadFactory 创建来一个 Thread，传入了一个 Runnable 对象，该 Runnable 重写的 run 代码比较长，不过重点仅仅是调用 NioEventLoop 类的 run 方法。

2. 使用 LinkedBlockingQueue 类初始化 taskQueue 。

其中newThread 方法的代码如下：

DefaultThreadFactory类

@Override
public Thread newThread(Runnable r) {
    Thread t = newThread(new DefaultRunnableDecorator(r), prefix + nextId.incrementAndGet());

    try {
    //判断是否是守护线程，并进行设置
        if (t.isDaemon()) {
            if (!daemon) {
                t.setDaemon(false);
            }
        } else {
            if (daemon) {
                t.setDaemon(true);
            }
        }
            //设置其优先级
        if (t.getPriority() != priority) {
            t.setPriority(priority);
        }
    } catch (Exception ignored) {
        // Doesn't matter even if failed to set.
    }
    return t;
}

protected Thread newThread(Runnable r, String name) {
    return new FastThreadLocalThread(r, name);
}

FastThreadLocalThread类

public FastThreadLocalThread(Runnable target, String name) {
    super(target, name);// FastThreadLocalThread extends Thread
}

到这里，可以看到底层还是借助于类似于Thread thread = new Thread(r)这种方式来创建线程。

关于NioEventLoop对象可以得到的点有，初始化了如下4个属性。

1. NioEventLoopGroup　（在父类SingleThreadEventExecutor中）

2. selector

3. provider

4. thread （在父类SingleThreadEventExecutor中）

结束

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