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：
- 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);
}
- 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;
}
- 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：
-
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。
-
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。
-
NioEventLoopGroup （在父类SingleThreadEventExecutor中）
-
selector
-
provider
-
thread （在父类SingleThreadEventExecutor中）
结束
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