Netty源码阅读之NioEventLoop简析

     在Netty中NioEventLoop以及NioEventLoopGroup是很重要的两个类，而NioEventLoopGroup主要是对NioEventLoop进行管理；首先来看一下这两个类的关系图（错综复杂）：

                                                                                                 图 1

1. NioEventLoopGroup初始化流程

通过分析NioEventLoopGroup的构造方法的调用栈我们能够看到在io.netty.channel.MultithreadEventLoopGroup的构造方法中进行了创建：

当未指定具体的线程数目的时候，Netty会提出一个默认的线程数：DEFAULT_EVENT_LOOP_THREADS

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

而该数值在同一类下的静态代码块中进行了设置：

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);
        }
    }

显而易见，默认的线程数量为2*cpu数目。

继续深入，打开io.netty.util.concurrent.MultithreadEventExecutorGroup这个类，查看其构造方法：

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) {
            executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());//创建线程执行器
        }
        
        //构造NioEventLoop的过程
        children = new EventExecutor[nThreads];

        for (int i = 0; i < nThreads; i ++) {
            boolean success = false;
            try {
                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);//生成线程选择器

        final FutureListener