netty源码分析-线程池

众所周知，netty是一款性能非常出色的nio框架，作为dubbo等众多优秀项目底层的数据传输框架，研究吃透它，对于我们今后的开发是绝对有益无害的，所以从今天开始我们就研究netty。本次分析基于netty4，请诸位看官自行下载jar包及源码。好了，我们今天说一下netty的线程池。
我们经常会看到netty的代码中有下面这一句。

EventLoopGroup workerGroup = new NioEventLoopGroup();

简单的new了一个事件的处理组（也没看官方怎么解释这个概念的，自己定义了一下吧，勿喷）。但他里面所作的事情却远不止看到的这么简单。这也是我们阅读源码的一个准则，不要忽略每一个你认为的不起眼的代码，也许他的作用是举足轻重的。他的具体实现

### io.netty.channel.nio.NioEventLoopGroup#NioEventLoopGroup()
/**
 * Create a new instance using the default number of threads, the default {@link ThreadFactory} and
 * the {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
 */
public NioEventLoopGroup() {
    this(0);
}

### io.netty.channel.MultithreadEventLoopGroup#MultithreadEventLoopGroup(int, java.util.concurrent.Executor, java.lang.Object...)
/**
 * @see MultithreadEventExecutorGroup#MultithreadEventExecutorGroup(int, Executor, Object...)
 */
protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) {
    super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, args);
}

### MultithreadEventLoopGroup.java:39
static {
    DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt(
            "io.netty.eventLoopThreads", NettyRuntime.availableProcessors() * 2));

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

### 表示代码出自的类及方法名，因为他们的相关性很大，所以我就放到同一个代码块中，防止思维跳跃太大，大家跟不上节奏。虽然现在初始化的时候设置了线程数为0，但是并不是最后的结果，经过了诸多的构造函数的调用及父类构造函数的引用,在这里做了一个转化，当为0时，会取DEFAULT_EVENT_LOOP_THREADS的值，而他的值,他取的是，如果设置io.netty.eventLoopThreads的值就取这个值，没有设置的话，会取默认值可用核数的两倍，同1比较去一个最大的进行赋值。最后我们到达了最好的构造函数

### io.netty.util.concurrent.MultithreadEventExecutorGroup#MultithreadEventExecutorGroup(int, java.util.concurrent.Executor, io.netty.util.concurrent.EventExecutorChooserFactory, java.lang.Object...)

/**
 * Create a new instance.
 *
 * @param nThreads          the number of threads that will be used by this instance.
 * @param executor          the Executor to use, or {@code null} if the default should be used.
 * @param chooserFactory    the {@link EventExecutorChooserFactory} to use.
 * @param args              arguments which will passed to each {@link #newChild(Executor, Object...)} call
 */
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) {   //如何executor为空，那么设置默认执行器为ThreadPerTaskExecutor
        executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
    }

    children = new EventExecutor[nThreads];   //MultithreadEventExecutorGroup是一个总的管理的类，具体和线程相关的都交给他的children进行处理，是一个EventExecutor的数组

    for (int i = 0; i < nThreads; i ++) {
        boolean success = false;
        try {
            children[i] = newChild(executor, args);   //初始化每一个EventExecutor的实例，下面会有详细解释
            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);  //创建线程的选择器，选择是有哪个线程来处理

    final FutureListener