眾所周知，netty是一款性能非常出色的nio框架，作為dubbo等眾多優(yōu)秀項(xiàng)目底層的數(shù)據(jù)傳輸框架，研究吃透它，對(duì)于我們今后的開(kāi)發(fā)是絕對(duì)有益無(wú)害的，所以從今天開(kāi)始我們就研究netty。本次分析基于netty4，請(qǐng)諸位看官自行下載jar包及源碼。好了，我們今天說(shuō)一下netty的線程池。
我們經(jīng)常會(huì)看到netty的代碼中有下面這一句。

EventLoopGroup workerGroup = new NioEventLoopGroup();

簡(jiǎn)單的new了一個(gè)事件的處理組（也沒(méi)看官方怎么解釋這個(gè)概念的，自己定義了一下吧，勿噴）。但他里面所作的事情卻遠(yuǎn)不止看到的這么簡(jiǎn)單。這也是我們閱讀源碼的一個(gè)準(zhǔn)則，不要忽略每一個(gè)你認(rèn)為的不起眼的代碼，也許他的作用是舉足輕重的。他的具體實(shí)現(xiàn)

### 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);
    }
}

### 表示代碼出自的類及方法名，因?yàn)樗麄兊南嚓P(guān)性很大，所以我就放到同一個(gè)代碼塊中，防止思維跳躍太大，大家跟不上節(jié)奏。雖然現(xiàn)在初始化的時(shí)候設(shè)置了線程數(shù)為0，但是并不是最后的結(jié)果，經(jīng)過(guò)了諸多的構(gòu)造函數(shù)的調(diào)用及父類構(gòu)造函數(shù)的引用,在這里做了一個(gè)轉(zhuǎn)化，當(dāng)為0時(shí)，會(huì)取DEFAULT_EVENT_LOOP_THREADS的值，而他的值,他取的是，如果設(shè)置io.netty.eventLoopThreads的值就取這個(gè)值，沒(méi)有設(shè)置的話，會(huì)取默認(rèn)值可用核數(shù)的兩倍，同1比較去一個(gè)最大的進(jìn)行賦值。最后我們到達(dá)了最好的構(gòu)造函數(shù)

### 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為空，那么設(shè)置默認(rèn)執(zhí)行器為ThreadPerTaskExecutor
        executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
    }

    children = new EventExecutor[nThreads];   //MultithreadEventExecutorGroup是一個(gè)總的管理的類，具體和線程相關(guān)的都交給他的children進(jìn)行處理，是一個(gè)EventExecutor的數(shù)組

    for (int i = 0; i < nThreads; i ++) {
        boolean success = false;
        try {
            children[i] = newChild(executor, args);   //初始化每一個(gè)EventExecutor的實(shí)例，下面會(huì)有詳細(xì)解釋
            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);  //創(chuàng)建線程的選擇器，選擇是有哪個(gè)線程來(lái)處理

    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);   //將這些子處理器設(shè)置為只讀，不能添加
}

在上面的代碼中都有關(guān)鍵步驟的注釋，總的來(lái)說(shuō)就是最后的臟活累活都不是這個(gè)Group干的，都交給自己內(nèi)部的children來(lái)干，都交給EventExecutor來(lái)干，我們看一下這個(gè)EventExecutor是如何實(shí)例化的

### io.netty.channel.nio.NioEventLoopGroup#newChild

@Override
protected EventLoop newChild(Executor executor, Object... args) throws Exception {
    return new NioEventLoop(this, executor, (SelectorProvider) args[0],
        ((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]);
}

### io.netty.channel.nio.NioEventLoop#NioEventLoop
NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,
             SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
    super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
    if (selectorProvider == null) {
        throw new NullPointerException("selectorProvider");
    }
    if (strategy == null) {
        throw new NullPointerException("selectStrategy");
    }
    provider = selectorProvider;
    final SelectorTuple selectorTuple = openSelector();
    selector = selectorTuple.selector;
    unwrappedSelector = selectorTuple.unwrappedSelector;
    selectStrategy = strategy;
}

EventExecutor是使用NioEventLoop進(jìn)行初始化的，這個(gè)NioEventLoop是SingleThreadEventLoop的子類，所以super調(diào)用的是SingleThreadEventLoop的構(gòu)造方法

/**
 * Create a new instance
 *
 * @param parent            the {@link EventExecutorGroup} which is the parent of this instance and belongs to it
 * @param executor          the {@link Executor} which will be used for executing
 * @param addTaskWakesUp    {@code true} if and only if invocation of {@link #addTask(Runnable)} will wake up the
 *                          executor thread
 * @param maxPendingTasks   the maximum number of pending tasks before new tasks will be rejected.
 * @param rejectedHandler   the {@link RejectedExecutionHandler} to use.
 */
protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
                                    boolean addTaskWakesUp, int maxPendingTasks,
                                    RejectedExecutionHandler rejectedHandler) {
    super(parent);
    this.addTaskWakesUp = addTaskWakesUp;
    this.maxPendingTasks = Math.max(16, maxPendingTasks);
    this.executor = ObjectUtil.checkNotNull(executor, "executor");
    taskQueue = newTaskQueue(this.maxPendingTasks);    //設(shè)置任務(wù)的處理隊(duì)列，后續(xù)任務(wù)會(huì)添加到這里面
    rejectedExecutionHandler = ObjectUtil.checkNotNull(rejectedHandler, "rejectedHandler");
}

OK,每一個(gè)處理子事件處理器都會(huì)有一個(gè)任務(wù)的隊(duì)列。目前為止線程池的初始化就告一段落了，感覺(jué)沒(méi)過(guò)癮，咱們就下一篇見(jiàn)。