平台工具MicroService技术框架Netty

Netty的NioEventLoop

NioEventLoop

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public final class NioEventLoop extends SingleThreadEventLoop {
    private Selector selector;
    private Selector unwrappedSelector;
    private SelectedSelectionKeySet selectedKeys;

    private final SelectorProvider provider;

    /**
     * Boolean that controls determines if a blocked Selector.select should
     * break out of its selection process. In our case we use a timeout for
     * the select method and the select method will block for that time unless
     * waken up.
     */
    private final AtomicBoolean wakenUp = new AtomicBoolean();

    private final SelectStrategy selectStrategy;

    private volatile int ioRatio = 50;
    private int cancelledKeys;
    private boolean needsToSelectAgain;

    ...
}

NioEventLoop 是Netty的核心类。

NioEventLoop 的首要职责就是为注册在它上的 channels 服务，发现这些 channels 上发生的新连接事件、读写等 I/O 事件，然后将事件转交 channel 流水线处理。

NioEventLoop 的职责

核心逻辑

NioEventLoop最核心的地方在于事件循环，具体代码在NioEventLoop.java在run方法中，如下逻辑：

  • 首先根据默认的选择策略DefaultSelectStrategy判断本次循环是否select，具体逻辑为：如果当前有任务则使用selectNow立刻查询是否有准备就绪的I/O；如果当前没有任务则返回SelectStrategy.SELECT，并将wakenUp设置为false，并调用select()进行查询。
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switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
 //    'if (wakenUp.get()) { ... }'. (OK)
    case SelectStrategy.CONTINUE:
        continue;
    case SelectStrategy.SELECT:
        select(wakenUp.getAndSet(false));
        // 'wakenUp.compareAndSet(false, true)' is always evaluated
        // before calling 'selector.wakeup()' to reduce the wake-up
        // overhead. (Selector.wakeup() is an expensive operation.)
        //
        // However, there is a race condition in this approach.
        // The race condition is triggered when 'wakenUp' is set to
        // true too early.
        //
        // 'wakenUp' is set to true too early if:
        // 1) Selector is waken up between 'wakenUp.set(false)' and
        //    'selector.select(...)'. (BAD)
        // 2) Selector is waken up between 'selector.select(...)' and
        // In the first case, 'wakenUp' is set to true and the
        // following 'selector.select(...)' will wake up immediately.
        // Until 'wakenUp' is set to false again in the next round,
        // 'wakenUp.compareAndSet(false, true)' will fail, and therefore
        // any attempt to wake up the Selector will fail, too, causing
        // the following 'selector.select(...)' call to block
        // unnecessarily.
        //
        // To fix this problem, we wake up the selector again if wakenUp
        // is true immediately after selector.select(...).
        // It is inefficient in that it wakes up the selector for both
        // the first case (BAD - wake-up required) and the second case
        // (OK - no wake-up required).

        if (wakenUp.get()) {
            selector.wakeup();
        }
        // fall through
    default:
}

cancelledKeys = 0;
needsToSelectAgain = false;
final int ioRatio = this.ioRatio;
if (ioRatio == 100) {
    try {
        processSelectedKeys();
    } finally {
        // Ensure we always run tasks.
        runAllTasks();
    }
} else {
    final long ioStartTime = System.nanoTime();
    try {
        processSelectedKeys();
    } finally {
        // Ensure we always run tasks.
        final long ioTime = System.nanoTime() - ioStartTime;
        runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
    }
}

SelectStrategy

SelectStrategy selectStrategy

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final class DefaultSelectStrategy implements SelectStrategy {
    static final SelectStrategy INSTANCE = new DefaultSelectStrategy();
    private DefaultSelectStrategy() { }
    @Override
    public int calculateStrategy(IntSupplier selectSupplier, boolean hasTasks) throws Exception {
        return hasTasks ? selectSupplier.get() : SelectStrategy.SELECT;
    }
}

Netty源码笔记

Reactor线程模型详解(二)NioEventLoop的执行

Netty源码分析之NioEventLoop