Netty源碼分析（五）：EventLoop

時間 2019-12-06

原文原文鏈接

上一篇主要介紹了一下EventLoopGroup，本篇詳細看下它的成員EventLoop。git

類結構

NioEventLoop繼承自 SingleThreadEventLoop,而 SingleThreadEventLoop又繼承自 SingleThreadEventExecutor。 SingleThreadEventExecutor內部持有一個Thread對象，是 Netty多線程的基礎。能夠認爲, 一個 NioEventLoop與一個特定的線程進行了綁定,而且在其生命週期內,綁定的線程都不會再改變。

SingleThreadEventExecutor

從名字就能夠看出來，SingleThreadEventExecutor是一個單線程事件執行器。主要作的事情就是線程的管理和事件的執行。github

線程管理

SingleThreadEventExecutor中定義了五種線程狀態：bash

/**
     * 未開始
     */
    private static final int ST_NOT_STARTED = 1;
    /**
     * 已開始
     */
    private static final int ST_STARTED = 2;
    /**
     * 關閉中
     */
    private static final int ST_SHUTTING_DOWN = 3;
    /**
     * 已關閉
     */
    private static final int ST_SHUTDOWN = 4;
    /**
     * 已終止
     */
    private static final int ST_TERMINATED = 5;
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這幾種狀態對應的方法有startThread、shutdownGracefully和shutdown。多線程

startThread

private void startThread() {
        if (state == ST_NOT_STARTED) {
            if (STATE_UPDATER.compareAndSet(this, ST_NOT_STARTED, ST_STARTED)) {
                try {
                    doStartThread();
                } catch (Throwable cause) {
                    STATE_UPDATER.set(this, ST_NOT_STARTED);
                    PlatformDependent.throwException(cause);
                }
            }
        }
    }
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startThread線程未開始時，嘗試更新線程狀態爲一開始，更新成功，則調用doStartThread方法啓動線程,子類的run方法就是在這裏調用的，好比說接下來的NioEventLoop。oop

shutdownGracefully

public Future<?> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit) {
        // 靜待時間須要>=0
        if (quietPeriod < 0) {
            throw new IllegalArgumentException("quietPeriod: " + quietPeriod + " (expected >= 0)");
        }
        // 超時時間不能小於靜待時間
        if (timeout < quietPeriod) {
            throw new IllegalArgumentException(
                    "timeout: " + timeout + " (expected >= quietPeriod (" + quietPeriod + "))");
        }
        // 必須設置時間單位
        if (unit == null) {
            throw new NullPointerException("unit");
        }
        // 關閉中直接返回終止Future
        if (isShuttingDown()) {
            return terminationFuture();
        }
        boolean inEventLoop = inEventLoop();
        boolean wakeup;
        int oldState;
        for (; ; ) {
            // 關閉中直接返回終止Future
            if (isShuttingDown()) {
                return terminationFuture();
            }
            int newState;
            wakeup = true;
            oldState = state;
            if (inEventLoop) {
                newState = ST_SHUTTING_DOWN;
            } else {
                switch (oldState) {
                    case ST_NOT_STARTED:
                    case ST_STARTED:
                        newState = ST_SHUTTING_DOWN;
                        break;
                    default:
                        newState = oldState;
                        wakeup = false;
                }
            }
            if (STATE_UPDATER.compareAndSet(this, oldState, newState)) {
                break;
            }
        }
        gracefulShutdownQuietPeriod = unit.toNanos(quietPeriod);
        gracefulShutdownTimeout = unit.toNanos(timeout);
        if (oldState == ST_NOT_STARTED) {
            try {
                doStartThread();
            } catch (Throwable cause) {
                STATE_UPDATER.set(this, ST_TERMINATED);
                terminationFuture.tryFailure(cause);

                if (!(cause instanceof Exception)) {
                    PlatformDependent.throwException(cause);
                }
                return terminationFuture;
            }
        }
        if (wakeup) {
            wakeup(inEventLoop);
        }

        return terminationFuture();
    }
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shutdownGracefully目的是讓正在執行的任務再執行一下子，同時拒絕新任務。quietPeriod和timeout這兩個時間會在confirmShutdown方法中用到，固然單位已經轉爲納秒了。ui

事件的執行

public void execute(Runnable task) {
        if (task == null) {
            throw new NullPointerException("task");
        }
        boolean inEventLoop = inEventLoop();
        if (inEventLoop) {
            addTask(task);
        } else {
            startThread();
            addTask(task);
            if (isShutdown() && removeTask(task)) {
                reject();
            }
        }
        if (!addTaskWakesUp && wakesUpForTask(task)) {
            wakeup(inEventLoop);
        }
    }
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NioEventLoop

NioEventLoop的核心操做都在它的run方法裏面：this

protected void run() {
        for (; ; ) {
            try {
                switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
                    case SelectStrategy.CONTINUE:
                        continue;
                    case SelectStrategy.SELECT:
                        // 重置wakenUp爲false並選擇任務
                        select(wakenUp.getAndSet(false));
                        if (wakenUp.get()) {
                            selector.wakeup();
                        }
                    default:
                }
                cancelledKeys = 0;
                needsToSelectAgain = false;
                final int ioRatio = this.ioRatio;
                // 當處理io用時佔比爲100%時
                if (ioRatio == 100) {
                    try {
                        processSelectedKeys();
                    } finally {
                        runAllTasks();
                    }
                } else {
                    final long ioStartTime = System.nanoTime();
                    try {
                        processSelectedKeys();
                    } finally {
                        final long ioTime = System.nanoTime() - ioStartTime;
                        runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                    }
                }
            } catch (Throwable t) {
                // 處理Loop異常
                handleLoopException(t);
            }
            try {
                // 處於關閉狀態
                if (isShuttingDown()) {
                    // 關閉全部
                    closeAll();
                    if (confirmShutdown()) {
                        return;
                    }
                }
            } catch (Throwable t) {
                // 處理Loop異常
                handleLoopException(t);
            }
        }
    }
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