• 五、NioEventLoop启动


    NioEventLoop执行概述:

    1、何时被启动

    当AbstractChannel.register()方法被执行时,该方法下有这样一段代码:

     if (eventLoop.inEventLoop()) {
                    register0(promise);
                } else {
                    try {
                        eventLoop.execute(new Runnable() {
                            @Override
                            public void run() {
                                register0(promise);
                            }
                        });
          ...
    

    其中的eventLoop.inEventLoop()为判断当前线程是否为eventLoop线程,此处是main线程,并且eventLoop线程在此处为null,所以返回false,并进入eventLoop.execute()方法,此方法在NioEventLoop没有实现,调用的是SingleThreadEventExecutor类中的execute()方法。

    boolean inEventLoop = inEventLoop();
            if (inEventLoop) {
                addTask(task);
            } else {
                startThread();
                addTask(task);
                if (isShutdown() && removeTask(task)) {
                    reject();
                }
            }
    

    inEventLoop返回为false,进入startThread()方法,继续进入doStartThread()方法,

     private void doStartThread() {
            assert thread == null;
            executor.execute(new Runnable() {
                @Override
                public void run() {
                    thread = Thread.currentThread();
                    ....
                    updateLastExecutionTime();
                    ....
                    SingleThreadEventExecutor.this.run();
    

    其中

    1. thread = Thread.currentThread():将当前线程与nioEventLoop绑定。
    2. 更新上次执行的时长。
    3. SingleThreadEventExecutor.this.run()代码,其中SingleThreadEventExecutor.this即为NioEventLoop对 象。在它的run方法封装了启动逻辑。

    即当 EventLoop.execute 第一次被调用时, 就会触发 startThread() 的调用, 进而导致了 EventLoop 所对应的 Java 线程的启动。

    2、NioEventLoop.run做了什么

    主要做了下面三件事:

    1. 检查是否有IO事件。
    2. 处理IO事件。
    3. 处理异步任务队列。
    protected void run() {
            for (;;) {
            ...
           select(wakenUp.getAndSet(false));
            ...
           processSelectedKeys();
           ...
           runAllTasks();
    

    select()方法执行逻辑

    • selectDeadLineNanos及任务穿插逻辑处理
       long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos);
                for (;;) {
                    long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
                    if (timeoutMillis <= 0) {
                        if (selectCnt == 0) {
                            selector.selectNow();
                            selectCnt = 1;
                            }
                            break;
                            }
        ...
                    if (hasTasks() && wakenUp.compareAndSet(false, true)) {
                        selector.selectNow();
                        selectCnt = 1;
                        break;
                    }
    
    1. 计算当前定时任务队列第一个定时任务的截止时间,当前时间 + 截止时间(NioEventLoop底层维持了一个定时任务队列,按照任务的截止时间正序排序)。
    2. 超时,判断有没有进行过select,如果没有则进行一次非阻塞的select操作,并退出当前循环。
    3. 没有超时,则检查当前任务队列(tailTasks和taskQueue)是否有任务,并将wakebUp通过CAS操作设为true。如果有则进行一侧非阻塞的select操作,并退出当前循环。
    • 阻塞式select
    int selectedKeys = selector.select(timeoutMillis);
    selectCnt ++;
    if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {
       // - Selected something,轮询到事件
       // - waken up by user, or用户线程唤醒
       // - the task queue has a pending task.任务队列有任务
       // - a scheduled task is ready for processing 定时任务队列有任务准备好执行
           break;//跳出当前循环
     }
    
    • 避免JDK空轮询的Bug
                  long time = System.nanoTime();
                    if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
                        // timeoutMillis elapsed without anything selected.
                        selectCnt = 1;
                    } else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
                            selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
                        // The selector returned prematurely many times in a row.
                        // Rebuild the selector to work around the problem.
                        logger.warn(
                                "Selector.select() returned prematurely {} times in a row; rebuilding Selector {}.",
                                selectCnt, selector);
    
                        rebuildSelector();
                        selector = this.selector;
    
                        // Select again to populate selectedKeys.
                        selector.selectNow();
                        selectCnt = 1;
                        break;
    
    1. 判断是否进行了一个空轮询(当前时间-阻塞时长>=开始时间,说明进行了阻塞),如果小于,则说明执行了一次空轮询。
    2. 判断空轮询次数是否大于512次(SELECTOR_AUTO_REBUILD_THRESHOLD在静态代码块中初始为512),如果大于512次则执行rebuildSelector()方法。
    3. 新建一个selector,并把之前oldSelector上的属性注册到新的selector上。
          ...
                newSelector = openSelector();
          ...
    
            // Register all channels to the new Selector.
            int nChannels = 0;
            for (;;) {
                try {
                    for (SelectionKey key: oldSelector.keys()) {
                        Object a = key.attachment();
                        try {
                            if (!key.isValid() || key.channel().keyFor(newSelector) != null) {
                                continue;
                            }
                            int interestOps = key.interestOps();
                            key.cancel();
                            SelectionKey newKey = key.channel().register(newSelector, interestOps, a);
                            if (a instanceof AbstractNioChannel) {
                                // Update SelectionKey
                                ((AbstractNioChannel) a).selectionKey = newKey;
                            }
                            nChannels ++;
                        } 
    

    3、处理IO事件

    • 先介绍下Netty对Selector的selectedKeys的优化
      SelectedSelectionKeySet继承自AbstractSet,内部维护了一个数组,并重写了其方法
    1. 先判断是否禁用优化,默认为false()。
    2. 创建优化后的SelectedSelectionKeySet,校验后通过反射替换select的selectedKeysField和publicSelectedKeysField。
      private Selector openSelector() {
            final Selector selector;
           ...
                selector = provider.openSelector();
          ...
            if (DISABLE_KEYSET_OPTIMIZATION) {
                return selector;
            }
            final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();
            Object maybeSelectorImplClass=Class.forName("sun.nio.ch.SelectorImpl",false,PlatformDependent.getSystemClassLoader());
                   ...
                        Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
                        Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");
                        selectedKeysField.setAccessible(true);
                        publicSelectedKeysField.setAccessible(true);
                        selectedKeysField.set(selector, selectedKeySet);
                        publicSelectedKeysField.set(selector, selectedKeySet);
               ...       
                selectedKeys = selectedKeySet;
        ...
            return selector;
        }
    
    • processSelectedKeys()处理IO事件
      private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {
                ...
                // 遍历selectedKeys,取得key之后将数组原位置置为空
                 selectedKeys[i] = null;
                 //取得绑定的Channel
                final Object a = k.attachment();
                ...
                }
                //调用处理方法
      private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
            final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
            //验证key是否合法
            if (!k.isValid()) {
                final EventLoop eventLoop;
                  ...
                    eventLoop = ch.eventLoop();
                  ...
                if (eventLoop != this || eventLoop == null) {
                    return;
                }
                // close the channel if the key is not valid anymore
                unsafe.close(unsafe.voidPromise());
                return;
            }
            //通过验证的进入处理逻辑
            try {
            //获取key事件类型并处理
                int readyOps = k.readyOps();
                // We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
                // the NIO JDK channel implementation may throw a NotYetConnectedException.
                if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                    // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
                    // See https://github.com/netty/netty/issues/924
                    int ops = k.interestOps();
                    ops &= ~SelectionKey.OP_CONNECT;
                    k.interestOps(ops);
                    unsafe.finishConnect();
                }
                // Process OP_WRITE first as we may be able to write some queued buffers and so free memory.
                if ((readyOps & SelectionKey.OP_WRITE) != 0) {
                    // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
                    ch.unsafe().forceFlush();
                }
                // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
                // to a spin loop
                if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                    unsafe.read();
                    if (!ch.isOpen()) {
                        // Connection already closed - no need to handle write.
                        return;
                    }
                }
            } catch (CancelledKeyException ignored) {
                unsafe.close(unsafe.voidPromise());
            }
        }
    

    4、runAllTasks()执行逻辑

    Netty的任务队列包括taskQueue、tailQueue和scheduledTaskQueue。

    • scheduledTaskQueue任务的添加。
      scheduledTaskQueue的任务是在当调用NioEventLoop(AbstractScheduledEventExecutor)的schedule方法时添加的。代码如下
      <V> ScheduledFuture<V> schedule(final ScheduledFutureTask<V> task) {
      //判断是否是当前线程执行添加操作,如果是直接添加,不是的话另起一个线程去执行添加任务
      //把添加定时任务也作为一个普通的task去执行,所有的任务都是在当前的NioEventLoop中执行,保证线程安全。
            if (inEventLoop()) {
                scheduledTaskQueue().add(task);
            } else {
                execute(new Runnable() {
                    @Override
                    public void run() {
                        scheduledTaskQueue().add(task);
        ...
            return task;
        }
    

    tips:scheduledTaskQueue是一个优先级队列,排序逻辑是按照截止日期排序,如果截止日期相同,则按照ID排序。

     public int compareTo(Delayed o) {
            if (this == o) {
                return 0;
            }
            ScheduledFutureTask<?> that = (ScheduledFutureTask<?>) o;
            long d = deadlineNanos() - that.deadlineNanos();
            if (d < 0) {
                return -1;
            } else if (d > 0) {
                return 1;
            } else if (id < that.id) {
                return -1;
            } else if (id == that.id) {
                throw new Error();
            } else {
                return 1;
            }
        }
    
    • 任务的聚合
      ioRatio默认是50,则runAllTask里时间是当前时间-处理IO开始时间
    final long ioStartTime = System.nanoTime();
        try {
            processSelectedKeys();
             } finally {
             // Ensure we always run tasks.
               final long ioTime = System.nanoTime() - ioStartTime;
               runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
            }
    

    fetchFromScheduledTaskQueue()-- 循环从定时任务中取出截止日期到期(截止日期<=现在时间-系统启动时间)的task,并添加到taskQueue中,然后循环执行任务。

     protected boolean runAllTasks(long timeoutNanos) {
            fetchFromScheduledTaskQueue();
            Runnable task = pollTask();
            if (task == null) {
            //如果任务都为空,则去执行tailQueue中的任务
                afterRunningAllTasks();
                return false;
            }
            final long deadline = ScheduledFutureTask.nanoTime() + timeoutNanos;
            long runTasks = 0;
            long lastExecutionTime;
            for (;;) {
                safeExecute(task);
                runTasks ++;
               ...
                //当执行任务到64个的时候,计算下执行事件是否超过deadline ,如果是就中止循环
                if ((runTasks & 0x3F) == 0) {
                    lastExecutionTime = ScheduledFutureTask.nanoTime();
                    if (lastExecutionTime >= deadline) {
                        break;
                    }
                }
                task = pollTask();
                if (task == null) {
                    lastExecutionTime = ScheduledFutureTask.nanoTime();
                    break;
                }
            }
            //执行收尾任务
            afterRunningAllTasks();
            //将lastExecutionTime 设置为系统启动时间
            this.lastExecutionTime = lastExecutionTime;
            return true;
        }
    
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  • 原文地址:https://www.cnblogs.com/demo-alen/p/13547222.html
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