[编织消息框架][netty源码分析]3 EventLoop 实现类SingleThreadEventLoop职责与实现

eventLoop是基于事件系统机制，主要技术由线程池同队列组成，是由生产/消费者模型设计，那么先搞清楚谁是生产者，消费者内容

SingleThreadEventLoop 实现

public abstract class SingleThreadEventLoop extends SingleThreadEventExecutor implements EventLoop {
    private final Queue<Runnable> tailTasks;

    @Override
    protected void afterRunningAllTasks() {
        runAllTasksFrom(tailTasks);
    }
}

SingleThreadEventLoop是个抽象类，从实现代码上看出很简单的逻辑边界判断

SingleThreadEventExecutor也是个抽象类，代码量比较大，我们先看重要的成员属性

public abstract class SingleThreadEventExecutor extends AbstractScheduledEventExecutor implements OrderedEventExecutor {
    //事件队列
    private final Queue<Runnable> taskQueue;
    //执行事件线程，可以看出只有一个线程只要用来记录executor的当前线程
    private volatile Thread thread;
    //主要负责监控该线程的生命周期，提取出当前线程然后用thread记录
    private final Executor executor;
    //用Atomic*技术记录当前线程状态
    private static final AtomicIntegerFieldUpdater<SingleThreadEventExecutor> STATE_UPDATER =
                AtomicIntegerFieldUpdater.newUpdater(SingleThreadEventExecutor.class, "state");
}

//启动线程做了比较判断
private void startThread() {
    if (STATE_UPDATER.get(this) == ST_NOT_STARTED) {
        if (STATE_UPDATER.compareAndSet(this, ST_NOT_STARTED, ST_STARTED)) {
            doStartThread();
        }
    }
}

private void doStartThread() {
    executor.execute(new Runnable() {
        @Override
        public void run() {
            //记录当前执行线程
            thread = Thread.currentThread();
            if (interrupted) {
                thread.interrupt();
            }

            boolean success = false;
            updateLastExecutionTime();
            try {
                //这里调用的是子类，注意子类是死循环不停的执行任务
                SingleThreadEventExecutor.this.run();
                success = true;
            } catch (Throwable t) {
                logger.warn("Unexpected exception from an event executor: ", t);
            } finally {
                //更改线程结束状态 省略部分代码
                for (;;) {
                    int oldState = STATE_UPDATER.get(SingleThreadEventExecutor.this);
                    if (oldState >= ST_SHUTTING_DOWN || STATE_UPDATER.compareAndSet(
                            SingleThreadEventExecutor.this, oldState, ST_SHUTTING_DOWN)) {
                        break;
                    }
                }
                try {
                    // 执行未完成任务同 shutdown hooks.
                    for (;;) {
                        if (confirmShutdown()) {
                            break;
                        }
                    }
                } finally {
                    try {
                        //最后清理操作，如 NioEventLoop实现 selector.close();
                        cleanup();
                    } finally {
                        //省略部分代码
                    }
                }
            }
        }
    });
}

protected final boolean runAllTasksFrom(Queue<Runnable> taskQueue) {
    Runnable task = pollTaskFrom(taskQueue);
    if (task == null) {
        return false;
    }
    for (;;) {
        //安全执行任务
        safeExecute(task);
        //继续执行剩余任务
        task = pollTaskFrom(taskQueue);
        if (task == null) {
            return true;
        }
    }
}

protected final Runnable pollTaskFrom(Queue<Runnable> taskQueue) {
    for (;;) {
        Runnable task = taskQueue.poll();
        //忽略WAKEUP_TASK类型任务
        if (task == WAKEUP_TASK) {
            continue;
        }
        return task;
    }
}

protected boolean runAllTasks(long timeoutNanos) {
    //先执行周期任务
    fetchFromScheduledTaskQueue();
    //从taskQueue提一个任务，如果为空执行所有tailTasks
    Runnable task = pollTask();
    //如果taskQueue没有任务，立即执行子类的tailTasks
    if (task == null) {
         afterRunningAllTasks();
        return false;
    }
    //计算出超时时间 = 当前 nanoTime + timeoutNanos
    final long deadline = ScheduledFutureTask.nanoTime() + timeoutNanos;
    long runTasks = 0;
    long lastExecutionTime;
    for (;;) {
        safeExecute(task);

        runTasks ++;
        //当执行任务次数大于64判断是否超时，防止长时间独占CPU
        if ((runTasks & 0x3F) == 0) {
            lastExecutionTime = ScheduledFutureTask.nanoTime();
            if (lastExecutionTime >= deadline) {
                break;
            }
        }

        task = pollTask();
        if (task == null) {
            lastExecutionTime = ScheduledFutureTask.nanoTime();
            break;
        }
    }

    afterRunningAllTasks();
    this.lastExecutionTime = lastExecutionTime;
    return true;
}

//SingleThreadEventLoop run 实现
public class DefaultEventLoop extends SingleThreadEventLoop {

    @Override
    protected void run() {
        for (;;) {
            Runnable task = takeTask();
            if (task != null) {
                task.run();
                updateLastExecutionTime();
            }

            if (confirmShutdown()) {
                break;
            }
        }
    }
}

我们可以在SingleThreadEventExecutor  两个runAllTasks 方法打上断点，看执行任务时调用逻辑

 本人为了搞清楚 taskQueue 同tailTasks 类型任务，在任务入队时打断点，分别为 SingleThreadEventLoop executeAfterEventLoopIteration方法同 SingleThreadEventExecutor offerTask方法

ServerBootstrap[bind address] ->

NioEventLoopGroup [register Channel] ->  [ChannelPromise] ->

NioEventLoop [build and push register task]

从调用链可以清晰看出，启动 netty server 绑定生成抽象 Channel 然后l转换成ChannelPromise，再调用注册实现register0

这里用了判断是否为当前线程，如果是不用加入队列马上执行，目前减少上下文切换开削

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

总结：

1.SingleThreadEventLoop 任务执行加了超时限制，目的防止当前线程长时间执行任务独占cpu

2.提交任务时做了减少上下文开削优化

3.执行任务优先级 1.周期任务 2.taskQueue 3.tailTasks

目前没有看到任何调用 SingleThreadEventLoop executeAfterEventLoopIteration 方法，估计是扩展处理。

4.用到Atomic*技术解决并发问题，从Executor提取当前线程，把单一线程维护交给Executor