Java 线程池系列-第三篇

第三篇我们要来讲下 Worker 这个“打工人”，线程池里实际在干活的同学
Worker 实现了 Runnable 接口，所以在前面介绍的 addWorker 中
线程启动其实就调用了下面的 run 方法，而 run 方法则调用了内部的
runWorker 方法

private final class Worker
    extends AbstractQueuedSynchronizer
    implements Runnable
{
    /**
     * This class will never be serialized, but we provide a
     * serialVersionUID to suppress a javac warning.
     */
    private static final long serialVersionUID = 6138294804551838833L;

/** Thread this worker is running in.  Null if factory fails. */
    final Thread thread;
    /** Initial task to run.  Possibly null. */
    Runnable firstTask;
    /** Per-thread task counter */
    volatile long completedTasks;

/**
     * Creates with given first task and thread from ThreadFactory.
     * @param firstTask the first task (null if none)
     */
    Worker(Runnable firstTask) {
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }

/** Delegates main run loop to outer runWorker  */
    // 启动线程 Worker
    public void run() {
        runWorker(this);
    }

介绍下这个 runWorker 方法
这里就会先处理 firstTask，如果 firstTask 是 null 则会去队列里取
里面会有两个钩子，一个是执行前，一个是执行后，注意这个循环是没有常规
退出逻辑的，说明是一直在往队列里获取，除非获取队列里的任务超时失败了
这样就到最后会去执行 processWorkerExit

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            // firstTask 不为空或者从队列里获取到任务
            while (task != null || (task = getTask()) != null) {
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    // 前置执行钩子
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        // 任务执行
                        task.run();
                    } catch (RuntimeException x) {
                        thrown = x; throw x;
                    } catch (Error x) {
                        thrown = x; throw x;
                    } catch (Throwable x) {
                        thrown = x; throw new Error(x);
                    } finally {
                        // 执行完后的钩子
                        afterExecute(task, thrown);
                    }
                } finally {
                    // 最后会执行到这，清空任务
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            // 循环的最后就会去退出Worker
            processWorkerExit(w, completedAbruptly);
        }
    }

而这个 processWorkerExit 也有个比较特殊的逻辑
照理我们会想到这里可能就是线程在处理完队列里的任务
以后，就会判断下是否要退出，如果是核心线程的话就不用
如果是非核心线程就会退出了，但是这里其实有个替换逻辑
就是最后有个 addWorker 调用

private void processWorkerExit(Worker w, boolean completedAbruptly) {
    if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
        decrementWorkerCount();

final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        completedTaskCount += w.completedTasks;
        workers.remove(w);
    } finally {
        mainLock.unlock();
    }

tryTerminate();

int c = ctl.get();
    // 判断状态
    if (runStateLessThan(c, STOP)) {
        if (!completedAbruptly) {
            // 核心线程是否也需要退出
            int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
            if (min == 0 && ! workQueue.isEmpty())
                min = 1;
            // 这里代表非核心线程就可以不用替换，也就是回收线程了
            if (workerCountOf(c) >= min)
                return; // replacement not needed
        }
        // 否则是替换线程
        addWorker(null, false);
    }
}

这里主要讲了线程 Worker 运行的逻辑