关于线程池创建、执行、销毁的原理及分析

private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();//获取当前线程池的控制状态。
            int rs = runStateOf(c);//获取当前线程池的运行状态。

            // Check if queue empty only if necessary.
            //下面这个条件判断用于判断线程池是否处于关闭状态，并且任务队列不为空。如果线程池的状态大于等于SHUTDOWN，并且不满足线程池状态为SHUTDOWN、首个任务为null且任务队列为空的条件，则返回false。这个判断是为了确保在线程池关闭时，不再添加新的工作线程。
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);//获取当前线程池中的工作线程数量。
                //这个条件判断用于判断工作线程数量是否达到上限。如果工作线程数量大于等于CAPACITY（工作线程数量的上限）或者大于等于核心线程数（如果core为true）或最大线程数（如果core为false），则返回false。这个判断是为了确保工作线程数量不超过线程池的限制。
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                    //尝试通过CAS（比较并交换）操作增加工作线程数量。如果成功增加工作线程数量，则跳出循环，继续执行后续逻辑。
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // 重新读取当前线程池的控制状态。
                //再次判断线程池的运行状态是否发生了变化。如果运行状态发生了变化，则继续重试内部循环。这个判断是为了处理在CAS操作过程中，线程池的状态发生了变化的情况。
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }
        ...创建线程的逻辑忽略...
}

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            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 {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        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 {
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?

        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            int wc = workerCountOf(c);

            // Are workers subject to culling?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
			//销毁线程需要满足这两个条件：1. （允许核心线程销毁 || 线程数大于核心线程数）&& 达到了销毁时间；2. 任务队列中没有任务了
            if ((wc > maximumPoolSize || (timed && timedOut)) 
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                // 这里返回了null，外层方法就跳出了while循环，从而结束该线程
                    return null;
                continue;
            }

            try {
            // 超时时间就是在这里设置的，如果允许超时销毁，那么就用poll进行拉取任务，超过了keepAliveTime就返回null。take是阻塞性等待
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }