Java线程池---addWorker方法解析

/*
* Methods for creating, running and cleaning up after workers
/
/*
* Checks if a new worker can be added with respect to current
* pool state and the given bound (either core or maximum). If so,
* the worker count is adjusted accordingly, and, if possible, a
* new worker is created and started running firstTask as its
* first task. This method returns false if the pool is stopped or
* eligible to shut down. It also returns false if the thread
* factory fails to create a thread when asked, which requires a
* backout of workerCount, and a recheck for termination, in case
* the existence of this worker was holding up termination.
*
* @param firstTask the task the new thread should run first (or
* null if none). Workers are created with an initial first task
* (in method execute()) to bypass queuing when there are fewer
* than corePoolSize threads (in which case we always start one),
* or when the queue is full (in which case we must bypass queue).
* Initially idle threads are usually created via
* prestartCoreThread or to replace other dying workers.
*
* @param core if true use corePoolSize as bound, else
* maximumPoolSize. (A boolean indicator is used here rather than a
* value to ensure reads of fresh values after checking other pool
* state).
* @return true if successful
*/

以上是addWorker方法的注释，大致意思如下：
该方法是用来创建，运行，清理Workers的。
检查是否一个新的Worker能否能够添加到当前状态以及给定的范围（包括corePoolSize以及maximumSize）的线程池中。如果可以的话，那么Worker的总数会根据添加的Worker来进行调整，并且如果可能的话，一个新的Worker会被创建，并且启动firstTask作为这个Worker的第一个任务。
当该方法返回false的时候，说明这个当前线程处于Stopped状态或者处于shut down状态或者创建线程失败的时候，会返回false。

private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);

        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;

        for (;;) {
            int wc = workerCountOf(c);
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();  // Re-read ctl
            if (runStateOf(c) != rs)
                continue retry;
            // else CAS failed due to workerCount change; retry inner loop
        }
    }

    Worker w = new Worker(firstTask);
    Thread t = w.thread;

    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        // Recheck while holding lock.
        // Back out on ThreadFactory failure or if
        // shut down before lock acquired.
        int c = ctl.get();
        int rs = runStateOf(c);

        if (t == null ||
            (rs >= SHUTDOWN &&
             ! (rs == SHUTDOWN &&
                firstTask == null))) {
            decrementWorkerCount();
            tryTerminate();
            return false;
        }

        workers.add(w);

        int s = workers.size();
        if (s > largestPoolSize)
            largestPoolSize = s;
    } finally {
        mainLock.unlock();
    }

    t.start();
    // It is possible (but unlikely) for a thread to have been
    // added to workers, but not yet started, during transition to
    // STOP, which could result in a rare missed interrupt,
    // because Thread.interrupt is not guaranteed to have any effect
    // on a non-yet-started Thread (see Thread#interrupt).
    if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted())
        t.interrupt();

    return true;
}

1. 获取当前线程池的状态，如果是STOP，TIDYING,TERMINATED状态的话，则会返回false，如果现在状态是SHUTDOWN，但是firstTask不为空或者workQueue为空的话，那么直接返回false。
2. 通过自旋的方式，判断要添加的Worker是否是corePool，如果是的话，那么则判断当前的workerCount是否大于corePoolsize，否则则判断是否大于maximumPoolSize，如果满足的话，说明workerCount超出了线程池大小，直接返回false。如果小于的话，那么判断是否成功将WorkerCount通过CAS操作增加1，如果增加成功的话。则进行到第3步，否则则判断当前线程池的状态，如果现在获取到的状态与进入自旋的状态不一致的话，那么则通过continue retry重新进行状态的判断。
3. 如果满足了的话，那么则创建一个新的Worker对象，然后获取线程池的重入锁后，判断当前线程池的状态，如果当前线程池状态为STOP,TIDYING,TERMINATED的话，那么调用decrementWorkerCount将workerCount减一，然后调用tryTerminate停止线程池，并且返回false。
4. 如果状态满足的话，那么则在workers中将新创建的worker添加，并且重新计算largestPoolSize，然后启动Worker中的线程开始执行任务。
5. 重新Check一次当前线程池的状态，如果处于STOP状态的话，那么就调用interrupt方法中断线程执行。