ThreadPoolExecutor关于Execute方法

ThreadPoolExecutor的两个核心集合：
一个是持有当前可用线程的集合HashSet<Worker> workers，
另一个是待执行的task的集合BlockingQueue<Runnable> workQueue

线程池之所以成为线程池是，我们在试图运行一个具体的task时，我们没有调用Thread.start()方法，而是通过worker线程对象来调用task中的run()方法。
注意start()会开启一个新的线程，而run()是运行在当前线程中的。

Execute()方法的逻辑

当我们调用ThreadPoolExecutor中的Execute方法时，线程池会对当前自身状态做出判断来决定是否创建新的worker来立即执行task，或者是将task放置在workQueue队列中。
Execute代码片段：

    int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);

在addWorker方法中，我们创建了一个封装task的worker对象。Worker内部类是一个Runnable接口的实现，addWorker创建完worker后，会启动worker自身持有的thread属性的start()方法来允许worker的run()方法。

        if (t != null) {
                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 rs = runStateOf(ctl.get());

                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }

而Worker类中run()方法调用的是ThreadPoolExecutor的runWorker(Worker w)方法，通过不断的查看持有task的集合workQueue中是否还有未执行的task来复用当前的worker线程。

        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();
                }
            }

关于processWorkerExit()方法的疑惑

在runWorker(Worker w)方法的最后，调用了processWorkerExit()方法。
这个方法的Javadoc是：

     * Performs cleanup and bookkeeping for a dying worker. Called
     * only from worker threads. Unless completedAbruptly is set,
     * assumes that workerCount has already been adjusted to account
     * for exit.  This method removes thread from worker set, and
     * possibly terminates the pool or replaces the worker if either
     * it exited due to user task exception or if fewer than
     * corePoolSize workers are running or queue is non-empty but
     * there are no workers.

其中将当前的worker()移除了workers集合，随之又创建一个idle的worker：

    if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
            decrementWorkerCount();

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

        tryTerminate();

        int c = ctl.get();
        if (runStateLessThan(c, STOP)) {
            if (!completedAbruptly) {
                int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
                if (min == 0 && ! workQueue.isEmpty())
                    min = 1;
                if (workerCountOf(c) >= min)
                    return; // replacement not needed
            }
            addWorker(null, false); // 创建一个idle线程
        }

那为什么要把原来的worker移走呢？
我暂时的理解是，因为worker执行完一个task后，它的状态为terminated，不可能再转变成了runnable，所以我们需要用一个idle的worker去替代它。

那么问题又来了，idle的worker在addWorker中又会被启动，如果它什么都不做，难道又会在绕进processWorkerExit()方法中被另一个idle的worker替代？

有待思考。