public void execute(Runnable command) {

        if (command == null)

            throw new NullPointerException();

        /*

         * Proceed in 3 steps:

         *

         * 1. If fewer than corePoolSize threads are running, try to

         * start a new thread with the given command as its first

         * task.  The call to addWorker atomically checks runState and

         * workerCount, and so prevents false alarms that would add

         * threads when it shouldn't, by returning false.

         *

         * 2. If a task can be successfully queued, then we still need

         * to double-check whether we should have added a thread

         * (because existing ones died since last checking) or that

         * the pool shut down since entry into this method. So we

         * recheck state and if necessary roll back the enqueuing if

         * stopped, or start a new thread if there are none.

         *

         * 3. If we cannot queue task, then we try to add a new

         * thread.  If it fails, we know we are shut down or saturated

         * and so reject the task.

         */

        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);

    }

    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);

        }

    }

        /**

         * Sets the next time to run for a periodic task.

         */

        private void setNextRunTime() {

            long p = period;

            if (p > 0)

                time += p;

            else

                time = triggerTime(-p);

        }

    void reExecutePeriodic(RunnableScheduledFuture<?> task) {

        if (canRunInCurrentRunState(true)) {
　　　　　　　//此处没有捕获异常，如果单个任务发生异常，会导致后续任务无法继续执行

            super.getQueue().add(task);

            if (!canRunInCurrentRunState(true) && remove(task))

                task.cancel(false);

            else

                ensurePrestart();

        }

    }