public void execute(Runnable command) { 
    // 如果传入的Runnable的空，就抛出异常
    if (command == null) 
        throw new NullPointerException();
    int c = ctl.get();
    
    if (workerCountOf(c) < corePoolSize) { 
        // 当前线程数小于核心线程数
        if (addWorker(command, true))
            // 增加线程成功 执行结束 
            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);
}