简介

看别人的博客, 不清楚为什么会入队.

code
   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 简单来说就是如果小于corePoolSize直接创建新线程
* 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)) { // 反正只要正在运行着, 就会直接入队了. 入队也会有四种判定方式, 一种是直接提交队列, 就是大于corePoolSize, 小于maxPoolSize创建新的线程. 一种是无界队列, 反正来的任务都等着执行, 还有有界, 就是大于corePoolSize, 且大于设定的有界个数, 才会去创建新的线程. 
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);
    }