思考一个问题：
线程池状态：核心线程数为 2，最大线程数为2，阻塞队列为有界队列长度为3
在使用线程池添加一个任务时，如果此时：阻塞队列为满，且核心线程的两个数同时完成，那此时任务会直接用核心线程执行吗？
一、线程池的核心参数：
- corePoolSize，核心线程数
- maximumPoolSize，最大线程数
- keeAliveTime，空闲存活时间
- unit，空闲存活时间单位
- workQueue，工作队列
- threadFactory，线程工场
- handle，拒绝策略
二、线程池的状态（占用ctl的前三位）
- RUNNING（-1<<29）
111；运行状态，线程池创建好之后就会进入此状态，如果不手动调用关闭方法，那么线程池在整个程序运行期间都是此状态。 - SHUTDOWN（0<<29）
000；关闭状态，不再接受新任务提交，但是会将已保存在任务队列中的任务处理完。 - STOP（1<<29）
001；停止状态，不再接受新任务提交，并且会中断当前正在执行的任务、放弃任务队列中已有的任务。 - TIDYING（2<<29）
010；整理状态，所有的任务都执行完毕后（也包括任务队列中的任务执行完），当前线程池中的活动线程数降为 0 时的状态。到此状态之后，会调用线程池的 terminated() 方法。 - TERMINATED（3<<29）
011；销毁状态，当执行完线程池的 terminated() 方法之后就会变为此状态。
状态变化图
三、execute()方法执行逻辑（提交任务）
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) {
//如果当前线程数小于核心线程数，则创建和核心线程并执行任务
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
//如果线程池为运行状态且workQueue未满,将任务加入阻塞队列
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
//如果线程池不是运行状态并且能够从阻塞队列中移除了该任务，则对该任务执行拒绝策略
reject(command);
else if (workerCountOf(recheck) == 0)
//如果工作线程为0则创建工作线程
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
四、addWorker() 方法（添加线程）
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (int c = ctl.get();;) {
// Check if queue empty only if necessary.
//检查线程池状态，如果是shutdown及以上的状态（shutdown，stop，tidying）且
//已停止或者没有任务或者工作队列为空
if (runStateAtLeast(c, SHUTDOWN)
&& (runStateAtLeast(c, STOP)
|| firstTask != null
|| workQueue.isEmpty()))
return false;
for (;;) {
//如果线程数达到核心线程数或者总线程数上限，则返回创建线程失败
if (workerCountOf(c)
>= ((core ? corePoolSize : maximumPoolSize) & COUNT_MASK)
)
return false;
//如果增加线程数成功，跳出retry循环
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
if (runStateAtLeast(c, SHUTDOWN))
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread;
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 c = ctl.get();
if (isRunning(c) ||
(runStateLessThan(c, STOP) && firstTask == null)) {
//如果线程池是运行状态或 处于shutdown状态且无任务
if (t.getState() != Thread.State.NEW)
//如果线程不是NEW状态则抛出异常
throw new IllegalThreadStateException();
//添加工作线程到workers集合并更新当前线程数量
workers.add(w);
workerAdded = true;
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
五、runWork()方法（执行任务）
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())
//如果线程池处于STOP状态或
//当前线程被中断且线程池至少是STOP状态去且工作线程没被中断，则中断工作线程
wt.interrupt();
try {
//任务执行前处理
beforeExecute(wt, task);
try {
task.run();
//任务执行后处理
afterExecute(task, null);
} catch (Throwable ex) {
afterExecute(task, ex);
throw ex;
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
//处理工作线程退出
processWorkerExit(w, completedAbruptly);
}
}
六、getTask()线程获取任务方法
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
// Check if queue empty only if necessary.
if (runStateAtLeast(c, SHUTDOWN)
&& (runStateAtLeast(c, STOP) || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
七、processWorkerExit()处理工作线程退出方法
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
} finally {
mainLock.unlock();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) {
//如果处于shutdown或者running状态
if (!completedAbruptly) {
//allowCoreThreadTimeOut 是否开启核心线程超时等待，默认为false
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
addWorker(null, false);
}
}
