一、线程池执行任务的流程
如果线程池工作线程数<corePoolSize，创建新线程执行task，并不断轮训t等待队列处理task。
如果线程池工作线程数>=corePoolSize并且等待队列未满，将task插入等待队列。
如果线程池工作流程数>=corePoolSize并且等待队列已满，且工作线程数<maximumPoolSize,创建新线程执行task。
如果线程池工作流程数>=corePoolSize并且等待队列已满，且工作线程数=maximumPoolSize,执行拒绝策略。
二、execute()原理
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.
* 如果运行的线程数小于corePoolSize，尝试创建一个新线程（Worker），并执行它的第一个任务command
* 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.
* 如果task成功插入等待队列，我们仍需要进行双重校验是否可以成功添加一个线程
（因为有的线程可能在我们上次检查以后已经死掉了）或者在我们进入这个方法后线程池已经关闭了
* 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.
如果等待队列已满，我们尝试新创建一个线程。如果创建失败，我们知道线程已关闭或者已饱和，因此我们拒绝改任务。
*/
int c = ctl.get();
//工作线程小于核心线程数，创建新的线程
if (workerCountOf(c) < corePoolSize) {
//创建新的worker立即执行command，并且轮训workQueue处理task
if (addWorker(command, true))
return;
c = ctl.get();
}
//线程池在运行状态且可以将task插入队列
//第一次校验线程池在运行状态
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//第二次校验，防止在第一次校验通过后线程池关闭。如果线程池关闭，在队列中删除task并拒绝task
if (! isRunning(recheck) && remove(command))
reject(command);
//如果线程数=0（线程都死掉了，比如：corePoolSize=0），新建线程且未指定firstTask，仅仅去轮训workQueue
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//线程队列已满，尝试创建新线程执行task，创建失败后拒绝task
//创建失败原因：1.线程池关闭；2.线程数已经达到maxPoolSize
else if (!addWorker(command, false))
reject(command);
}
1. addWorker(Runnable firstTask, boolean core)
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
//外层循环判断线程池的状态
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);//线程池状态
// Check if queue empty only if necessary.
//线程池状态:RUNNING = -1、SHUTDOWN = 0、STOP = 1、TIDYING = 2、TERMINATED = 3
//线程池至少是shutdown状态
if (rs >= SHUTDOWN &&
//除了线程池正在关闭（shutdown），队列里还有未处理的task的情况，其他都不能添加
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
//内层循环判断是否到达容量上限，worker+1
for (;;) {
int wc = workerCountOf(c);//worker数量
//worker大于Integer最大上限
//或到达边界上限
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//CAS worker+1
if (compareAndIncrementWorkerCount(c))
break retry;//成功了跳出循环
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs) //如果线程池状态发生变化，重试外层循环
continue retry;
// else CAS failed due to workerCount change; retry inner loop
// CAS失败workerCount被其他线程改变，重新尝试内层循环CAS对workerCount+1
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
final ReentrantLock mainLock = this.mainLock;
w = new Worker(firstTask); //1.state置为-1，Worker继承了AbstractQueuedSynchronizer
//2.设置firstTask属性
//3.Worker实现了Runable接口，将this作为入参创建线程
final Thread t = w.thread;
if (t != null) {
//addWorker需要加锁
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int c = ctl.get();
int rs = runStateOf(c);
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);//workers是HashSet<Worker>
//设置最大线程池大小
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
addWorker(Runnable firstTask, boolean core)
参数：
firstTask： worker线程的初始任务，可以为空
core： true：将corePoolSize作为上限，false：将maximumPoolSize作为上限
addWorker方法有4种传参的方式：
1、addWorker(command, true)
2、addWorker(command, false)
3、addWorker(null, false)
4、addWorker(null, true)
在execute方法中就使用了前3种，结合这个核心方法进行以下分析
1、线程数小于corePoolSize。判断workers(HashSet<Worker>)大小，如果worker数量>=corePoolSize 返回false，否则创建worker添加到workers，并执行worker的run方法（执行firstTask并轮询tworkQueue）；
2、线程数大于corePoolSize且workQueue已满。如果worker数量>=maximumPoolSize返回false，否则创建worker添加到workers，并执行worker的run方法（执行firstTask并轮询tworkQueue）；
3.、没有worker存活，创建worker去轮询workQueue，长度限制maximumPoolSize。
4、prestartAllCoreThreads()方法调用，启动所有的核心线程去轮询workQueue。因为addWorker是需要上锁的，预启动核心线程可以提高执行效率。
2. ThreadPoolExecutor 内部类Worker
/**
* Class Worker mainly maintains interrupt control state for
* threads running tasks, along with other minor bookkeeping.
* This class opportunistically extends AbstractQueuedSynchronizer
* to simplify acquiring and releasing a lock surrounding each
* task execution. This protects against interrupts that are
* intended to wake up a worker thread waiting for a task from
* instead interrupting a task being run. We implement a simple
* non-reentrant mutual exclusion lock rather than use
* ReentrantLock because we do not want worker tasks to be able to
* reacquire the lock when they invoke pool control methods like
* setCorePoolSize. Additionally, to suppress interrupts until
* the thread actually starts running tasks, we initialize lock
* state to a negative value, and clear it upon start (in
* runWorker).
* 1.Worker类主要负责运行线程状态的控制。
* 2.Worker继承了AQS实现了简单的获取锁和释放所的操作。来避免中断等待执行任务的线程时，中断正在运行中的线程（线程刚启动，还没开始执行任务）。
* 3.自己实现不可重入锁，是为了避免在实现线程池控状态控制的方法，例如：setCorePoolSize的时候中断正在开始运行的线程。
* setCorePoolSize可能会调用interruptIdleWorkers()，该方法中会调用worker的tryLock()方法中断线程，自己实现锁可以确保工作线程启动之前不会被中断
*/
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L; /** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** Per-thread task counter */
volatile long completedTasks; /**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker //状态置为-1，如果中断线程需要CAS将state 从0->1，以此来保证能只中断从workerQueue getTask的线程
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
} /** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this); //首先执行w.unlock,就是把state置为0，对该线程的中断就可以进行了
} // Lock methods
//
// The value 0 represents the unlocked state.
// The value 1 represents the locked state. protected boolean isHeldExclusively() {
return getState() != 0;
}
//在setCorePoolSize/shutdown等方法中断worker线程时需要调用该方法，确保中断的是从workerQueue getTask的线程
protected boolean tryAcquire(int unused) {
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
} protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
setState(0);
return true;
} public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); } //调用tryRelease修改state=0，LockSupport.unpark(thread)下一个等待锁的线程
public boolean isLocked() { return isHeldExclusively(); } void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}