Java线程池ThreadPoolExecuter：execute（）原理⼀、线程池执⾏任务的流程
1. 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。
2. 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。
3. 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。
4. 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 = ();
    //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 = ();
}
    　//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 = ();
        //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 = ();
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 = ();  // Re-rea
d 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 = ();
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;
}
}
java线程池创建的四种
} 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) {
}
}
}
}

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