java.util.concurrent.Executors 创建线程池常用封装类，不过一般不推荐使用。因为很多线程池参数无法定制化。
java.util.concurrent.ThreadPoolExecutor 推荐直接使用线程池创建类。参数构造一目了然。
创建线程池构造参数详解
corePoolSize 核心线程数，只要线程池中有效线程数小于这个值，执行任务时直接创建线程并运行maximumPoolSize 线程池允许创建的最大线程数keepAliveTime 多余空闲线程数最大存活时间，一旦超过直接回收。这里通过阻塞队列的带有超时时间poll方法实现unit 超时时间单位workQueue 工作队列，尚未执行的任务会先丢进工作队列threadFactory 创建线程的工程类，可以自定义工程类实现线程名生成规则的自定义handler 线程解决策略，默认是向外层抛异常处理
allowCoreThreadTimeOut 注，构造器没有对此参数的初始化，默认为false即不对核心线程数进行回收，设置为true即为超过存活时间核心线程数也进行回收线程池主要参数详解
runstate 当前线程池运行状态，有下面几个状态：
running运行状态， 接受新任务和处理队列中的任务
shundown关闭状态，不接收新任务，但是还要处理队列中的任务
stop停止状态， 不接受新任务，也不处理队列中的任务
tidying过渡状态， 所有任务被中断，workcount为0，然后过渡到tidying状态
terminated中止状态 终端状态
workercount 当前线程池有效线程数
ctl 线程池控制状态，默认运行状态，由上面runstate和workercount按位或产生
提交任务主要代码块如下：
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
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);如上分三步走
1. 当前池中线程数少于核心线程数，直接添加任务
2. 线程池处于运行状态下，直接把任务丢进工作队列里，这里有个double check防止线程池不在运行状态。 如果当前线程数没有了即都被回收掉了，重新创建。
3. 工作队列已满，直接创建线程执行任务，若创建失败，执行拒接策略
创建线程任务部分代码
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
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 rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
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;
}
任务在线程池中执行代码：
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())
wt.interrupt();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
这里需要注意的是getTask()方法，主要是从工作队列里取执行任务。
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= 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;
}
}
}
这里需要特别注意， workQueue.poll(keepAliveTime,TimeUnit.NANOSECONDS)和workQueue.take()的使用
线程池关闭操作
shutdown() 尝试关闭，只是尝试获取每个工作线程的锁然后终端线程，并不保证一定会关闭
shutdownNow（） 强制关闭，强制中断线程，但如果线程无法中断，也是无法强制关闭的，一般不会出现此问题
awaitTermination（） 等待线程池进入中止状态。一般上述两个关闭最好在配合此等待方法使用
。
