ThreadPoolExecutor作为多线程开发的核心类，也是各大厂的面试必问题目，身为一个Javaer，是有必要去详细的了解一下他的运行原理的。
ThreadPoolExecutor的构造方法源码（看不懂英文的同学出门右转）：
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}几个重要的参数的含义是必须要掌握的，本文不再一一叙述，下面主要介绍一下通过ThreadPoolExecutor是如何创建线程的。
首先看一下几个比较重要的全局变量与方法：
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }ThreadPoolExecutor是通过ctl变量来保存当前的线程池的一些信息的，ctl是一个原子整型的变量，保证了其独有的原子性，实际保存的是一个32位的二进制数（在win平台上，下文中只以32位为基准），保存的信息如下图：
几个比较重要的全局变量如下：
2个常量：
- COUNT_BITS：整型变量的长度减3，用于移位用，可以保证数字的前3位用来标识状态。
- CAPACITY：容量，用于保存当前线程池中的线程数量，是一个前3位为0，后面全部29位全部为1的数字（0001 1111 1111 1111 1111 1111 1111 1111）。这样其他的数字和他做与操作时，前3位保证为0，后29位保留原来的数字。
5个状态常量：RUNING < SHUTDOWN < STOP < TIDYING < TERMINATED，通过左移位29位，空位补0的方式，保证一个前3位的状态标识。
3个常用方法：
- runStateOf：参数通常为当前ctl的值（通过ctl.get()获取），然后让其与~CAPACITY（1110 0000 0000 0000 0000 0000 0000 0000）做&运算，结果为前3位保留原有值，后29位为0，得出状态信息。
- workerCountOf：同上，不过与CAPACITY（0001 1111 1111 1111 1111 1111 1111 1111）做&运算，结果为前3位为0，后29位为原值，得出线程数量信息。
- ctlOf：参数为runState（后29位都为0）、workerCount（前3位都为0），进行或操作后，前3位保留runState的值，后29位保留workerCount的值，即为当前线程池的全部状态信息。
后续的操作中，会经常性的出现以上的变量及方法，请各位谨记。
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