/**
 * The main pool control state, ctl, is an atomic integer packing
 * two conceptual fields
 *   workerCount, indicating the effective number of threads
 *   runState,    indicating whether running, shutting down etc
 *
 * In order to pack them into one int, we limit workerCount to
 * (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2
 * billion) otherwise representable. If this is ever an issue in
 * the future, the variable can be changed to be an AtomicLong,
 * and the shift/mask constants below adjusted. But until the need
 * arises, this code is a bit faster and simpler using an int.
 *
 * The workerCount is the number of workers that have been
 * permitted to start and not permitted to stop.  The value may be
 * transiently different from the actual number of live threads,
 * for example when a ThreadFactory fails to create a thread when
 * asked, and when exiting threads are still performing
 * bookkeeping before terminating. The user-visible pool size is
 * reported as the current size of the workers set.
 *
 * The runState provides the main lifecycle control, taking on values:
 *
 *   RUNNING:  Accept new tasks and process queued tasks
 *   SHUTDOWN: Don‘t accept new tasks, but process queued tasks
 *   STOP:     Don‘t accept new tasks, don‘t process queued tasks,
 *             and interrupt in-progress tasks
 *   TIDYING:  All tasks have terminated, workerCount is zero,
 *             the thread transitioning to state TIDYING
 *             will run the terminated() hook method
 *   TERMINATED: terminated() has completed
 *
 * The numerical order among these values matters, to allow
 * ordered comparisons. The runState monotonically increases over
 * time, but need not hit each state. The transitions are:
 *
 * RUNNING -> SHUTDOWN
 *    On invocation of shutdown(), perhaps implicitly in finalize()
 * (RUNNING or SHUTDOWN) -> STOP
 *    On invocation of shutdownNow()
 * SHUTDOWN -> TIDYING
 *    When both queue and pool are empty
 * STOP -> TIDYING
 *    When pool is empty
 * TIDYING -> TERMINATED
 *    When the terminated() hook method has completed
 *
 * Threads waiting in awaitTermination() will return when the
 * state reaches TERMINATED.
 *
 * Detecting the transition from SHUTDOWN to TIDYING is less
 * straightforward than you‘d like because the queue may become
 * empty after non-empty and vice versa during SHUTDOWN state, but
 * we can only terminate if, after seeing that it is empty, we see
 * that workerCount is 0 (which sometimes entails a recheck -- see
 * below).
 */
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; }

RUNNING    -- 对应的高3位值是111。
SHUTDOWN   -- 对应的高3位值是000。
STOP       -- 对应的高3位值是001。
TIDYING    -- 对应的高3位值是010。
TERMINATED -- 对应的高3位值是011。

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));