線程池之ThreadPoolExecutor源碼解析

1.變量

ThreadPoolExecutor先定義了這幾個常量，初看時一臉懵逼，其實它就是用int的二進制高三位來表示線程池的狀态，

先回顧一下位運算：

1. <<’左移：右邊空出的位置補0，其值相當于乘以2。
2. ‘>>’右移：左邊空出的位，如果是正數則補0，若為負數則補0或1，取決于所用的計算機系統OS X中補1。其值相當于除以2。
3. 負數二進制由它的絕對值取反後加1

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    private static final int COUNT_BITS = Integer.SIZE - 3;// 29
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1; // 1<<29=00100000 00000000 00000000 00000000   再減1=00011111 11111111 11111111 11111111
　　 // 1的二進制是001，取反是110，再加1是111,111就是-1的二進制，再左移29，
    private static final int RUNNING    = -1 << COUNT_BITS;// 11100000 00000000 00000000 00000000
    private static final int SHUTDOWN   =  0 << COUNT_BITS;// 00000000 00000000 00000000 00000000
    private static final int STOP       =  1 << COUNT_BITS;// 00100000 00000000 00000000 00000000
    private static final int TIDYING    =  2 << COUNT_BITS;// 01000000 00000000 00000000 00000000
    private static final int TERMINATED =  3 << COUNT_BITS;// 01100000 00000000 00000000 00000000