线程池之ThreadPoolExecutor源码解析

Posted 2022-10-02 akaneblog

1.变量

ThreadPoolExecutor先定义了这几个常量，初看时一脸懵逼，其实它就是用int的二进制高三位来表示线程池的状态，

先回顾一下位运算：

1. <<’左移：右边空出的位置补0，其值相当于乘以2。
2. ‘>>’右移：左边空出的位，如果是正数则补0，若为负数则补0或1，取决于所用的计算机系统OS X中补1。其值相当于除以2。
3. 负数二进制由它的绝对值取反后加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   再减1=00011111 11111111 11111111 11111111
　　 // 1的二进制是001，取反是110，再加1是111,111就是-1的二进制，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