我试图了解ThreadPoolExecutor类,发现该类中声明了一些最终变量,无法理解其用法。

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;    //536870911     00011111111111111111111111111111

// RUN_STATE is stored in the high-order bits
private static final int RUNNING    = -1 << COUNT_BITS;         //-536870912    11100000000000000000000000000000
private static final int SHUTDOWN   =  0 << COUNT_BITS;         //0             00000000000000000000000000000000
private static final int STOP       =  1 << COUNT_BITS;         //536870912     00100000000000000000000000000000
private static final int TIDYING    =  2 << COUNT_BITS;         //1073741824    01000000000000000000000000000000
private static final int TERMINATED =  3 << COUNT_BITS;         //1610612736    01100000000000000000000000000000

以上是最终变量及其二进制和十进制值。

然后我发现了使用这些变量的两种方法:
private static int runStateOf(int c)     { return c & ~CAPACITY; }  // RUN_STATE & ~CAPACITY = RUN_STATE
private static int workerCountOf(int c)  { return c & CAPACITY; }   // RUN_STATE & CAPACITY = 0
private static int ctlOf(int rs, int wc) { return rs | wc; }

方法之前的评论是我观察到的输出。

现在在ThreadPoolExecutor#execute(runnable)方法中,

它正在使用If fewer than corePoolSize threads are running语句进行以下计算
int c = ctl.get();
if (workerCountOf(c) < corePoolSize)

我试图理解,在这种情况下,workerCountOf(c)的值可以大于corePoolSize。
如您所见,ctl的初始值为RUNNING。

此外,还有一些方法可以原子地递增和递减ctl值,
private boolean compareAndIncrementWorkerCount(int expect) {
    return ctl.compareAndSet(expect, expect + 1);
}

private boolean compareAndDecrementWorkerCount(int expect) {
    return ctl.compareAndSet(expect, expect - 1);
}

现在假设有5个线程正在运行,所以ctl = RUNNING + 5

即使是workerCountOf(ctl.get()) = 0

作为((RUNNING+5) & CAPACITY) = 0

谁能解释我创建这些最终变量的原因及其用途?

workerCountOf()方法实际上如何不返回正在运行的线程?

我肯定错过了什么。

谢谢

最佳答案

如您所见,Java使用int ctl字段存储池的当前状态和线程数。状态存储在高三位中,所有其他位用于存储线程数。按位掩码CAPACITY用于将它们彼此分开:

  • CAPACITY = 00011111111111111111111111111111
  • ~CAPACITY = 11100000000000000000000000000000

  • 因此,
  • ctl & CAPACITY保留低29位,并将高3位清零;结果是当前线程数
  • ctl & ~CAPACITY保留较高的三位并将所有其他零清零。结果是池运行状态

  • 正如您正确地注意到的那样,具有五个线程的运行池具有ctl = (RUNNING + 5),其二进制表示形式111000...000101。因此,应用CAPACITY掩码会将最高的三个位清零,并为您提供000000...000101值,即5,而不是0。

    10-06 02:07