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。