线程可以因为各种各样的原因终止并进入可运行状态(因为堵塞)。
A thread might cease to runnable (that is, because blocked) for a variety of reasons.
当前运行队列中线程的数目。代表线程处于可运行状态，但CPU还未能执行。
The amount of threads in the run queue. These are threads that are runnable, but the CPU is not available to execute them.
一旦被堵塞的线程进入可运行状态，它将会被放回适当的可运行池中。
When a blocked thread becomes runnable, it is placed back into the appropriate runnable pool.
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