Thread.yield 和 Thread.sleep 是没有任何同步语义的

http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html 17.3

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Thread.sleep causes the currently executing thread to sleep (temporarily cease execution) for the specified duration, subject to the precision and accuracy of system timers and schedulers. The thread does not lose ownership of any monitors, and resumption of execution will depend on scheduling and the availability of processors on which to execute the thread.

It is important to note that neither Thread.sleep nor Thread.yield have any synchronization semantics. In particular, the compiler does not have to flush writes cached in registers out to shared memory before a call to Thread.sleep or Thread.yield, nor does the compiler have to reload values cached in registers after a call to Thread.sleep or Thread.yield.

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For example, in the following (broken) code fragment, assume that this.done is a non-volatile boolean field:

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while (!this.done)
    Thread.sleep(1000);

The compiler is free to read the field this.done just once, and reuse the cached value in each execution of the loop. This would mean that the loop would never terminate, even if another thread changed the value of this.done.