以下是

Java Concurrency in Practice关于该主题的内容：

The JVM can implement blocking either via spin-waiting (repeatedly

trying to acquire the lock until it succeeds) or bysuspending the

blocked thread through the operating system. Which is more efficient

depends on the relationship between context switch overhead and the

time until the lock becomes available; spin-waiting is preferable for

short waits and suspension is preferable for long waits. Some JVMs

choose between the two adaptively based on profiling data of past wait

times, but most just suspend threads waiting for a lock.

而且(也就是IMO,最重要的一点)：

Don’t worry excessively about the cost of uncontended synchronization. The basic mechanism is already quite fast, and JVMs can perform additional optimizations that further reduce or eliminate the cost. Instead, focus optimization efforts on areas where lock contention actually occurs.