Does thread.yield() lose the lock on object if cal

问题:

I understand that Thread.currentThread().yield() is a notification to thread scheduler that it may assign cpu cycle to some other thread of same priority if any such is present. My question is: If current thread has got lock on some object and calls yield(), will it loses that lock right away? And when thread scheduler finds out there is no such thread to assign cpu cycle, then the thread which has called yield() will again be in fight to get lock on the object which it has lost earlier??

I couldn't find it in javadoc and forums [http://www.coderanch.com/t/226223/java-programmer-SCJP/certification/does-sleep-yield-release-lock] have 50-50 answers.

I think yield() (lets say thread1) should release lock because if some thread (lets say thread2) of same priority wants to operate on same object, then it can have chance when thread scheduler eventually assign cup to thread2.

回答1:

No. Thread.yield() is not like Object.wait(). It just gives up control to allow a thread switch. It will have no effect on the concurrency of your program.

There is no guarantee which thread the scheduler will run after a yield.

回答2:

In Java Language specification
17.3 Sleep and Yield
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.

My comment:

In java's early days, when it did not really supported parallel executions, but only concurrent (green threads), yield() was suspending the current thread, and the jvm was picking up another thread to resume. Now-days, yield does not have much meaning as usually the tread scheduling is on OS level.

So, yield is just a hint to the JVM that current thread wants to take a rest and nothing else, it is up to the thread scheduler to decide what to do. yield does not have any synchronization semantic. If thread holds lock, it will continue to hold it.

回答3:

Only wait methods of the Object class release the intrinsic lock of the current instance (the thread may have other locks acquired, they don't get released). Yield, sleep, join do not bother about locks. However, join is a little more special, you are guaranteed to see all the changes made by the thread you're waiting for to finish.