synchronized lock grant order when multiple thread ask for it?

Question

When multiple thread try to acquire the lock on synchronized when lock is already acquired by one thread. My understanding was that lock will be given in order of acquire lock request.

But as per the book O'Reilly Java threads lock will be given that is best for platform. Thats very abstract statement.I think platform mainly meant OS here. My question what is the criteria based on which JVM decides what is best for platform and how developer accounts it while doing programming?

Update:- i know i can use Lock object with fairness argument. But just want to know how does it work with synchronized locks?

why don't you manage on your own the locks to be sure everything works as you want? Look for tutorials on java Lock object — piacente.cristian, Feb 28 '14 at 17:06
Is this a generic question or are you interested in enforcing a lock acquisition ordering? — Giovanni Botta, Feb 28 '14 at 17:22
Just a warning: fair locking comes with a performance penalty. There's added overhead to it. — David Ehrmann, Feb 28 '14 at 17:45

score 2 · Accepted Answer · answered Feb 28 '14 at 17:21

2

synchronized acquisition follows a non-fair lock policy. That is, threads that enter first while blocking may not be the first to acquire. If you want a fair lock use a new ReentrantLock(true)

answered Feb 28 '14 at 17:21

John Vint

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Semaphore also allows fair locking, but ReentrantLock is a better replacement for synchronized. – David Ehrmann Feb 28 '14 at 17:44

Enno Shioji · Answer 2 · 2014-02-28T17:38:27.327

As of JDK6 (in HotSpot JVMs), it uses an algorithm called biased locking. Have a look at this white paper by Oracle, esp. the section on biased locking. They cite this paper that further describes the details of the algorithm.

As for how the developer should account for this, IMO the only important part is that it's unfair. You prob. never have to worry about anything else unless you are coding a high frequency trading platform or something.

You should generally favor unfair locking against fair locking unless you have a reason, as the former typically has higher throughput.

score 0 · Answer 3 · answered Feb 28 '14 at 17:34

My understanding was that lock will be given in order of acquire lock request.

I believe that's only true for green threads (which no one really uses anymore).

My question what is the criteria based on which JVM decides what is best for platform and how developer accounts it while doing programming?

I don't think the JVM "decides" at runtime. The threading model will just be compiled into the JVM.

synchronized lock grant order when multiple thread ask for it?

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