8. Fair lock and unfair lock of Java lock

Why is the algorithm so difficult 2021-02-23 18:39:48
fair lock unfair lock java


Fair lock : It means that multiple threads acquire locks according to the order in which they apply for locks . It's like queuing up to buy food , first come , first served , First come, first served , It's fair , That's the queue

Not fair lock : The order in which multiple threads acquire locks , It's not in the order of applying for locks . It is possible that the thread that applied first gets the lock before the thread that applied first , In a highly concurrent environment , It may cause priority reversal , Or hungry threads ( That is, a thread can't wait for the lock )


Under contract ReentrantLock The creation of can specify the boolean Type to get fair lock or unfair lock , Default is unfair lock

Lock lock = new ReentrantLock(true);
Create a reentrant lock .true Fair lock ,false It means unfair lock . The default is false.


Fair lock : In a concurrent environment , Each thread will first check the waiting queue maintained by the lock when acquiring the lock , If it is empty , Or wait for the first one in the queue . Take the lock , Otherwise it will join the waiting queue , In the future according to FIFO The rules of get themselves from the queue .

Not fair lock : Unfair locks are rude , Come up and try to own the lock , If the attempt fails , We'll use a fair lock .


The advantage of unfair lock is that throughput is larger than fair lock , about synchronized for , It's also a kind of unfair lock .

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