Syncronized lock upgrade process

synchronized

synchronized Is an object lock ( It's the object that locks, not the reference ), Action granularity is the object ,java Every object in can be locked ( Only one thread can be locked successfully at a time ), And stored inside the object markword Mark the lock status .

stay jdk The old version used heavyweight locks , But in jdk The lock upgrade that appears only after the update .

Locks are generally divided into 4 Transition state ： unlocked -> Biased locking -> Lightweight lock -> Heavyweight lock .

The process of lock upgrading is irreversible .

  Biased locking ：

  When there is only one thread a When you get the lock , At this time, the lock is a bias lock , This is the time markword Point to a, When a When you ask for a lock , There is no need for any operation , This can save a lot of time .

Lightweight lock ：

When a thread A After getting the lock , This time comes the thread B To apply for the lock , At this time, there is a competitive relationship , At this time, lightweight locks appear , If A At this time, the lock is still used ,B Then it spins ,B Think A It won't take long , If so B Come after A No longer in use , This is the time markword Direct to B. Generally, there are many cases like this , It's better to save cpu Utilization ratio .

  Heavyweight lock ：

It's similar to the lightweight lock above , But the difference is , When A In use , Now markword It's pointing A Of , Now B coming ,CDE Here we are , Now BCDE Will compete with each other , But the competition is stronger than the lightweight lock , At this time, heavyweight locks depend on the operating system Mutex lock Realization , At this time, the thread waiting for the lock hangs , When the lock is released, the operating system wakes up and tries to acquire the lock again , With the help of the operating system , Cause user mode kernel mode switching , The time cost of this process is relatively high .

In primitive times synchronzie The heavyweight lock is used , The use of cpu Not very efficient , Up to now, the lock upgrade process is used .