Synchronized lock upgrade

author : TaoYuanming of the Western Wei Dynasty
Blog : https://blog.springlearn.cn/

TaoYuanming of the Western Wei Dynasty
Don't laugh at the young Jianghu dream , Who doesn't dream of Jianghu

One 、 Heavyweight lock

What is heavyweight lock ？

That is, applying for resources must go through kernel( The kernel is also called the operating system ), call .

Two 、 Lightweight lock

Lightweight lock , Is not through the operating system . Lightweight locks are called relative to heavyweight locks , It can also be called Le Guan lock .

stay Java The optimistic lock is cas operation (compare and swap) According to English translation, it means comparison and Exchange .
The bottom layer is called Unsafe The method inside , You can see that these methods are native Method .

By looking at jvm Source code , notice c++ The code has an assembly language support cas

however , This article cmpchg1 It's not atomic , Click in lock_if_mp(%4) in

The ultimate realization is lock cmpxchg Instructions ： It means that the hardware will lock a North Bridge bus when executing the following instructions .（ Equivalent to locking the bus , But it is lighter than the lock bus ） The following problems have been solved

3、 ... and 、 Biased locking

seeing the name of a thing one thinks of its function , It will favor the first thread to access the lock , If in operation , The synchronization lock has only one thread access , There is no multi-threaded contention , The thread does not need to trigger synchronization , In this case , Will add a biased lock to the thread .
To put it bluntly , There is no competition , What lock is it called . Is to add a mark . Think no one is competing for you .

Four 、 Lock upgrade steps

Biased locking -> Lightweight lock -> Heavyweight lock

stay jdk1.6 Before synchronized Direct is a heavyweight lock , One finished, all is finished .
jdk After optimization , The concept of lock escalation .

So in fact synchronized Implementation process of :

1. testing Mark Word Is it the current thread ID, If it is , Indicates that the current thread is in a biased lock
2. If not , Then use CAS Will be the current thread of ID Replace Mard Word, If successful, it means that the current thread gets biased lock , Set the bias flag 1( When the previous thread is just released , This can succeed , Otherwise, look at 3)
3. If you fail , It means that there is competition , Undo the bias lock , Then upgrade to lightweight lock .
4. The current thread uses CAS Put the Mark Word Replace with lock record pointer , If it works , Current thread gets lock
5. If you fail , Indicates that other threads are competing for locks , The current thread attempts to acquire a lock using spin .
6. If the spin succeeds, it's still in a lightweight state .
7. If spin fails , Upgrade to heavyweight lock .

Translated into vernacular :

1. When no one competes with you, it is a biased lock , When cas failed , It means someone is competing with you , At this time, the lock is upgraded from biased lock to lightweight lock .

2. In the state of lightweight lock , There are still many threads to compete , So at this time cas It will be more serious and waste cpu perform . Upgrade to a heavyweight lock .
   Secondly, other waiting threads enter the blocking state .

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