Briefly describe synchronized and lock upgrade

1、Synchronized lock

​ adopt Synchronized Implement synchronization mechanism , It belongs to a pessimistic lock , When operating data , Determine whether the object is locked , If it is locked, it enters the blocking state until the occupied thread is released , If there is no lock or the current thread already has a lock on the operation object, lock it , Release the data after operation .

​ When a class object is locked , By Synchronized The modified method will cause blocking , Not Synchronized The modified method will not cause blocking .

​ Synchronized Solve the problem of locking yourself through Reentry ; Because it locks by judging that the object has no lock or that the current thread has the lock of the current object , If there is no reentrant class, two synchronous methods calling each other will cause deadlock .

​ Synchronized It's also a kind of unfair lock , Because it doesn't allocate locks according to the time of applying for locks , Instead, the lock is obtained in the form of competition , This improves execution performance .

Method lock ：synchronized When modifying a method , Each instance object corresponds to a lock .

Object lock ：synchronized Decorate a method or block of code , Each instance object corresponds to a lock .

Kind of lock ：synchronized Modify static methods or static code blocks , All instance objects share the same lock , We call it a class lock .

In a class , There are both class lock methods and object lock methods . When multithreading two methods , Will threads block .

2、 Lock escalation

From low to high is ： Biased locking 、 Lightweight lock ( spinlocks )、 Heavyweight lock , Locks can only be upgraded, not demoted ;

Why do I need to upgrade locks ： ** When a thread obtains the same lock multiple times , If you have to compete for locks every time, it will increase a lot of unnecessary costs , in order to Reduce CPU The cost of , Provides execution efficiency .**

Implementation of upgrade lock , Depends on... In the object header Mark Word, It will record the status of the first thread to obtain the lock ID, And lock status ;

• Biased locking ： When a thread A After getting the lock , The thread A Of ID Record to object header , And the lock is upgraded to bias lock ( Lock sign -01( No lock or bias is 01), Whether the preference is 1); Subsequent threads A When acquiring the lock of the object again , Find out The object is in a biased lock state , Thread in object header ID With threads A Agreement , At this time, the bias lock will be obtained directly ;
• Lightweight lock ： When another thread B When you get the lock , The lock is found to be in a biased state , The thread B Will pass CAS Try to scramble for the lock 【 Modify the thread in the object header ID】( Judging threads A Survival , If you live, you can't modify ), If the partial lock fails to grab the lock , Lightweight locks tend to be upgraded , If the lock grabbing is successful, execute the code ;
• Heavyweight lock ： Lightweight locks compete for locks by spinning , And it is applicable to the case that the thread does not hold the lock for a long time **( Because blocking threads requires CPU state , It costs a lot , So spin )**; When the number of spins of the thread exceeds the threshold （ The default is 10） In order to prevent cpu Idle , Will upgrade the spin lock to a heavyweight lock , And block the thread that does not acquire the lock ;

Lightweight lock modification thread ID The way ：

​ After the deflection lock is upgraded to a lightweight lock , Threads B Lock the markword Copy to the thread itself markword In the space , And then through cas To set the thread in the lock object ID value ;