One interview question a day - the underlying implementation of synchronize and the lock upgrade process

Scope of action

synchronized You can modify static methods 、 Decorated instance method 、 Decorated code block

Method underlying principle

jvm Through... In the method table structure in the method constant pool ACC_SYNCHRONIZED Flag to distinguish whether it is a synchronous method , If it's a synchronous method , The thread will hold monitor, And then execute the method , At the end of the execution （ Whether the execution is normal or abnormal ） Release monitor

The underlying principles of code blocks

utilize monitorenter and monitorexit These two bytecode instructions . They are at the beginning and end of the synchronization block, respectively . When jvm Execute to monitorenter When the command , The current thread is trying to get monitor Ownership of objects , If not locked or held by the current thread , Just lock the counter +1; When executed monitorexit When the command , Lock counter -1; When the lock counter is 0 when , The lock is released . If you get monitor Object failed , The thread is blocked , Until the other thread releases the lock .

Lock escalation

jdk1.6 The default is to open the biased lock ,synchronize When the first thread enters , The default is modified to bias lock , Will be the current thread of ID Update to the... Of the object header makeword The thread of ID in , Add the timestamp of the biased lock and modify the flag of the biased lock to 1
When a biased lock is upgraded to a lightweight lock, there is competition between threads , Threads 1 Delayed exit code block , Threads 2 And get this lock again , And threads 2 adopt cas Spin has never been successful , At this point, it will be upgraded to a lightweight lock , If the thread is spinning 1 After execution, the lock is obtained , At this point, it will not be upgraded to a lightweight lock
Upgrading a lightweight lock to a heavyweight lock is a time when the competition between threads is particularly fierce , There are many threads competing for the same lock , Threads 1 Executing code , The remaining threads are spinning and waiting for the lock , When the number of spins reaches the set maximum, the lock is still not obtained , At this point, it is upgraded to a heavyweight lock , Heavyweight locks block the rest of the waiting threads , When the thread that obtains the lock releases the lock, it wakes up the blocked thread , Open a new round of lock competition .