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1 AQS (AbstractQuenedSynchronizer)

• AQS Maintenance state volatile int state （ReentrantLock Take it as the number of re-entry times of the lock ） And a FIFO Thread waiting queue , When a multi thread contention resource is blocked, it will enter this queue ; After the thread at the head of the queue is executed , It calls its successor thread
• Threads pass through CAS Change status symbol state, Success is the success of the lock , If it fails, it will enter the waiting queue , Waiting to be awakened
• AQS use spinlocks The mechanism of

2 ReentrantLock Unfair locking process

• An unfair lock is a new thread that follows AQS The thread at the head of the queue competes for the lock , Other threads in the queue still queue normally
• Fair lock is strictly enforced FIFO, New threads can only join the tail of the queue

1. Unfair lock attempts to lock , The perform tryAcquire() The process is ： Check state Field , if 0, Indicates that the lock is not occupied , Try to occupy the lock ; If not 0, Check whether the current lock is occupied by itself , If you are occupied by yourself , Update state Field , Reentry times plus 1
2. If none of the above works , Failed to acquire lock , Enter the waiting queue
3. The thread entering the waiting queue attempts to acquire the lock （ The top thread is qualified to try ）, If successful, it becomes the new head node of the queue , If the acquisition fails, try to suspend
4. The prerequisite for a thread to be able to suspend after it is queued is , The state of its precursor node is SIGNAL, Status as SIGNAL The node will wake up the next node after leaving the queue

3 ReentrantLock and synchronized Similarities and differences

• In response to interrupt ： If a thread A Executing code in lock , Another thread B Waiting to acquire the lock , It may be due to the long waiting time , Threads B Don't want to wait , Want to deal with other things first , We can let it interrupt itself or interrupt it in another thread , This is the interruptible lock .Lock In the process of waiting for the lock, you can use interrupt To interrupt the waiting
• Overtime waiting ： Specify the timeout waiting time , Avoid threads waiting indefinitely for lock release
• Fair lock and unfair lock ： Fair lock is when multiple threads try to acquire the same lock at the same time , The order in which the locks are acquired is the order in which the threads are reached , Non fair locks allow threads to “ Jump the queue ”, Specifically, the new thread competes with the thread of the team leader for lock