AQS SOS AQS with me

ReentrantLock Brief introduction

First take ReentrantLock Say something
There are AQS Implementation class of abstract queue NonfairSync and FairSync They implement unfair and fair locks respectively
An abstract queue is a two-way queue There are node node Every node Node stores the current thread also pre and next and waitStatus The status value

ReentrantLock Realization principle

In a word The spin (for) cas park
The abstract queue will not be initialized when there is no lock competition, and there will be no additional memory consumption
An empty... Is initialized when lock contention exists node node thread=null Then initialize your own thread node node hold pre Point to the head node
A standard is maintained here
Queue header thread Always empty
The thread holding the lock is never in the queue That is, it does not participate in the queuing mechanism

When you join the team, you will put the last node Node waitStatus Change it to -1 Indicates that the previous node is allowed to sleep , ( Why didn't the last node sleep itself Because I don't know I'm asleep. Haha, the explanation is far fetched, but it conforms to the actual life )
Successful lock snatching will make you the head of the queue node Node drop Become the head node And put yourself node Node thread empty

If it's the first person in line ( Second in the queue ) Just try cas Lock up
If not the first person in line It's too late to grab the lock

The following supplement
Locking process
1. Use a variable state As the flag bit of the lock , The default is 0, Indicates that all threads can be locked at this time , Pass when locked cas take state from 0 Turn into 1,cas Successful execution means successful locking
2. When a thread takes possession of a lock , At this time, there are other threads to lock , Determine whether the thread that is currently trying to grab the lock is the thread that is occupying the lock ？ yes ： Reentrant lock ,state+1, When released state-1, use state Indicates the number of locks no ： Locking failed , Put the thread in the waiting queue , And blocking
3. Is there anything else you can optimize ？ When placed in the waiting queue , See if there are any other threads ？ Yes , The lock is occupied , And the current thread cannot be used to rob , Just block it directly When you put it in the queue , adopt cas Try again to acquire a wave of locks , If successful , There is no need to block , Improved efficiency
Unlocking process
1. adopt cas Yes state-1, If it's a re-entry lock , One release minus one , When state=0 When, the lock is released .2. Wake up the thread in the waiting queue
The process of joining the team
When a thread is put in a queue , What should I do next ？
If you are asked to write whether it is directly put into the queue, you are done ？ but Doug Lea Here's how it works
1. If the current thread is the second node in the queue, try to rob the lock again （ If it's not the second node, you don't have to grab it , It's not the turn ）, This avoids frequent blocking and wakeup of threads , Improved efficiency
2. Set the alarm clock , Let the previous thread wake itself up （ I'll talk about it later , That is, change the of the previous node waitStatus）
3. parkAndCheckInterrupt() This method calls LockSupport.park(this); Will block the thread that currently competes for the lock , Until there are threads unlock Get out of the team and unpark Next blocked thread , Only then does the method call return Thread.interrupted(); Why call ( Reset thread break flag ) Methods? Use this to determine whether a blocked thread is interrupted by another thread , If interrupted, return to true Jump out of the way And then go ahead with acquire() Internal selfInterrupt(); Restore the thread of the awakened contention lock to the thread interrupt state
4. lock.lock() Unable to accept the message that the thread is interrupted
lock.lockInterruptibly() It can accept the message that the thread is interrupted while waiting for lock snatching

Blocking
Out of the team
When A Thread release lock , Wake up in the queue B Threads ,A The thread is removed from the queue
Who will do the task of leaving the team ？ It is done by the awakened thread , namely B Threads

Linked articles
https://mp.weixin.qq.com/s/uUAUGQ_WGHJzhJAyBmcWYg