Interview question: talk about your understanding of AQS

Hello, I'm Qingwei , A delusion 30 Beipiao programmers with an annual salary of millions before the age of .
This column is the common interview questions of big factories that I sorted out , I will give an answer , I hope I can help you
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link : Explore the principles from the source code

link : Java Concurrent programming

link : SQL– Knowledge and principle

In the interview of many big factories , Interviewers have relatively high assessment requirements for concurrent programming , Simply speaking , If you don't understand concurrent programming , Then it's hard for you to pass the interview for a high paying position in a large factory .

Talk to you about AQS The understanding of the

  AQS yes AbstractQueuedSynchronizer For short , It is the core component in concurrent programming , It is J.U.C In bag Underlying implementation of multiple components , It can be used to implement multithreaded synchronizer , The underlying implementation of multiple components in the package , Such as Lock、 CountDownLatch、Semaphore And so on AQS.

   essentially ,AQS Two locking mechanisms are provided , Exclusive lock , And Shared lock .
   Exclusive lock , This is when multiple threads compete for the same shared resource , Only one thread is allowed to access this at one time Shared resources , That is, only one thread in multiple threads can obtain lock resources , such as Lock Medium ReentrantLock The implementation of reentry lock uses AQS Exclusive lock function in .
   Shared locks are also called read locks , That is to allow multiple threads to obtain lock resources at the same time , such as CountDownLatch and Semaphore It's all about using AQS Shared lock function in .

  AQS In the maintenance of a state Field , Represents lock preemption . And to provide for state Of cas operation . And the method of providing a locked template , such as tryAcquire, You can reproduce and implement the relevant logic .
   meanwhile , Threads that cannot be grabbed need to queue and wait , therefore AQS There is a thread waiting in the queue . The most important thing in it is a two-way linked list . The data structure of the node is node,node Store thread information , as well as node The state of . At the same time, it provides relevant operation methods for two-way lists .

   If the thread fails to preempt the lock , Will enter AQS Waiting queue in , also park.

   At the same time, it provides relevant methods for releasing locks , Releasing the lock will wake up the related thread . Lock preemption .