How dare you write a resume that is proficient in concurrent programming? Why do you use a two-way linked list in AQS?

A job 4 Programmer of , The resume says proficient in concurrent programming , And also read AQS（
AbstractQueuedSynchronizer） Source code , Then the interviewer only asked such a question ：“AQS Why use a two-way linked list structure ”？, Then it collapsed ！

Actually AQS Everyone is no stranger , It is J.U.C A very important thread synchronizer in the package . today , Let me share with you my understanding .

in addition , I put all the videos I shared in previous periods into one 500 page PDF Supporting documents for interview question analysis , I hope this can improve the passing rate of all fans ,

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1、 Cause analysis

First , Double linked list has two pointers , A pointer points to the front node , A pointer to the successor node . therefore , Bidirectional linked lists can support constants O(1) Find the precursor node in the case of time complexity . therefore , When inserting and deleting a two-way linked list , It is simpler than a one-way linked list 、 Efficient .

From the characteristics of the two-way linked list , In my submission AQS There are three reasons for using a two-way linked list ：

The first 1 One reason , Threads that do not compete for locks join the blocking queue , And the premise of blocking waiting is , The leading node of the node where the current thread is located is in normal state , This design is to avoid the problem that there are abnormal threads in the linked list, which makes it impossible to wake up subsequent threads .

therefore , Before thread blocking, you need to judge the status of the front node , If there is no pointer to the front node , You need from Head The node starts to traverse , Very low performance .

 

The first 2 One reason , stay Lock There's one in the interface ,lockInterruptibly() Method , This method indicates that threads blocked by locks are allowed to be interrupted .

in other words , Threads that do not compete for locks join the synchronization queue and wait , Is to allow external threads to pass interrupt() Method to trigger wakeup and interrupt . This is the time , The status of the interrupted thread will be changed to CANCELLED. And marked as CANCELLED Thread in state , There is no need to compete for locks , But it still exists in the two-way linked list .

This means that in the subsequent lock competition , You need to remove this node from the linked list , Otherwise, the thread blocked by the lock will not wake up normally . under these circumstances , If it's a one-way list , You need from Head Nodes start to traverse down one by one , Find and remove the node with abnormal state . Similarly, the efficiency is relatively low , It will also lead to competition between lock wake-up operations and traversal operations .

The first 3 One reason , To avoid the overhead of thread blocking and wakeup , So the thread just added to the linked list , First, I will try to compete for the lock by spinning .

But in fact, according to the design of fair lock , Only the next node of the head node is necessary to compete for locks , after

Continued node contention locks are of little significance . otherwise , It will cause herding , That is, a large number of threads try to compete for locks before blocking, which brings relatively large performance overhead .

therefore , To avoid this problem , Nodes added to the linked list before trying to compete for locks , You need to judge whether the front node is the head node , If it's not the head node , There is no need to trigger lock competition . So this will involve the search of the front node , If it's a one-way list , Then the implementation of this function will be very complex .

This problem , There may be 99% No one can answer . Yes AQS Without deep understanding , May not know how to answer . Understand why a technology is designed like this , The key is what kind of problems it needs to solve .

Last , I sorted all the interview questions I shared in previous periods into 1 Share 10W Document of words , I hope this can improve the passing rate of all fans

I was delayed by programming Tom, Only dry goods without water ！ Your support is my biggest motivation ！ Pay attention to me , The interview is no longer difficult ！

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