[Java Interview] Please talk about what happened to AQS?

Hi everyone, my name is Mic.

The market environment this year is really difficult. Many people who have worked for one year, the difficulty of the interview is equivalent to a person with 4 years of experience.

The more this is the case, the stronger we should be, in order to gain more opportunities in the face of adversity.

Today, a fan with one year of experience was asked about the "implementation principle of AQS" and came to me for help.

Check out the expert's answer to this question below.

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Expert:

AQS is a very core abstract class in the JUC package. It provides a queue synchronizer for multi-threaded access to shared resources.

In the JUC package, many components rely on AQS to synchronize and wake up threads, such as Lock, Semaphore, CountDownLatch, and so on.

AQS consists of two core parts:

• A volatile modified state variable, as a race condition
• FIFO thread waiting queue maintained by doubly linked list structure

Its specific working principle is that multiple threads implement race conditions by modifying this state shared variable.

Threads that fail to compete are added to the FIFO queue and blocked,

After the thread that has preempted the competitive resource is released, the subsequent threads wake up in an orderly manner according to the FIFO order.

AQS provides two resource sharing methods,

One is the exclusive resource, only one thread can obtain the competitive resource at the same time. For example, ReentrantLock uses this method to implement exclusive locks

The other is to share resources. At the same time, multiple threads can obtain competitive resources at the same time. CountDownLatch or Semaphore is a way to use shared resources to wake up multiple threads at the same time.

Summarize

In actual development, if we need to implement some special mutually exclusive scenarios,

It is a bit troublesome to use ReentrantLock directly, then you can integrate AQS yourself and customize the implementation logic of multi-threaded competition.

This question mainly examines the understanding of the basics of Java.

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