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In the previous article, we talked about the principles of atomic classes used in Java concurrency and optimization of Java 8. For details, see article: How can Java 8 optimize CAS performance? .
In this article, we discuss one of the most powerful interview questions: what is your understanding of AQS?
Before, I had a feedback from my classmates. I went to an Internet company for an interview, and the interviewer asked me this question when talking about it. At that time, the student was probably hurt in 10,000 points…
First of all, a lot of people really don’t know what AQS are, probably haven’t even heard of it. Or someone has heard of the term AQS, but may not even know how to spell it.
Or, worse, might say: AQS? Is it a thought? How do we usually develop to use AQS?
In general, many students are estimated to have a sense of AQS in the clouds, if you use a search engine to check what AQS is? Read a few articles, it is estimated to give up directly, because of the dense text, it is not understand!
Therefore, based on the above pain points, let’s this article, with the most simple plain words with N more than hand-drawn pictures, to tell you what AQS is? So that you don’t feel overwhelmed when asked this question in an interview.
The relationship between ReentrantLock and AQS
First let’s see what it feels like to use Java and ReentrantLock to lock and release the lock.
After all, this is the use of Java concurrency basic API, should be everyone has learned, so let’s directly look at the code:
The above code should not be difficult to understand, it is nothing more than a Lock object, then Lock and release the Lock.
Now, you might ask, what does this have to do with AQS? It’s a big deal! Because many APIS under Java concurrent package are based on AQS to achieve lock and lock release functions, AQS is the basic class of Java concurrent package.
For example, ReentrantLock and ReentrantReadWriteLock are all implemented based on AQS.
So what does AQS stand for? AbstractQueuedSynchronizer, abstract queue synchronizer. To draw a picture, let’s look at the relationship between ReentrantLock and AQS.
Let’s look at the picture above. To put it bluntly, already contains an internal AQS object, namely AbstractQueuedSynchronizer types of objects. This AQS object is the key core component of ReentrantLock’s ability to lock and release locks.
ReentrantLock ReentrantLock ReentrantLock ReentrantLock ReentrantLock
Ok, so now what happens if a thread comes along and tries to lock using ReentrantLock’s lock() method?
This AQS object has a core variable called state, which is of type int and represents the locked state. In the initial state, the value of this state is 0.
In addition, there is a key variable inside the AQS, which is used to record which thread is currently locked. In the initialization state, this variable is null.
Thread 1 then runs in and calls the lock() method of ReentrantLock to try to lock it. This locking process directly changes the state value from 0 to 1 with the CAS operation.
If you don’t know what CAS is, take a look at the previous article and discuss how Java 8 can optimize CAS performance.
If no one has locked before, then the state value must be 0, and thread 1 can successfully lock.
Once thread 1 has successfully locked, you can set the current locked thread to itself. So if you look at the picture below, it’s thread 1 running up to lock.
In fact, if you look at this, you should get a sense of what is called AQS. To put it bluntly, it is a core component in the package, which has the state variable, the lock thread variable and other core things, and maintains the lock state.
As you can see, ReentrantLock is just an external API, and the lock mechanism implementation in the kernel relies on the AQS component.
The ReentrantLock starts with Reentrant, meaning it is a Reentrant lock.
ReentrantLock means that you can perform lock() and unlock() multiple times on a ReentrantLock object. This is called ReentrantLock.
Now that you understand the state variable, you know how to do reentrant locking!
In fact, thread 1 can re-enter the lock once, so it can re-enter the lock multiple times. Each time the lock is added, the value of state is incremented by 1, and nothing else changes.
And then, if thread 1 locks, what happens if thread 2 comes along and locks?
** Let’s see how mutual exclusion of locks is implemented. ** Thread 2 runs and sees, oops! State is not zero, right? The CAS operation will fail to change state from 0 to 1, because state is currently 1, indicating that someone has locked it!
And then thread 2 will look and say, did you lock it? Of course not, the ** “lock thread” ** variable explicitly records that thread 1 is holding the lock, so thread 2 will fail to lock.
Here’s a picture to give you a taste of the process:
Thread 2 will then put itself in a queue in the AQS because it failed in its attempt to lock, and will wait for thread 1 to release the lock so that it can try again
So you can see that AQS is so central! AQS also has a wait queue for threads that fail to lock!
Again, here’s a picture to give you a taste of it:
Thread 1 then releases the lock after executing its own business logic code! If the state value is 0, then the lock will be released completely, and the “lock thread” variable will also be set to NULL!
The whole process is shown below:
Next, thread 2 is awakened from the head of the waiting queue to try locking again.
Good! Thread 2 now attempts to lock again, again using the CAS operation to change state from 0 to 1, which will be successful. After success, the lock will be successful, and the state will be set to 1.
In addition, the “lock thread” is set to thread 2 itself, and thread 2 itself disconnects from the wait queue.
And then one last picture, just to show you how it works.
Four,
OK, this article so far, basically through ReentrantLock lock and lock release process, to tell you clearly its underlying dependence on the core principle of AQS.
Basically, you will understand this article, and you will no longer worry about being asked in an interview: tell me about your understanding of AQS.
In fact, a summary of AQS is a basic component and send packets, used to achieve all kinds of locks, all kinds of synchronization components. It contains the state variable, locking threads, wait queues, and other core components of concurrency.
What is volatile about Java concurrent interview Questions? How can Java 8 optimize CAS performance? What is your understanding of AQS? What are fair locks and unfair locks for Concurrent interview questions? Stay tuned for a chat on Java concurrent interview questions about read/write lock optimization in the microservice registry? Stay tuned for
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