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This article mainly explains "how to understand Java multithreaded optimistic lock and CAS mechanism". The content of the article is simple and clear, and it is easy to learn and understand. please follow the editor's train of thought to study and learn "how to understand Java multithreaded optimistic lock and CAS mechanism".

Catalogue

Pessimistic lock and optimistic lock

1. Pessimistic lock

2. Optimistic lock

II. CAS mechanism

Pessimistic lock and optimistic lock 1. Pessimistic lock

Pessimistic lock is based on a pessimistic attitude class to prevent all data conflicts, it is a preventive attitude to lock the data before modifying the data, and then read and write the data. No one can operate on the data until it releases the lock, and the data can not be locked until the previous person releases the lock, and then the data can be locked. Synchronized is a pessimistic lock, and once this thread gets a lock, other threads that need a lock hang up.

Features: the exclusivity and correctness of the data can be fully guaranteed, because each request will lock the data first, then operate the data, and then unlock it, and the process of locking and releasing the lock will cause consumption, so the performance is not high.

2. Optimistic lock

Optimistic locking is to maintain an optimistic attitude towards data conflicts, and the operating data will not be locked when operating the data (which allows multiple tasks to operate on the data in parallel). It is only when the data is submitted that there is a mechanism to verify whether there is a conflict. CAS operates on optimistic locks, each time without locking and assuming that there is no conflict to complete an operation, and if it fails because of a conflict, retry until it succeeds.

Features: optimistic lock is a concurrent type of lock, which does not lock the data but realizes the locking function through the business. not locking the data means allowing multiple requests to access the data at the same time. At the same time, it also saves the process of locking and unlocking the data, which saves the pessimistic locking operation, so it can improve the performance of the operation to a certain extent. However, in the case of very high concurrency, it will lead to a large number of request conflicts, which will cause most operations to return without success and waste resources, so in high concurrency scenarios, the performance of optimistic locks is not as good as pessimistic locks.

II. CAS mechanism

The full name of the CAS mechanism is Compare And Swap, which translates to compare and exchange. There are three variables in the CAS mechanism, the memory address address, the old expected value oldvalue, and the new value newvalue to be modified. When performing a CAS operation, first check and compare whether the memory address is consistent with the old expected value, and if the same returns true, otherwise it returns false. You can see the following code for a good understanding.

AtomicInteger in the code is an atomic operation class, count.compareAndSet (11 count 10) is an atomic operation, he is an atomic operation, he first needs to compare whether the original count value is 11, if it is 11, change it to 10, if thread 1 and thread 2 enter the code, but thread 1 first triggers CAS, changing the count value to 10, then thread 2 finds that the count value is no longer equal to 10 when it is executed to the CAS mechanism, then the compareAndSet function will return false Enter the else to continue run (). After thread 1 sleeps for 5 seconds, after changing the count value to 11, thread 2 enters the compareAndSet function again and finds that the count value becomes 11, then the value is changed to 10 and the true value is returned. As a result, the optimistic lock is realized.

Public class AtomiIntegerTestimplements Runnable {private AtomicInteger count = new AtomicInteger (11); public static void main (String [] args) {AtomiIntegerTest ast = new AtomiIntegerTest (); Thread thread1 = new Thread (ast); Thread thread = new Thread (ast); thread1.start (); thread.start ();} @ Override public void run () {System.out.println ("thread:" + Thread.currentThread (). GetName () + " Count: "+ count.get (); if (count.compareAndSet (11Mag10)) {System.out.println (Thread.currentThread (). GetName () +"; modified successfully "+ count.get ()); try {Thread.sleep (5000);} catch (InterruptedException e) {e.printStackTrace ();} count.set (11) } else {System.out.println ("retry mechanism thread:" + Thread.currentThread (). GetName () + "; flag:" + count.get ()); try {Thread.sleep;} catch (InterruptedException e) {e.printStackTrace ();} run ();}

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