Interview - CAS of Java Concurrency: Master the principles, advantages and disadvantages, and application scenario analysis to avoid race problems

1. Introduction and principle of CAS

The CAS (Compare-and-Swap) operation in multithreading is a common concurrency control method for atomically updating the value of shared variables. Its core idea is to determine whether the update operation can be performed by comparing whether the value on the memory address is equal to the expected value, thereby avoiding the race condition problem under multi-threaded conditions. It is a CPU instruction-level operation that ensures data consistency. Contains three values, the memory address (V), the expected value (A), and the new value (B). The AB thread updates the variable value. The AB thread first obtains the variable value, and then generates a new value. The variable value is placed in the memory V, the original value is expected to be A (backup value), and the new value is B. If V=A update the new value. Guaranteed value consistency in multi-threaded situations. The main purpose is to ensure that the value taken by the thread at that time, when the memory is to be updated, the value is still the same and has not been changed by other threads.

Specifically, the multithreaded CAS operation includes the following steps:

1. Get the value and expected value of the current shared variable.
2. Compare whether the current value of the shared variable is equal to the expected value, and if they are equal, update the value of the shared variable to the new value to be written.
3. If the current value of the shared variable is not equal to the expected value, it means that other threads have modified the value of the shared variable at this time, so the current thread needs to reacquire the latest value of the shared variable, and repeat step 2.

In a concurrent environment, multi-threaded CAS operations can guarantee the atomic operation of shared variables, and at the same time avoid the overhead of thread blocking and context switching caused by traditional lock mechanisms. Therefore, multi-threaded CAS operations are widely used in various high-concurrency scenarios, such as database transactions, distributed systems, and so on.

2. Related interview questions

1. What is a CAS operation?

Answer: The CAS (Compare-and-Swap) operation is a concurrency control method used to ensure the atomic update of shared variables. Its core idea is to determine whether the update operation can be performed by comparing whether the value on the memory address is equal to the expected value.

1. What are the advantages of CAS operations?

Answer: CAS operation has the following advantages:

• It can avoid the race condition problem under the condition of multithreading.
• It does not cause the overhead of thread blocking and context switching like traditional lock mechanisms.
• With high concurrency performance and adaptability, it can be applied to various high concurrency scenarios.

1. What are the disadvantages of CAS operations?

Answer: CAS operation also has some disadvantages:

• There may be an ABA problem, that is, when the CAS operation is performed, the value of the shared variable has changed from A to B and back to A, but because the value is compared instead of the identifier, it is impossible to determine whether there are other threads in the change process Modified this shared variable.
• Since CAS operations are implemented based on hardware instructions, their performance may be affected by hardware limitations. At the same time, in a high-concurrency environment, the success rate of CAS operations may also be reduced.
• CAS operation does not support multi-step operation. If multi-step operation is required, it must be realized by cyclic CAS and other methods.

1. How to avoid ABA problems?

Answer: You can use version number or timestamp to solve the ABA problem. Specifically, every time a CAS operation is performed, in addition to comparing whether the values ​​are equal, it is also necessary to compare whether the version number or timestamp is consistent, so as to ensure that the shared variable has not been modified by other threads.

1. What are the applications of CAS operations in Java?

Answer: In Java, CAS operations are widely used in various high-concurrency scenarios. For example, methods in classes such as AtomicInteger, AtomicBoolean, and AtomicReference are implemented based on CAS operations. At the same time, after Java 8, new atomic operation classes LongAdder and DoubleAdder were introduced to replace the old versions of AtomicLong and AtomicDouble to improve concurrency performance.