Beware of Java multi-threading traps: Explore the causes and solutions to ABA problems!

1. Concept

CAS (Compare and Swap) is an optimistic locking mechanism. It is an atomic operation based on hardware instructions that can ensure safe access to shared variables by multiple threads without using traditional mutex locks. In Java, we can use the Atomic class and the AtomicReference class to implement CAS operations. These classes provide a series of atomic update methods, such as compareAndSet, getAndSet, incrementAndGet, etc. CAS operation is very useful in many high-concurrency applications, but it also has some potential problems, such as ABA problem. So what is an ABA problem?

The ABA problem is a typical problem that may be encountered when using CAS (Compare and Swap) operations. It refers to the fact that the value of a shared variable changes from A to B and then from B back to A during the operation, and the CAS operation may mistakenly believe that no other thread has modified this value. This can lead to misjudgment of CAS operations, which may cause potential problems.

For example, suppose there are two threads T1 and T2 that perform CAS operations on a shared variable V with an initial value of A. Thread T1 first changes the value of V from A to B, and then changes it from B back to A, and thread T2 may perform a CAS operation during this period. Since the expected value is A, the CAS operation will succeed, although T1 has no effect on V during the period. Many changes were made.

The solution to the ABA problem is to use version numbers or tags. In this way, in a CAS operation, not only the values ​​of shared variables need to be compared, but also the version numbers or tags need to be compared. The CAS operation will only succeed if both the value and the version number match.

2. Related questions

Here are some common interview questions and answers related to ABA issues:

1. What is the ABA problem?
   Answer: The ABA problem is a problem that may be encountered when using CAS operations. It refers to the value of a shared variable changing from A to B during the operation. , and then changes from B back to A, and the CAS operation may mistakenly believe that no other thread has modified this value.
2. Why does ABA problem occur?
   Answer: The reason for the ABA problem is that during the CAS operation, the value of the shared variable changed many times, but eventually returned to the original value, making the CAS Operations have difficulty identifying these intermediate changes.
3. How to solve ABA problem?
   Answer: ABA problems are usually solved by introducing version numbers or tags. In a CAS operation, not only the values ​​need to be compared, but also the version number or tag to ensure that the operation is performed in the correct context.
4. How to solve ABA problem using AtomicStampedReference in Java?
   Answer: In Java, you can use AtomicStampedReference classes to solve ABA problems. It allows you to include a tag in a CAS operation to track versions of shared variables. By comparing values ​​and markers, you can avoid ABA problems.

import java.util.concurrent.atomic.AtomicStampedReference;

public class ABADemo {

    // 创建一个初始值为100，版本号为0的原子引用
    static AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(100, 0);

    public static void main(String[] args) {
        // 创建一个线程，用CAS操作将100变成101，再变成100
        new Thread(() -> {
            // 获取初始版本号
            int stamp = atomicStampedReference.getStamp();
            System.out.println(Thread.currentThread().getName() + " 第一次版本号：" + stamp);
            // 暂停一秒，让另一个线程也获取到同样的版本号
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // 尝试用CAS操作将100变成101，期望版本号为stamp，更新版本号为stamp+1
            atomicStampedReference.compareAndSet(100, 101, stamp, stamp + 1);
            System.out.println(Thread.currentThread().getName() + " 第二次版本号：" + atomicStampedReference.getStamp());
            // 尝试用CAS操作将101变成100，期望版本号为stamp+1，更新版本号为stamp+2
            atomicStampedReference.compareAndSet(101, 100, stamp + 1, stamp + 2);
            System.out.println(Thread.currentThread().getName() + " 第三次版本号：" + atomicStampedReference.getStamp());
        }, "t1").start();

    }
}

Output result:

t1 第一次版本号：0
t1 第二次版本号：1
t1 第三次版本号：2

1. Examples how to use version numbers to solve ABA problems.
   Answer: Suppose there is a shared variable V with an initial value of A and a version number of 1. Thread T1 first changes the value of V from A to B and increments the version number to 2. Thread T2 then tries to change the value of V from A to C, but due to a mismatch in version numbers (expected version number 1, actual 2), the CAS operation fails, even though the value is A.
2. Under what circumstances are ABA problems particularly likely to occur?
   Answer: ABA problems are particularly likely to occur in scenarios where shared variable values ​​need to be modified frequently, especially when multiple threads operate shared variables at the same time. After the thread is modified, change it back to the original value.