Atomicity

Incremental and Atomicity

In this code, we define a Counter class with a count variable and an increment method that increases the value of the count variable by 1.

Main


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public class Counter {
    int count;  // Declare a variable

    public void increment() {  // In this method we increment
        count++;
    }
}

It may seem like incrementing is a single operation, but it is actually composed of 3 operations:

But at the machine code level, these are several operations:

1. Reading the count value;

2. Increasing the value by 1;

3. Writing the new value back to count.

Note

If multiple threads perform this operation simultaneously without synchronization, the result might be incorrect because one thread might start incrementing while another thread is still writing the increment result to memory. This issue is known as race conditions.

How to Avoid Atomicity Violations?

The following approaches can address atomicity issues in multithreaded programming in Java:

Using synchronization: Synchronization controls access to shared resources by using the synchronized keyword, which can be applied to methods or code blocks.

Using atomic classes: Java's java.util.concurrent.atomic package offers classes for atomic operations. These classes use low-level synchronization mechanisms, like CAS (Compare-And-Swap), to ensure atomicity without locks. (We will explore these classes in more detail later)

Use of high-level classes and collections: Java provides high-level synchronized data structures such as ConcurrentHashMap and CopyOnWriteArrayList that offer safe access from multiple threads. (We will review these classes in more detail later)

1. Which of the following approaches help to ensure atomicity of operations in Java?

2. Why is the increment operation (incrementing a value by 1) not atomic in multithreaded programming?

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