What is the purpose of the 'volatile' keyword in Java?

Understanding the 'volatile' Keyword in Java

In Java, the volatile keyword is used to modify the value of a variable by multiple threads. It is also used to ensure that reading and writing operations to the variable are atomic. But, what does that mean exactly, and how does it work in practical contexts? Let's dive in to learn more.

The volatile Keyword and Thread-Safety

A volatile variable in Java is used in multithreaded programs. When a variable is declared as volatile, it ensures that all reads from and writes to that variable are made directly to and from the main memory. This guaranties that the most recent change to a volatile variable by any thread is visible to all other threads - this makes it thread-safe in terms of visibility.

Consider the following example:

public class ExampleThread implements Runnable {
    private volatile int count = 0;

    @Override
    public void run() {
        for(int i=0; i<50; i++)
            incrementCount();
    }

    public void incrementCount(){
       count++;
    }

    public int getCount() {
        return count;
    }
}

In this case, multiple threads accessing count would always see the most recent update to its value due to its volatile designation.

When to Use the volatile Keyword

It's important to note that volatile doesn't provide atomicity of compound operations like count++. If you need to read-update-write as an atomic operation, use synchronized, java.util.concurrent.atomic package or Locks. volatile is only appropriate when the updates do not rely on the current state of the variable.

Despite the limitations, volatile plays a crucial role in the Java Memory Model's synchronization process, specifically with the visibility and ordering of changes to variables across threads.

Using volatile Keyword: Best Practices

While its uses are very specific, volatile is a powerful tool in a Java developer's toolbox. Here are some key things to remember:

1. Use volatile variables when you want to read and write to the variable

2. You should not use volatile for compound operations like i++

3. volatile does not provide the atomicity of the compound operation. Make sure to use other synchronization methods in those cases.

4. Each read or write to a volatile variable creates a "memory barrier", which can affect performance in high concurrency scenarios.

All things considered, volatile is an important keyword to understand and use correctly, particularly as it pertains to concurrency in Java. By employing thoughtful strategies and having a clear understanding of its limitations and strengths, volatile will be a beneficial tool for managing multi-thread handling in your Java programs.