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Explore the role of the `volatile` keyword in Java, particularly in multithreading scenarios involving the `main()` thread. Uncover the significance of inter-thread visibility and learn why the `volatile` keyword might not be necessary in certain conditions.
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Understanding the volatile Keyword: Inter-Thread Visibility and the main() Thread

When working with Java's multithreading capabilities, you may encounter scenarios where thread visibility can be perplexing. A common question arises: Why is the volatile keyword not needed for inter-thread visibility when one of the threads involved is the main() thread?

In this guide, we’ll explain this intricate aspect of Java threading and help you grasp the reasoning behind it. Let's dive into the example provided in the question to understand how it all works.

The Example Code

Consider the Java snippet below:

[[See Video to Reveal this Text or Code Snippet]]

Key Points of the Example

The stopRequested Variable: It's declared as a regular static boolean variable and not marked as volatile.

Expected Behavior: Ideally, when stopRequested is changed to true in the main() thread, the backgroundThread should read the new value and stop its execution.

Outcome: Surprisingly, when executed, the backgroundThread stops after printing several numbers and displays "Stopping the thread!!".

Why Does This Happen Without volatile?

The Java Language Specification does not guarantee that changes made by one thread are visible to other threads without proper synchronization. Here’s a deeper dive into these concepts:

Understanding Visibility with volatile

The volatile keyword guarantees that any read by one thread is always the latest write made by another thread. It adds a safety net against what the Java Memory Model may otherwise allow, ensuring visibility of shared variables.

If stopRequested were marked as volatile, both threads would be assured that they have the most current value at all times.

Absence of volatile - An Unpredictable Outcome

In the absence of synchronization (i.e., no use of volatile), there is no guaranteed happen-before relationship between the write (in the main() thread) and the read (in the backgroundThread). This implies:

The backgroundThread could read the old value of stopRequested, leading to unexpected results during execution.

However, in our current example, you might see the backgroundThread stopping. Yet, this is not guaranteed behavior and can change based on various factors, including optimizations by the Java Virtual Machine (JVM).

Testing the Limits

To illustrate how unreliable unsynchronized variables can be, consider running the loop without any printing:

[[See Video to Reveal this Text or Code Snippet]]

When tested under the same conditions, you may find that this time the thread does not terminate. The lack of any output meant that the Just-In-Time (JIT) compiler applied optimizations, which caused it to behave differently.

Conclusion

In conclusion, even if your code appears to function correctly without using the volatile keyword, relying on such behavior is dangerous.

Thread Safety: Always use volatile when you need inter-thread visibility.

Robustness: Consider conforming to proper synchronization practices to ensure your code is reliable, especially in more complex multithreaded environments.

By understanding the nuances of the Java Memory Model and the importance of the volatile keyword, you can write more robust and error-free multithreaded applications. Remember, it’s not just about making it work—it's about ensuring it works predictably under all conditions.