Understanding IllegalMonitorStateException in Java: When to Use this vs. .class as a Monitor

Learn why using `this` and `WaitTest.class` as synchronized monitors produce different results in Java multithreading and how to fix the `IllegalMonitorStateException`.
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This video is based on the question https://stackoverflow.com/q/66151797/ asked by the user 'andrewlawrence' ( https://stackoverflow.com/u/15189267/ ) and on the answer https://stackoverflow.com/a/66151961/ provided by the user 'dreamcrash' ( https://stackoverflow.com/u/1366871/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding IllegalMonitorStateException in Java: When to Use this vs. .class as a Monitor

In the world of Java multithreading, developers often encounter the IllegalMonitorStateException. This can particularly happen when dealing with synchronization in threads. Recently, a programmer faced this issue when trying to use the .class literal as a synchronized monitor in their code intended to print numbers using threads. Let us delve into this problem and how it can be solved.

The Problem

The user provided two versions of their code for printing numbers from 0 to 99 using two threads. In one case, they utilized this as the monitor for synchronization, and the code ran without issues. However, when they switched to using WaitTest.class as the monitor, they encountered an IllegalMonitorStateException. Here are the main points:

Working Code: synchronized (this) worked without issues.

Problematic Code: synchronized (WaitTest.class) led to an exception.

Understanding IllegalMonitorStateException

What Is IllegalMonitorStateException?

This exception is thrown when a thread attempts to wait on or notify an object's monitor (lock) without first owning that monitor. It usually indicates a problem with the way synchronization has been implemented in the code.

Why Does This Happen?

Using this: When calling wait() and notify() on this, you are implicitly using the same object that was synchronized (synchronized (this)). This means the thread holds the monitor lock for that specific instance of the class, allowing it to safely call these methods.

Using .class: When using WaitTest.class as the synchronized monitor, you are locking at the class level. However, inside the synchronized block, calling wait() and notify() on this refers back to the instance rather than the class monitor. Since the thread does not hold the monitor lock for this, the IllegalMonitorStateException is thrown.

The Solution

To resolve the issue, the code needs to be modified so that the calls for wait() and notify() are made on the same monitor that has been locked. Here’s how to do it:

Updated Code Snippet

Replace the following:

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

With this:

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

Here is the corrected version of the code:

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

Conclusion

The IllegalMonitorStateException can be a common source of frustration for Java developers, especially when dealing with synchronized blocks in multithreaded applications. Through understanding the nuances of how Java handles synchronization and monitors, you can avoid these pitfalls. Always make sure that when you wait or notify on an object, you hold the lock on that same object. By following the corrected usage of WaitTest.class.wait() and WaitTest.class.notify(), you'll successfully eliminate the exception and achieve your desired functionality in your Java application.

By mastering synchronization and threading in Java, you're not just solving current issues but also setting a strong foundation for any multithreaded applications you develop in the future.

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