Understanding Why actionPerformed Is Executed by Two Threads

This guide explains why the `actionPerformed` method in Java Swing applications might get invoked by multiple threads, along with solutions to ensure your game handles inputs correctly.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why actionPerformed is executed by two Threads?

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Why actionPerformed Is Executed by Two Threads: A Simple Guide

In the world of Java Swing development, especially when creating games with responsive input, encountering threading issues can become quite frustrating. A common problem game developers face is when the actionPerformed method is executed by more than one thread despite the input being activated just once. If you've ever wondered why this happens and how to prevent it, you've come to the right place!

The Problem Explained

For instance, in a simple game designed with Swing, a developer encountered an issue where their showMenu() method was being called multiple times by two different threads when the 'm' key was pressed just once. The primary question here is: Why is this happening?

Key Symptoms

The issue was specifically observed when the game loop was running. Upon pressing the 'm' key, the console output indicated that the showMenu call was coming from two threads. For example:

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

Analyzing the Cause

The key to understanding this problem lies in how Java handles threading, particularly with Swing applications. Here’s the breakdown of what’s happening:

Game Loop Functionality: The processGameLoop() method is crucial because it runs in its own thread and is responsible for the game's main loop. Within its implementation, it handles rendering and waits until the optimal time to render the next frame.

Thread Interruption: When the game loop gets interrupted (for instance, when stopping the game), this interruption triggers the catch block of the try-catch statement within the processGameLoop(), leading to the invocation of showMenu() again. This is evident in the snippet:

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

Not Swing Thread-Safe: The key issue here is that the game loop isn't thread-safe regarding Swing UI operations, as it’s calling the Swing components (like showMenu()) from a background thread. In Swing, it is crucial to handle UI updates on the Event Dispatch Thread (EDT), and not directly from other threads.

How to Prevent It

To resolve this issue and ensure that the actionPerformed method is consistently executed by a single thread, follow these best practices:

Use the Event Dispatch Thread (EDT)

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

Review the Game Loop Implementation

Ensure that the game loop is correctly managing its thread state and not inadvertently creating conditions where interruptions lead to concurrent UI calls.

Wrap Methods to Handle Actions Safely

Make sure that methods invoked from event listeners also respect threading constraints:

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

Conclusion

By following these approaches, you can avoid the frustrating and unpredictable behavior of method calls like actionPerformed being executed by multiple threads in your Java Swing applications. Keeping your threading models in check when dealing with Swing UI components is essential for building smooth and interaction-friendly applications, especially for games.

By recognizing the importance of thread management in Java Swing, developers can enhance the quality and reliability of their applications. Happy coding, and may your game run smoothly with a seamless user experience!