Why Does My JAR File Fail to Load the Main Class, and How Can I Fix It?

Running JAR Files: Uncovering the Main-Class Key

In the quest to execute JAR files, one may encounter pitfalls. This article delves into the enigmatic world of JAR file execution, uncovering the intricacies that underlie the "Failed to load main class" error.

As exemplified by the user's experience, simply double-clicking on a JAR file or running it from a DOS prompt without specifying a Main-Class is insufficient. To remedy this issue, we delve into the concept of the Main-Class, a vital entry within the JAR file manifest.

To construct a JAR file that runs flawlessly, adhere to the following steps:

1. Create two files: the source code (e.g., Test.java) containing the main method and a text file (e.g., manifest.mf) containing the manifest.
2. Ensure the manifest.mf file declares the Main-Class attribute, specifying the fully qualified name of the class containing the main method.

3. Execute the following commands:

   • Compile the source code: javac Test.java.
   • Create the JAR file with the manifest: jar cfm test.jar manifest.mf Test.class.
   • Execute the JAR file: java -jar test.jar.

By following these steps, you can banish the "Failed to load main class" error and successfully run your JAR files. Remember, the Main-Class serves as the entry point into the JAR file, guiding the execution of your application.

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