How to Fix ClassNotFoundException When Using URLClassLoader in Java скачать в хорошем качестве

How to Fix ClassNotFoundException When Using URLClassLoader in Java

Learn how to resolve the `java.lang.ClassNotFoundException` error when dynamically loading classes with URLClassLoader in Java. --- This video is based on the question https://stackoverflow.com/q/76146141/ asked by the user 'Unknown BL4NK' ( https://stackoverflow.com/u/8344003/ ) and on the answer https://stackoverflow.com/a/76146214/ provided by the user 'Sweeper' ( https://stackoverflow.com/u/5133585/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. 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: URLClassLoader is unable to load class in java Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- How to Fix ClassNotFoundException When Using URLClassLoader in Java Working with Java, especially when dealing with dynamic loading of classes, can sometimes be tricky. A common issue that many developers face is the infamous ClassNotFoundException. This guide will take you through a typical scenario and provide an in-depth solution to resolve this problem when using URLClassLoader in Java. The Problem: Unable to Load Class Imagine you have written a Java source file and you want to load it dynamically at runtime using the URLClassLoader. You may follow all the right steps—compiling the Java file, creating a JAR, and attempting to load the class. However, you might encounter the error message: [[See Video to Reveal this Text or Code Snippet]] This error indicates that the Java runtime is unable to locate the Equity class defined in the ubl4nk package. Below is an overview of the steps leading up to this error so you can understand the context better. Steps Leading to the Error Java Source File: Your initial Java source file (Equity.java) includes packages from external JAR files (lib1.jar and lib2.jar). [[See Video to Reveal this Text or Code Snippet]] Compilation: You compile your file using: [[See Video to Reveal this Text or Code Snippet]] Creating a JAR File: Next, you package your compiled class into a JAR: [[See Video to Reveal this Text or Code Snippet]] Loading the Class: You attempt to load your class using URLClassLoader: [[See Video to Reveal this Text or Code Snippet]] The Solution: Proper JAR Packaging The root cause of the ClassNotFoundException lies in how the JAR file was created. When you used the command jar -cf myjar.jar *.class, it only included the Equity.class file at the root of the JAR, omitting its package structure. Correctly Packaging Your Class To ensure that your class is correctly recognized with its package structure, follow these steps to create your JAR file with the necessary hierarchy: Include the Whole Directory: Instead of just the class file, run: [[See Video to Reveal this Text or Code Snippet]] This command includes all files in the current directory, preserving the package structure. Or, Define the Package Folder: If you'd rather specify the package directory (assuming ubl4nk is the folder name), you can run: [[See Video to Reveal this Text or Code Snippet]] Conclusion By following the correct steps to create your JAR file with the appropriate package structure, you can resolve the ClassNotFoundException. It's crucial to ensure that the package hierarchy is maintained when building your JAR, enabling the URLClassLoader to find and load your classes without issues. With this understanding, you can confidently work with dynamic class loading in Java and avoid common pitfalls associated with classpath configurations. Happy coding!