Troubleshooting URLClassLoader: Loading Classes from External JAR Files in Java скачать в хорошем качестве

Troubleshooting URLClassLoader: Loading Classes from External JAR Files in Java

Learn how to effectively use `URLClassLoader` to load classes from external JAR files in Java and resolve common issues that return null. --- This video is based on the question https://stackoverflow.com/q/73445524/ asked by the user 'OreoFanatics' ( https://stackoverflow.com/u/7434507/ ) and on the answer https://stackoverflow.com/a/73445764/ provided by the user 'DrHopfen' ( https://stackoverflow.com/u/6825678/ ) 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 trying to get a class from external jar always returning null 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. --- Troubleshooting URLClassLoader: Loading Classes from External JAR Files in Java When working with Java, it’s common to need to load classes dynamically from external JAR files. However, if you find that your attempts to load a class using URLClassLoader always return null, you are not alone. This issue can be frustrating, especially when you have double-checked everything from file paths to class names. In this guide, we will explore a typical scenario that leads to this problem and offer solutions to help you overcome it. The Problem Imagine you are trying to load a specific class named PlumberConfig located within the myapp.jar file. The structure of your JAR file looks like this: [[See Video to Reveal this Text or Code Snippet]] Your code for loading this class using URLClassLoader might look like this: [[See Video to Reveal this Text or Code Snippet]] Despite your efforts, calling new URLClassLoader() seems to return null, preventing you from loading the class. Understanding the Solution The good news is that the issue you are facing is a common misunderstanding regarding the use of URLClassLoader. Let’s break down the solution into clear steps. 1. Verify Your Setup Before diving deep into fixes, confirm the following: Correct Path: Ensure that the path to the JAR file is accurate. Sometimes, a simple typo can lead to issues. Class Availability: Check the actual existence of your class within the JAR file. You can open the JAR file using tools such as WinRAR or any archive manager. 2. Use the Correct Method One of the main issues is misunderstanding how getName() works. Contrary to what you might have expected, getName() is not defined on URLClassLoader. Instead, you should be using getURLs() to retrieve the URLs associated with the class loader. Here’s how you can modify your code: [[See Video to Reveal this Text or Code Snippet]] This will display the URLs that the URLClassLoader is aware of, giving you insight into whether your JAR file is correctly loaded. 3. Load the Class Properly With the above changes, if everything else is set up correctly, you should proceed to load the class like this: [[See Video to Reveal this Text or Code Snippet]] This checks if the class has been loaded successfully and logs the right message accordingly. Conclusion In summary, when using URLClassLoader, make sure you are aware of the methods available for use. Always verify your paths and the presence of the class within the JAR. By using getURLs() instead of getName(), you can debug and resolve issues effectively. Armed with this knowledge, you can now comfortably load classes from your external JAR files without running into frustrating null returns. Happy coding!