Скачать или смотреть Resolving NoClassDefFoundError in Jetty with Custom ClassLoaders

Discover how to properly configure custom ClassLoaders in Jetty to avoid `NoClassDefFoundError` and ensure your web services access their own JAR files seamlessly.
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This video is based on the question https://stackoverflow.com/q/62859674/ asked by the user 'yjatip' ( https://stackoverflow.com/u/1907747/ ) and on the answer https://stackoverflow.com/a/62862523/ provided by the user 'SKumar' ( https://stackoverflow.com/u/11244881/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving NoClassDefFoundError in Jetty with Custom ClassLoaders: A Guide

When developing web services using RESTEasy and Jetty, you may encounter a frustrating issue: your custom ClassLoader is not correctly utilized, leading to java.lang.NoClassDefFoundError. This can happen when your services are unable to find and access specific classes that are supposed to be loaded from designated JAR files. In this post, we will walk you through the problem and its solution step by step.

The Problem: NoClassDefFoundError in Jetty

Imagine that you are trying to create multiple web services. Each one is supposed to have its own unique set of JAR files, loaded from specific directories. You write a custom ClassLoader, assign it to Jetty's ServletContextHandler, and everything seems to be in place. However, when you call your web service, you receive the following error:

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

This error indicates that the class cannot be loaded, typically due to visibility issues arising from how ClassLoaders operate in Java.

Understanding ClassLoader Hierarchy

By default, Java uses a Parent-First strategy, which means it searches for classes in the following order:

Bootstrap Class Loader

Ext Class Loader

System Class Loader

Custom Class Loader

In your case, the Dummy class is being loaded using the System ClassLoader, while the HikariConfig class resides in your custom ClassLoader directory. Classes loaded via a ClassLoader can only access classes visible to it from its parent ClassLoader. When your Dummy class is using the system ClassLoader, it cannot see classes that exist in your custom ClassLoader.

Solution: Use the Proper ClassLoader

To resolve this NoClassDefFoundError, you need to ensure that your web services can access the.classes loaded in your custom ClassLoader. Here's how you can achieve that:

Step 1: Inject the ServletContext

Modify your Dummy class to inject the ServletContext. This will allow you to access the custom ClassLoader associated with the ServletContext:

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

Step 2: Load Class from the Custom ClassLoader

In the test() method, load the HikariConfig class using the injected servletContext.getClassLoader(). By doing this, you're ensuring that the service looks for the class in the correct ClassLoader, which should solve the visibility issue you were experiencing.

Conclusion

By adjusting how you load classes in your web service with Jetty and your custom ClassLoader, you can eliminate the occurrences of NoClassDefFoundError. Understanding the ClassLoader hierarchy and correctly utilizing the ServletContext allows your services to access their intended libraries without issues.

If you're new to ClassLoaders, the learning curve may seem steep, but with these strategies in place, you'll be better prepared to manage dependencies across different web services in your Java applications.

Keep experimenting and learning! Your understanding of Java ClassLoader mechanisms will only grow stronger with time.