Jar Loader

What is it?

Easy migration to Java 9+ for applications that extend the class path by reflective access to the class loader.

Does your code work under Java 8 but fail under Java 9+ with an error stating that the system class loader is not an instance of URLClassLoader or that the method addURL could not be found? Then this is for you.

How to use it

Three steps:

1. Add jar-loader.jar to your build libraries and application class path.
2. Add -javaagent:path/to/jar-loader.jar to the VM options passed to the java command used to start your application.
3. Where your code previously used reflection to invoke addURL on the system class loader, instead call ca.cgjennings.jvm.JarLoader.addToClassPath(File jarFile).

Notes

Jar Loader uses an official, documented method for extending the class path at runtime that has been supported since Java 6. It falls back to attempting reflective access if that fails. This method differs from URLClassLoader in that it only supports local JAR files. If your code relies on addURL to download code over a network, you will have to download it yourself and write it to a local file.

If you move the compiled JarLoader class to a different JAR file, it must include a MANIFEST.MF with the following line:

Premain-Class: ca.cgjennings.vm.JarLoader

Image credit: icon adapted from an icon by Eucalyp.