Plugin based systems are really common. Jenkins, Jira, wordpress, whatever. Recently I built a plugin workflow for a system at work and have been mired in the joys of the class loader. For the uninitiated, a class in Java is identified uniquely by the class loader instance it is created from as well as its fully qualified class name. This means that
foo.bar class loaded by class loader A is not the same as
foo.bar class loaded by class loader B.
There are actually some cool things you can do with this, especially in terms of code isolation. Imagine your plugins are bundled as shaded jars that contain all the internal dependencies. By leveraging class loaders you can isolate potentially conflicting versions of libraries from the host application and the plugin. But, in order to communicate to the host layer, you need a strict set of shared interfaces that the … Read more