java plugin specification via properties file

Question

I have a project which I want to add plugins. I have all the interfaces/factories/etc. setup (my gateway interface is called ApplicationMonitorFactory), I just need to make a way to locate/activate the plugin. My configuration file is a java properties file.

I think what I need to do is:

1. find a good way to specify a set of one or more plugins
2. for each plugin, run it

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1. find a good way to specify a set of one or more plugins

something like:

application.plugins=foo-monitor.jar,bar-monitor.jar

I think maybe it's just best to specify a list of jar files; for each jar file specified, the implication is that it contains one or more classes which implement ApplicationMonitorFactory, and these are the ones that will be instantiated. (I might also add an annotation @ApplicationMonitorPlugin so that a .jar file can have a test ApplicationMonitorFactory that does not get instantiated)

Does this sound reasonable?

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2. for each plugin, run it

I did this once a while back, and if I remember right I think I need to use a custom classloader to add the appropriate .jar file to the classpath dynamically. Or is there an easier way?

Answer 1

Could I suggest using OSGI instead? If it's a serverside project, something like Apache Karaf gives you quite a lot out of the box in terms of plugin deployment and specification.

To answer the questions based on what you have at the moment:

1. find a good way to specify a set of one or more plugins

The properties file approach is fine. You may want to just be able to drop plugins into a folder that you monitor if you want hot deploy. Just having 1 jar file for a plugin does limit plugin developers to packaging all of their dependencies into a single jar file (maven shade plugin is useful for this). The annotation approach should work (the approach that Servlet 3.0 uses). Using OSGI, you'd have a manifest file with a Bundle-Activator property that would reference the plugin class that should be instantiated.

2. for each plugin, run it

Yes, you would need to fire up a class loader for the Jar files. This is where things get a bit hairier. It's easy enough to do but Class loading has all sorts of gotcha's. This is where OSGI would really help, even though it is a bit of an upfront cost.