Question

I’ve inherited a Java project which is aimed at being a platform for multi-agent development. The idea is that you can create “applications” which run on the platform. I’m looking for a way to distribute the base platform such that the end users can add applications (likely packaged in jars) to a predefined folder, add a reference to the existence of said jar to some manifest file that the platform reads, and can therefore access files from it.

I’ve looked at the following answers, but these seem to require that the platform code know the names of classes at design-time:

• Read .jar file in Java applet

• Is there a way to load classes jars and packages on run time?

Ideally, I’d like the following structure to work:

• Main directory: platform.jar, apps.manifest, Apps (directory)
• Apps/App1/app1.jar, Apps/App2/app2.jar, …, Apps/AppN/appN.jar

apps.manifest will contain entries for each app, which will include the path to the jar, and the name of the class which is the main Application (implementing a common interface) object.

In my main code in platform.jar, I would like to read the manifest, notice that there are N applications, and be able to call ApplicationOne appOne = new ApplicationOne(this) where ApplicationOne is the name of the primary application object in app1.jar, and this is a reference to the object from platform.jar which holds the reference to the application.

If all the jars (platform.jar and the application jars) were all in the user’s classpath, would it all work as above? If so, is there a way to temporarily add a directory (and all subdirectories) to the user’s classpath in Java, which I could run when I run platform.jar? I ask temporarily because the enclosing folder should be movable on the file system.

Any advice on the issue appreciated, as well as any advice on improving the question so it’s clearer and easier to read.

Thanks

Answer 1

Sounds like the ServiceLoader API may be of use.

A service is a well-known set of interfaces and (usually abstract)
classes. A service provider is a specific implementation of a service.
The classes in a provider typically implement the interfaces and
subclass the classes defined in the service itself. Service providers
can be installed in an implementation of the Java platform in the form
of extensions, that is, jar files placed into any of the usual
extension directories.

See this blog entry on its usage