Question

Conceptually, what am I doing when I’m loading a new Jar? Is URLClassloader the only choice? How should I form those URLs to point to a subdirectory containing more jars.

If anyone is feeling super-generous, some demonstration code to do the following would be really helpful (let’s assume “jars/A.jar” contains “myClass” which we want to instantiate):

1. load some jar from a subdirectory
2. return a particular class from it
3. instantiate that class

Answer 1

Conceptually, what am I doing when I’m loading a new Jar?

Conceptually, you are not loading a new JAR. Rather, you are defining a class loader which will load code and other resources from the JAR file on demand.

Is URLClassloader the only choice?

In theory, you could implement your own subclass of ClassLoader, but that’s not necessary for what you are trying to do.

How should I form those URLs to point to a subdirectory containing more jars.

This is the nub of your problem I think. The URLClassLoader constructors interpret the URL[] argument as follows:

“Any URL that ends with a ‘/’ is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be downloaded and opened as needed.”

In the first case of the quoted text above, the directory is assumed to be the root of tree containing resources to be loaded.

But I gather you are trying to set up a class loader that will load from all JAR files in a given directory. To do that, you need to:

1. Read the directory and build a list of the File objects for any JAR files.
2. Create an array to hold the same number of URL instances.
3. For each JAR file File, use File.toURL() to create a URL and add to the array. (Using File.toURL() means that you will get kosher “file:” URLs for the JAR files that will work on your platform.)
4. Create the URLClassLoader using the URL array.