You concept of "package" is at fault.  That's to say: you have a mental
"picture" of what they are and how they work which is only approximately
connected with reality.

I could trivially create a "package" which generated random new classes every
10 seconds for as long as the application is running.  I could trivially create
a classloader which /always/ found a class for any possible classname it was
asked for -- again generating classes on the fly (and probably randomly).  In
either case your question has no meaning at all.

If you want to restrict yourself to less dynamic ideas of what a package is,
and what sorts of classloader you are going to use, then the question may
become feasible to answer -- for instance, you could simply include a list of
classes (maybe indexed by interfaces) as a readable text resource in the same
JAR.  Alternatively, you could discover the location from which the classloader
was loading .class files (if there was any concept of "location") and use
location-specific code to discover what .class files are available there.  E.g.
reading the table-of-contents of a JAR file considered as a ZIP file, or
listing (recursively) the contents of a directory.  Note that this may not be
possible; for instance if the "location" is an HTTP base-URL.

   -- chris

Class.forName(name, false, null) won't do that.

piotr

Of course, it should.

See the output of running the following agent:

import java.lang.instrument.Instrumentation;
import java.lang.reflect.Proxy;
import java.util.ArrayList;
import java.util.Arrays;

/**
 * After building agent's jar with duly manifest run it with:
 *   java -javaagent:agent.jar ClassesAgent
 *
 * @author pk
 */
public class ClassesAgent {

    private static Instrumentation inst;

    public static void premain(String agentArgs, Instrumentation inst) {
        ClassesAgent.inst = inst;
    }

    public static Class[] getAllClassesAssignableFrom(Class...
interfaces) {
        ArrayList<Class> found = new ArrayList<Class>();
        loop: for (Class c : inst.getAllLoadedClasses()) {
            for (Class<?> ic : interfaces)
                if (!ic.isAssignableFrom(c))
                    continue loop;
            found.add(c);
        }
        return found.toArray(new Class[found.size()]);
    }

    interface Test {}

    public static void main(String[] args) throws Exception {
        System.out.println("lookup of classes implementing " +
Test.class + "...");
        Class[] classes;
        classes = getAllClassesAssignableFrom(Test.class);
        System.out.println("classes found: " + Arrays.toString(classes));
        Class tic = Proxy.getProxyClass(
                Test.class.getClassLoader(), Test.class);
        System.out.println("just generated: " + tic);
        classes = getAllClassesAssignableFrom(Test.class);
        System.out.println("classes found: " + Arrays.toString(classes));
    }

}

piotr