I have written a library routine for this, based on code by
 Ralf Ullrich.

 For example, to find all classes implementing »java.util.Map«,
 one sets a filter accepting only classes which
 »java.util.Map« is assignable from:

public boolean accepts( final java.lang.Class class_ )
{ return java.util.Map.class.isAssignableFrom( class_ ); }

 One also needs to provide a starting class to find the jar, which
 is done as follows.

public java.lang.String entryPath(){ return "java.lang.Object"; }

 The jar containing this type will be chosen for exploration.

 In the following example, the client does not need to specify
 this »entryPath«, as "java.lang.Object" is the default
 obtained by extending
 »de.dclj.ram.java.lang.reflect.Finder.DefaultSpecification«.

 The example client is:

public class Main
{ /* based on an idea and on code by Ralf Ullrich from 2006 */

 public static void main( final java.lang.String[] args )
 { new de.dclj.ram.java.lang.reflect.Finder
   ( new de.dclj.ram.java.lang.reflect.Finder.DefaultSpecification()
     {
       public boolean isClassFinder(){ return true; }

       public boolean accepts( final java.lang.Class class_ )
       { return java.util.Map.class.isAssignableFrom( class_ ); }

       }).inspectJar(); }}

class java.lang.ProcessEnvironment
class java.rmi.server.RemoteObjectInvocationHandler$MethodToHash_Maps$1
class java.security.AuthProvider
(...)
class java.util.Properties
class java.util.Hashtable
interface java.util.Map

 The library »ram.jar« is an early publication in alpha state,
 it is experimental, changing, and mostly undocumented. See:

http://www.purl.org/stefan_ram/pub/ram-jar

 Known issues:

   - I have ideas to improve the interface, which are not yet
     implemented.

   - To compile the library from the sources, one might need
     to take care of some minor bugs: If I remember
     correctly, some »private«s need to  be replaced by
     »public«.

The correct way to implement this type of feature is using
java.util.ServiceLoader (since Java 6). This has been the recommended
approach since at least Java 1.4, but ServiceLoader was added to make it
easier.

Mark Thornton

I think there are two approaches:

1/ manually scan all classes available in your jar file
2/ specify your implementation somewhere, for example in your
manifest.mf file.

Cheers
K