From Anton...

I've downloaded your sources and compiled them myself to see if I could narrow it down... and I did!

The problem lies in the class com.avaje.ebean.server.util.ClassPathSearch in partical this method:
private void initClassPaths() {
try {
classPaths = ((java.net.URLClassLoader) classLoader).getURLs();
} catch (ClassCastException e) {
classPaths = System.getProperty("java.class.path", "").split(File.pathSeparator);
}
if (logger.isLoggable(Level.FINER)) {
String msg = "Classpath " + Arrays.toString(classPaths);
logger.finer(msg);
}
}

The classloader resin 2.1.17 is giving us is the com.caucho.java.CompilingClassLoader which in particular is not an instance of java.net.URLClassLoader, so you get the classcastexception your code is catching. But the system-property java.class.path is not filled with the web-application jars/classes, so that's the reason for not running under resin 2.1.17. Resin 3.x gives you the option to specify classloaders via their config-files, so it can probably work without adjustments for resin 3.x, but I need resin 2.1.17 for some applications we have in production....

I've tried searching for a general way besides getUrls() to get all the resource-paths from a classloader, but I didn't find anything alike. So for my application I've hacked the method like this:
private void initClassPaths() {
try {
classPaths = ((java.net.URLClassLoader) classLoader).getURLs();
} catch (ClassCastException e) {
try {
classPaths = classLoader.getClass().getMethod("getClassPath").invoke(classLoader).toString().split(File.pathSeparator);
} catch (Throwable ex) {
classPaths = System.getProperty("java.class.path", "").split(File.pathSeparator);
}
}
if (logger.isLoggable(Level.FINER)) {
String msg = "Classpath " + Arrays.toString(classPaths);
logger.finer(msg);
}
}
This works because com.caucho.java.CompilingClassLoader implements the interface com.caucho.util.CauchoClassLoader which amongst others specifies this method:
public String getClassPath();

Do you know any general ways to retrieve all resource-paths from a classloader? That whould certainly make my hack unneccesary....