At runtime, find all classes in a Java application that extend a base class

Question

I want to do something like this:

List animals = new ArrayList();

for( Class c: list_of_all_classes_available_to_my_app() )
   i         


        

Answer 1

You could use ResolverUtil (raw source) from the Stripes Framework
if you need something simple and quick without refactoring any existing code.

Here's a simple example not having loaded any of the classes:

package test;

import java.util.Set;
import net.sourceforge.stripes.util.ResolverUtil;

public class BaseClassTest {
    public static void main(String[] args) throws Exception {
        ResolverUtil resolver = new ResolverUtil();
        resolver.findImplementations(Animal.class, "test");
        Set> classes = resolver.getClasses();

        for (Class clazz : classes) {
            System.out.println(clazz);
        }
    }
}

class Animal {}
class Dog extends Animal {}
class Cat extends Animal {}
class Donkey extends Animal {}

This also works in an application server as well since that's where it was designed to work ;)

The code basically does the following:

• iterate over all the resources in the package(s) you specify
• keep only the resources ending in .class
• Load those classes using ClassLoader#loadClass(String fullyQualifiedName)
• Checks if Animal.class.isAssignableFrom(loadedClass);