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

use this

public static Set<Class> getExtendedClasses(Class superClass)
{
    try
    {
        ResolverUtil resolver = new ResolverUtil();
        resolver.findImplementations(superClass, superClass.getPackage().getName());
        return resolver.getClasses();  
    }
    catch(Exception e)
    {Log.d("Log:", " Err: getExtendedClasses() ");}

    return null;
}

getExtendedClasses(Animals.class);

Edit:

• library for (ResolverUtil) : Stripes

Answer 2

Using OpenPojo you can do the following:

String package = "com.mycompany";
List<Animal> animals = new ArrayList<Animal>();

for(PojoClass pojoClass : PojoClassFactory.enumerateClassesByExtendingType(package, Animal.class, null) {
  animals.add((Animal) InstanceFactory.getInstance(pojoClass));
}

Answer 3

The Java way to do what you want is to use the ServiceLoader mechanism.

Also many people roll their own by having a file in a well known classpath location (i.e. /META-INF/services/myplugin.properties) and then using ClassLoader.getResources() to enumerate all files with this name from all jars. This allows each jar to export its own providers and you can instantiate them by reflection using Class.forName()

Answer 4

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<Animal> resolver = new ResolverUtil<Animal>();
        resolver.findImplementations(Animal.class, "test");
        Set<Class<? extends Animal>> classes = resolver.getClasses();

        for (Class<? extends Animal> 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);

Answer 5

One way is to make the classes use a static initializers... I don't think these are inherited (it won't work if they are):

public class Dog extends Animal{

static
{
   Animal a = new Dog();
   //add a to the List
}

It requires you to add this code to all of the classes involved. But it avoids having a big ugly loop somewhere, testing every class searching for children of Animal.

Answer 6

Unfortunately this isn't entirely possible as the ClassLoader won't tell you what classes are available. You can, however, get fairly close doing something like this:

for (String classpathEntry : System.getProperty("java.class.path").split(System.getProperty("path.separator"))) {
    if (classpathEntry.endsWith(".jar")) {
        File jar = new File(classpathEntry);

        JarInputStream is = new JarInputStream(new FileInputStream(jar));

        JarEntry entry;
        while( (entry = is.getNextJarEntry()) != null) {
            if(entry.getName().endsWith(".class")) {
                // Class.forName(entry.getName()) and check
                //   for implementation of the interface
            }
        }
    }
}

Edit: johnstok is correct (in the comments) that this only works for standalone Java applications, and won't work under an application server.