Use Java reflection to find classes that implement an interface from a package

How do you find a set of Classes that implement and interface? You do this (well, I did this).

final List<Class<?>> processorCandidates = ReflectionHelper.findClassesImpmenenting(HostDataProcessor.class, HostDataProcessor.class.getPackage());

And use this helper class:

public class ReflectionHelper {

public static List<Class<?>> findClassesImpmenenting(final Class<?> interfaceClass, final Package fromPackage) {

if (interfaceClass == null) {
Debug.println("Unknown subclass.");
return null;
}

if (fromPackage == null) {
Debug.println("Unknown package.");
return null;
}

final List<Class<?>> rVal = new ArrayList<Class<?>>();
try {
final Class<?>[] targets = getAllClassesFromPackage(fromPackage.getName());
if (targets != null) {
for (Class<?> aTarget : targets) {
if (aTarget == null) {
continue;
}
else if (aTarget.equals(interfaceClass)) {
Debug.println("Found the interface definition.");
continue;
}
else if (!interfaceClass.isAssignableFrom(aTarget)) {
Debug.println("Class '" + aTarget.getName() + "' is not a " + interfaceClass.getName());
continue;
}
else {
rVal.add(aTarget);
}
}
}
}
catch (ClassNotFoundException e) {
Debug.println("Error reading package name.");
Debug.printStackTrace(e, Debug.LOW_LEVEL);
}
catch (IOException e) {
Debug.println("Error reading classes in package.");
Debug.printStackTrace(e, Debug.LOW_LEVEL);
}

return rVal;
}

/**
* Load all classes from a package.
*
* @param packageName
* @return
* @throws ClassNotFoundException
* @throws IOException
*/
public static Class[] getAllClassesFromPackage(final String packageName) throws ClassNotFoundException, IOException {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
assert classLoader != null;
String path = packageName.replace('.', '/');
Enumeration<URL> resources = classLoader.getResources(path);
List<File> dirs = new ArrayList<File>();
while (resources.hasMoreElements()) {
URL resource = resources.nextElement();
dirs.add(new File(resource.getFile()));
}
ArrayList<Class> classes = new ArrayList<Class>();
for (File directory : dirs) {
classes.addAll(findClasses(directory, packageName));
}
return classes.toArray(new Class[classes.size()]);
}

/**
* Find file in package.
*
* @param directory
* @param packageName
* @return
* @throws ClassNotFoundException
*/
public static List<Class<?>> findClasses(File directory, String packageName) throws ClassNotFoundException {
List<Class<?>> classes = new ArrayList<Class<?>>();
if (!directory.exists()) {
return classes;
}
File[] files = directory.listFiles();
for (File file : files) {
if (file.isDirectory()) {
assert !file.getName().contains(".");
classes.addAll(findClasses(file, packageName + "." + file.getName()));
}
else if (file.getName().endsWith(".class")) {
classes.add(Class.forName(packageName + '.' + file.getName().substring(0, file.getName().length() - 6)));
}
}
return classes;
}
}