How do you find all subclasses of a given class in Java?

Question

How does one go about and try to find all subclasses of a given class (or all implementors of a given interface) in Java? As of now, I have a method to do this, but I find it quite inefficient (to say the least). The method is:

1. Get a list of all class names that exist on the class path
2. Load each class and test to see if it is a subclass or implementor of the desired class or interface

In Eclipse, there is a nice feature called the Type Hierarchy that manages to show this quite efficiently. How does one go about and do it programmatically?

Answer 1

There is no other way to do it other than what you described. Think about it - how can anyone know what classes extend ClassX without scanning each class on the classpath?

Eclipse can only tell you about the super and subclasses in what seems to be an "efficient" amount of time because it already has all of the type data loaded at the point where you press the "Display in Type Hierarchy" button (since it is constantly compiling your classes, knows about everything on the classpath, etc).

Answer 2

It should be noted as well that this will of course only find all those subclasses that exist on your current classpath. Presumably this is OK for what you are currently looking at, and chances are you did consider this, but if you have at any point released a non-final class into the wild (for varying levels of "wild") then it is entirely feasible that someone else has written their own subclass that you will not know about.

Thus if you happened to be wanting to see all subclasses because you want to make a change and are going to see how it affects subclasses' behaviour - then bear in mind the subclasses that you can't see. Ideally all of your non-private methods, and the class itself should be well-documented; make changes according to this documentation without changing the semantics of methods/non-private fields and your changes should be backwards-compatible, for any subclass that followed your definition of the superclass at least.

Answer 3

Don't forget that the generated Javadoc for a class will include a list of known subclasses (and for interfaces, known implementing classes).

Answer 4

The reason you see a difference between your implementation and eclipse is because you scan each time, while eclipse ( and other tools ) scan only once ( during project load most of the times ) and create an index. Next time you ask for the data he doesn't scan again, but look at the index.

Answer 5

Scanning for classes is not easy with pure Java.

The spring framework offers a class called ClassPathScanningCandidateComponentProvider that can do what you need. The following example would find all subclasses of MyClass in the package org.example.package

ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(true);
provider.addIncludeFilter(new AssignableTypeFilter(MyClass.class));

// scan in org.example.package
Set<BeanDefinition> components = provider.findCandidateComponents("org/example/package");
for (BeanDefinition component : components)
{
    Class cls = Class.forName(component.getBeanClassName());
    // use class cls found
}

This method has the additional benefit of using a bytecode analyzer to find the candidates which means it will not load all classes it scans.