How Can I Dynamically Find All Subclasses of a Base Class in Java at Runtime?

Dynamically Identifying Subclasses in Java Applications

Java programs utilize .class files that are loosely federated. This makes it challenging to dynamically identify all subclasses that extend a base class at runtime. However, there is a mechanism available to mitigate this limitation.

org.reflections Library

The org.reflections library offers a powerful solution for discovering subclasses at runtime. It utilizes annotations and reflection to introspect classes and their relationships. The following code snippet demonstrates its usage:

Reflections reflections = new Reflections("com.mycompany");
Set<Class<? extends MyInterface>> classes = reflections.getSubTypesOf(MyInterface.class);

where "com.mycompany" is the package to scan for classes. This code captures all subtypes of MyInterface within the specified package.

Example Usage

For instance, consider the following code:

public static void main(String[] args) throws IllegalAccessException, InstantiationException {
    Reflections reflections = new Reflections("java.util");
    Set<Class<? extends List>> classes = reflections.getSubTypesOf(java.util.List.class);
    for (Class<? extends List> aClass : classes) {
        System.out.println(aClass.getName());
        if(aClass == ArrayList.class) {
            List list = aClass.newInstance();
            list.add("test");
            System.out.println(list.getClass().getName() + ": " + list.size());
        }
    }
}

This code iterates through all subtypes of List in the java.util package. Specifically, it identifies ArrayList and demonstrates the ability to create an instance with reflection. The output will include the following:

java.util.LinkedList
java.util.AbstractSequentialList
java.util.Vector
java.util.ArrayList: 1

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