How Can I Discover All Subclasses of a Given Class at Runtime in Java?

Reflection-Based Class Discovery in Java

In Java applications, determining all classes extending a base class at runtime poses a challenge due to the nature of .class file proliferation. This article explores various approaches to overcome this limitation.

Using Java Reflection and Third-Party Libraries

Through reflection, it is possible to traverse an application's classpaths to locate classes. Libraries like org.reflections provide convenient methods for this task. For instance:

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

This code retrieves all classes that extend MyInterface within the specified package.

Example Application

Consider an application that needs to create objects of all classes that extend Animal. Using the above technique, we can achieve this:

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 will print the names of all List implementations and create an instance of ArrayList to verify its functionality.

Conclusion

While there are inherent challenges in Java due to its class-based structure, the use of reflection and libraries like org.reflections provides practical methods for runtime class discovery, empowering developers to extend functionality and dynamically create instances of classes based on their inheritance relationships.

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