How Can I Find Runtime Classes Extending a Specific Base Class in Java?

Finding Runtime Classes that Inherit from a Base Class in Java

In Java, detecting classes within a running application that extend a specified base class has been a longstanding challenge. Traditionally, developers maintained a list of classes that needed to be instantiated, potentially requiring updates when new classes were added or removed.

However, recent advancements in Java make this task much more straightforward. The org.reflections library provides a convenient mechanism to introspect and retrieve classes based on inheritance relationships. Here's how it works:

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

In this example, the Reflections instance is configured to examine packages starting with "com.mycompany." The getSubTypesOf method returns a Set of all classes that extend the MyInterface interface.

Another example demonstrates how to find and instantiate concrete classes:

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());
    }
}

In this code, the library is used to find subclasses of java.util.List. The loop iterates over the classes, prints their names, and for the ArrayList class, creates an instance and verifies its size after adding an element.

Previously, this functionality was not natively supported in Java. However, with the advent of reflection libraries like org.reflections, developers can now dynamically discover and instantiate classes based on inheritance relationships, enabling more flexible and extensible code bases.

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