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

Finding Classes Extending a Base Class at Runtime in Java

At runtime, you may need to find all classes within your Java application that inherit from a specific base class. This can be useful for implementing dynamic class loading or adding functionality without modifying existing code.

In Java, it is not straightforward to retrieve a list of all available classes in the application's classpath at runtime. However, there are libraries that can facilitate this task.

One such library is org.reflections, which provides methods for scanning packages and discovering classes. Using org.reflections, you can obtain a set of classes that extend a given base class:

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

This code scans the "com.mycompany" package recursively, looking for classes that implement the "MyInterface" interface. You can then iterate over the resulting set of classes and instantiate objects as needed.

Another notable library for this purpose is ClassGraph. It uses a different approach to discover classes, providing a comprehensive API for querying and inspecting the classpath. With ClassGraph, you can obtain a list of classes extending a base class using the following code:

ClassGraph classGraph = new ClassGraph();
ClassInfoList classInfos = classGraph.getSubclasses(BaseClass.class);

ClassGraph scans the classpath thoroughly, including jar files and class loaders. It offers additional features such as filtering by annotation or method presence.

These libraries provide a convenient way to dynamically load and instantiate child classes at runtime. By avoiding hardcoding a list of classes, you can easily add or remove functionality without altering the base class or application code.

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