How Can I Dynamically Discover and Inspect Java Classes at Runtime from JARs or Folders?

Discovering Classes at Runtime from Folders or JARs

Envision a Java tool that delves into the structure of a Java application, extracting meaningful insights. To achieve this, the tool must scan .class files from a specified source (JAR/WAR or folder). It then employs reflection to inspect class methods and their attributes. However, this task has proven elusive.

Numerous solutions based on URLClassloader exist, allowing the loading of specific classes from directories or archives. However, none seem capable of identifying and loading classes without prior knowledge of their names or package structures.

Addressing the Crux of the Issue

Upon closer examination, the challenge lies not in acquiring all class files through recursive scanning techniques but rather in obtaining Class objects for each discovered file.

Leveraging the Power of JarFile and URLClassLoader

The provided code effectively utilizes Java NIO to load all classes from a JAR file without requiring any prior knowledge of their existence. The process involves:

1. Constructing a JarFile object representing the JAR in question
2. Enumerating the JarFile's entries to identify .class files
3. Creating a URLClassLoader instance to load the extracted class names as defined in the respective JarEntries

Considering an Alternative: Javassist

As suggested by commentators, Javassist offers another viable option. By initializing a ClassPool before the loop, you can create CtClass objects in lieu of loading classes with a class loader. CtClass objects provide access to an array of class attributes, including methods, fields, nested classes, and more.

This comprehensive solution empowers developers with the ability to dynamically load and inspect classes at runtime, providing a powerful foundation for sophisticated code analysis and manipulation tools.

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