Over the years, there have been many third party libraries for programmatically generating Java byte codes, such as ASM, BCEL, and Javassist. These libraries are used to implement code transformations such as dependency injection or instrumentation, and more generally to dynamically generate behavior in the Java virtual machine. Since Java 24, there is a standard API for this purpose in the core platform. In this presentation, I will give a brief introduction into the byte codes of the JVM, followed by some usage examples. I will then review the structure of the Class File API and demonstrate a couple of simple programs for analyzing byte codes and generating new ones from scratch, before progressing to more intricate tasks that require rewriting existing code. One demo will add logging to an existing class, and another will provide automatic memoization of recursive methods, similar to the Python @functools.cache decorator.

Jfokus

A conference talk covering the Java Class File API (JEP 484), a new standard API in the JDK for parsing, generating, and transforming Java bytecode. The speaker explains bytecode basics, walks through practical examples including dumping strings from class files, adding logging via bytecode instrumentation, and implementing a memoization decorator similar to Python's @cache. Key topics include the API's immutable tree model, builder pattern with replay support, transformation convenience methods, the Java instrumentation agent mechanism for load-time bytecode rewriting, and how AI coding assistants (ChatGPT, Claude Code) struggle with this relatively new and uncommon API.

The Java Class File API, by Cay Horstmann