Why Java Is Introducing a New Classfile API to Replace ASM

Java community is drafting a Classfile API proposal to provide an API for parsing, generating, and transforming Java class files, initially as an internal replacement for ASM in the JDK and later as a public API.

The proposal notes that class file generation, parsing, and inspection are ubiquitous in the Java ecosystem; many tools and libraries need to handle class files, and frameworks often perform on‑the‑fly bytecode instrumentation, transformation, and generation. The JDK should offer an accurate, complete, up‑to‑date, high‑performance API for reading, writing, and converting class files.

The API aims to replace ASM as a runtime dependency without unacceptable performance loss, and eventually replace the internal "classreader" library used by the compiler and JDK tools. It is expected that many applications and frameworks will adopt this library to effectively replace ASM, cglib, or other bytecode libraries.

Design goals include representing all class‑file entities (such as methods and fields) with immutable objects and providing user‑driven navigation. The motivation includes tighter JDK integration, reducing version mismatches between frameworks and the running JDK, keeping up with rapid JVM evolution, and reflecting language improvements since ASM was first written.

For more details, see the proposal: https://bugs.openjdk.org/browse/JDK-8280389