Class-File API (2nd preview)

Java 23 Course · lesson 10 of 15 · 4 min read

A supported way to read and write bytecode, replacing everyone's bundled copy of ASM.

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Key points

  • Frameworks generate and rewrite bytecode constantly - proxies, mocks, instrumentation.
  • They all shade a copy of ASM, which must be updated for every new class-file version.
  • JEP 466 gives the JDK its own API that always understands the class file format it ships with.
  • It is immutable and tree-shaped: parse a ClassModel, transform it, write new bytes.
  • You will rarely call it directly, but it is why your framework upgrades should get less painful.

The Class-File API exists so tool authors stop shipping a bytecode library that lags every JDK release.

sun.misc.Unsafe memory access deprecated Flexible constructor bodies (2nd preview)

This is a reading copy. The full lesson — with the visual explainer, the interactive lab and a Run button for the code — lives in the Java 23 Course course, and every lesson in it is listed on the Java 23 Course contents page.