2026-04-22: Fun with IKVM, multi-release jars, and jackson-core
(This article is also available via the short link tomrei.ch/ikvm-mrjar)
I've been using IKVM to include some Java native code in a .NET application. It's a great tool that lets you run Java libraries directly in .NET without having to rewrite everything from scratch.
However, I ran into an issue when trying to import jackson-core. The problem is that jackson-core is packaged as a Multi-Release JAR (MRJAR), and it contains classes with a version of 65 or higher. IKVM doesn't like this and fails to process the jar.
The fix turned out to be straightforward: I recompiled the jar with the multi-release flag set to false. Once I did that, IKVM was able to import and work with jackson-core without any issues.
I've included the modified jar here in case it helps anyone else running into the same problem: