How To Use Multi-release JARs To Target Multiple Java Versions // nipafx

Multi-release JARs (MR-JARs) let you ship a single JAR containing bytecode for multiple Java versions. Java 8 and older JVMs load classes from the JAR root, while Java 9+ JVMs first check META-INF/versions/${version} and fall back to the root. Creation uses the jar --release flag. Best practices include keeping the oldest-version code complete and self-contained in the root, and using MR-JARs primarily for API differences rather than language features, since language features spread across a codebase and require too many duplicate files. The added complexity of IDE/build configuration, keeping variants in sync, and version-specific testing means MR-JARs should only be used when there's a clear payoff.