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javac plugin (Java Compiler Plugin)

A javac plugin is a Java-compiler extension mechanism that runs during compilation and can inspect, analyse, or transform the compilation unit. Distinct from a compiler plugin in other languages by virtue of running inside javac; distinct from an annotation processor by operating on already-attributed trees (post-attribution hooks).

Role in the wiki

Meta built a javac plugin to collect runtime nullability telemetry across Java/Kotlin interop:

"We decided to borrow an idea from the Kotlin compiler and create a Java compiler plugin that helps us collect runtime nullability data. This plugin allows us to collect data on all return types and parameters that are receiving/returning a null value and are not annotated as such."

Canonical wiki instance of concepts/runtime-nullability-telemetry. The collected data drives codemods that backfill @Nullable annotations into the Java code, improving accuracy for both eventual translation and Java code that Meta "may never translate."

The architectural choice is notable: when static analysis runs out of road, Meta instruments production rather than continuing to refine the analyser.

Seen in

• sources/2024-12-18-meta-translating-10m-lines-of-java-to-kotlin — the runtime-nullability-plugin instance.

Related

• systems/nullsafe — the static-analysis tool the javac plugin complements.
• systems/kotlinator — the consumer of the annotation backfill.
• concepts/runtime-nullability-telemetry — the concept this plugin instantiates.
• concepts/interlanguage-null-safety — the problem space.