Modular Java

[sgammon]

Summary

This changeset attempts to discover the minimal set of changes necessary to add module-info.java definitions to Protocol Buffers. Fixes and closes #16133.

This is a draft PR for now. I'm looking for feedback and to setup an integration testsuite downstream with popular Protobuf-based projects to make sure things work as expected.

Note

Re-filing after a bad rebase on #16178.

PR Tree

This PR is applied on top of the following:

• fix(java): fixup test codegen for maven build #16176
• fix: pin bazel → 6.3.2 #16171

Relevant PRs + issues upstream

• Guava + JPMS google/guava#7094
• feat: add jpms definition for annotations google/error-prone#4311
• feat: support jpms in annotations module google/j2objc#2302
• Add module-info.java google/guava#2970

JAR Structure

JARs issued as part of the Protobuf Java release now have Multi-Release: true instead of Automatic-Module-Name within their MANIFEST.MF, making them eligible to be considered as Multi-Release JARs (or, MRJARs). Such JARs can include a module-info.class without interfering with bytecode targeting at JDK8 or earlier, where modules are not supported yet. These JARs are thus called "modular MRJARs."

For example, after building Protobuf Java with bazel build //java:release, vim bazel-bin/java/core/amended_lite_mvn-project.jar shows:

JAR structure:

META-INF/MANIFEST.MF
META-INF/
META-INF/versions/
META-INF/versions/9/
META-INF/versions/9/module-info.class
# ...

In the MANIFEST.MF:

Modular Java downstream

For projects that build with Protobuf Java on the classpath, no change is experienced by the end-user; for projects which build on modulepath:

module my.module {
  requires com.google.protobuf;
  requires com.google.protobuf.kotlin;  // optional
  requires com.google.protobuf.util;  // optional
}

Open module

Protobuf, Protobuf Util, and Protobuf Kotlin are expressed as open modules. This is the case because Protobuf Java is often used reflectively by library consumers. By default, all packages provided by Modular Java protobuf artifacts are open as a result.

Exports + Lite Variants

Protobuf Java Core, Protobuf Java Util, and Protobuf Kotlin all reside in one package path each, which is extremely lucky and convenient for Modular Java because there is no split package problem.

Kotlin Lite/Kotlin and Java Core/Java Lite are meant to be used exclusively within the classpath, but there is no enforcement possible for this case from the compiler or VM, at least not in a consistent way across environments. Building on the modulepath solves this because Kotlin Lite/Kotlin and Java Core/Java Lite share a module coordinate and export an identical package path.

Thus, only one of (Kotlin/Kotlin Lite) and (Java Core/Java Lite) can be used at a time when building on the modulepath. This guarantee is enforced by the compiler and by the JVM at runtime.

Changelog

• feat(java): support for modular java
• fix: transition to rules_kotlin