Creating a JAR natively in Starlark

BUILD.bazel

load("@bazel_skylib//rules:write_file.bzl", "write_file")


genrule(
    name = "example",
    outs = ["example.jar"],
    cmd = """
        OLD_CWD=$$(pwd) && cd $(RULEDIR) && \\
        $$OLD_CWD/$(JAVABASE)/bin/jar cmfv $(rootpath :manifest) \\
                                $$OLD_CWD/$@ $(rootpath :main_class)
        """,
    srcs = [
        ":main_class",
        ":manifest",
    ],
    toolchains = ["@bazel_tools//tools/jdk:current_java_runtime"],
)

genrule(
    name = "main_class",
    outs = ["Main.class"],
    cmd = "$(JAVABASE)/bin/javac $(SRCS) -d $$(dirname $@)",
    srcs = [
        "Main.java",
    ],
    toolchains = ["@bazel_tools//tools/jdk:current_java_runtime"],
)

write_file(
    name = "manifest",
    out = "META-INF/MANIFEST.MF",
    # extra line needed for line break
    content = ["Manifest-Version: 1.0", "Main-Class: Main", ""],
)

Main.java

public class Main {

    public static void main(String[] args) {
        System.out.println("Hello World!");
    }

}

MODULE.bazel

bazel_dep(name = "bazel_skylib", version = "1.7.1")

The idea for this is to have a pkg_jar in rules_pkg that can natively create JAR rather than going through pkg_zip.