bazel-expert

bazel-expert

Keyword: bazel | Platforms: gemini,claude,codex

General Bazel & Starlark Expert Skill - Expert guidance on writing idiomatic Bazel rules and optimizing build performance.

Architectural Mandates

• Hermeticity: Actions must only depend on declared inputs
• Granularity: Break large targets into smaller java_library or starlark rules
• Bzlmod: Always use Bzlmod for external dependency management
• Visibility: Default visibility = ["//visibility:private"], explicitly widen only as needed

rules_java & Java Toolchains

Toolchain Setup (Bzlmod)

# MODULE.bazel
bazel_dep(name = "rules_java", version = "7.12.4")

java_toolchains = use_extension("@rules_java//java:extensions.bzl", "toolchains")
java_toolchains.toolchain(version = "21")
use_repo(java_toolchains, "remotejdk21_linux")

register_toolchains("@remotejdk21_linux//:jdk")

BUILD.bazel Examples

# Library target with proper granularity
java_library(
    name = "user-service",
    srcs = glob(["src/main/java/**/*.java"]),
    resources = glob(["src/main/resources/**"]),
    deps = [
        "//common/utils:json-utils",
        "@maven//:com_google_guava_guava",
        "@maven//:jakarta_inject_jakarta_inject_api",
    ],
    visibility = ["//services:__subpackages__"],
)

# Test target with testonly deps
java_test(
    name = "user-service-test",
    srcs = glob(["src/test/java/**/*.java"]),
    test_class = "com.example.UserServiceTest",
    deps = [
        ":user-service",
        "@maven//:org_junit_jupiter_junit_jupiter",
        "@maven//:org_assertj_assertj_core",
        "@maven//:org_mockito_mockito_core",
    ],
)

Do vs Don't

# BAD - monolithic target, slow incremental builds
java_library(
    name = "everything",
    srcs = glob(["src/**/*.java"]),
    deps = ["@maven//:all-the-things"],
    visibility = ["//visibility:public"],
)

# GOOD - granular targets, fast incremental builds
java_library(
    name = "user-model",
    srcs = ["src/main/java/com/example/User.java"],
    deps = ["@maven//:jakarta_validation_jakarta_validation_api"],
)

java_library(
    name = "user-repository",
    srcs = ["src/main/java/com/example/UserRepository.java"],
    deps = [":user-model", "@maven//:io_quarkus_quarkus_hibernate_orm_panache"],
)