Add Kover code coverage support by oliviernotteghem · Pull Request #1527 · bazel-contrib/rules_kotlin

oliviernotteghem · 2026-03-18T21:54:58Z

Summary

This PR adds Kover (JetBrains' Kotlin code coverage tool) as an alternative to JaCoCo for code coverage in rules_kotlin.

Why Kover?

Feature	JaCoCo	Kover
Instrumentation	Build-time bytecode modification	Runtime JVM agent
Kotlin support	Limited (misses inline functions)	Native Kotlin awareness
Recompilation needed	Yes	No
Mixed Java/Kotlin	Problematic	Works well

New Toolchain Attributes

define_kt_toolchain(
    name = "kotlin_toolchain",
    experimental_kover_enabled = True,
    experimental_kover_agent = "@maven//:org_jetbrains_kotlinx_kover_jvm_agent",
    experimental_kover_exclude = ["com.example.generated.*"],
    experimental_kover_exclude_annotation = ["Generated"],
    experimental_kover_exclude_inherited_from = ["android.app.Activity"],
)

Usage

bazel coverage //your/kotlin/test_target

Output files are created alongside test outputs:

*-kover_report.ic - Binary coverage data
*-kover_metadata.txt - Metadata for Kover CLI report generation

Generate reports with:

java -jar kover-cli.jar report @path/to/kover_metadata.txt --html report/

Changes

kotlin/internal/jvm/kover.bzl (NEW) - Core Kover integration logic
kotlin/internal/jvm/compile.bzl - Disable JaCoCo instrumentation when Kover enabled
kotlin/internal/jvm/impl.bzl - Kover support in kt_jvm_test
kotlin/internal/jvm/kt_android_local_test_impl.bzl - Kover support for Android local tests
kotlin/internal/toolchains.bzl - New toolchain configuration attributes

Notes

Kover agent is added to both -Xbootclasspath/a: and -javaagent: to support code using the bootstrap classloader
Report generation is done outside Bazel (test actions are terminal)
For mixed Java/Kotlin sourcesets, JaCoCo instrumentation must be disabled

Test plan

Unit tests for get_kover_jvm_flags function
Toolchain builds successfully
Manual testing with actual Kotlin test targets

Usage

To enable Kover in your project:

  define_kt_toolchain(
      name = "kotlin_toolchain",
      experimental_kover_enabled = True,
      experimental_kover_agent = "@maven//:org_jetbrains_kotlinx_kover_jvm_agent",
      experimental_kover_exclude = ["com.example.generated.*"],
      experimental_kover_exclude_annotation = ["Generated"],
  )

  Then run: bazel coverage //your/kotlin/test_target

Android Code Duplication

kt_android_local_test_impl.bzl duplicates significant logic from rules_android. This needs to be kept in sync manually. There's ongoing work with Google to add better extensibility points to rules_android.

Summary This commit integrates Kover (JetBrains' Kotlin code coverage tool) into rules_kotlin as an alternative to JaCoCo. Kover uses a JVM agent for runtime instrumentation, eliminating the need for build-time bytecode modification. --- File-by-File Changes 1. kotlin/internal/jvm/kover.bzl (NEW - 167 lines) Purpose: Core Kover integration logic. ┌───────────────────────────────────┬─────────┬───────────────────────────────────────────────────┐ │ Function │ Lines │ Description │ ├───────────────────────────────────┼─────────┼───────────────────────────────────────────────────┤ │ is_kover_enabled(ctx) │ 49-50 │ Checks if Kover is enabled via toolchain │ ├───────────────────────────────────┼─────────┼───────────────────────────────────────────────────┤ │ get_kover_agent_file(ctx) │ 52-63 │ Retrieves Kover agent JAR from toolchain │ ├───────────────────────────────────┼─────────┼───────────────────────────────────────────────────┤ │ get_kover_jvm_flags(...) │ 65-72 │ Generates -javaagent:kover.jar=file:args.txt flag │ ├───────────────────────────────────┼─────────┼───────────────────────────────────────────────────┤ │ create_kover_agent_actions(...) │ 74-99 │ Creates .ic output file and args file for agent │ ├───────────────────────────────────┼─────────┼───────────────────────────────────────────────────┤ │ create_kover_metadata_action(...) │ 102-167 │ Generates metadata file for Kover CLI reports │ └───────────────────────────────────┴─────────┴───────────────────────────────────────────────────┘ Key implementation details: - Uses ctx.actions.run_shell hack to declare the .ic file (created by agent at runtime) - Filters source files and classfiles to include only same-package dependencies - Supports exclusions by class pattern, annotation, and inherited class --- 2. kotlin/internal/jvm/compile.bzl (+5 lines) Changes: # Added import load("//kotlin/internal/jvm:kover.bzl", _is_kover_enabled = "is_kover_enabled") # Modified instrumentation flag args.add("--instrument_coverage", ctx.coverage_instrumented() and not _is_kover_enabled(ctx)) Effect: Disables JaCoCo bytecode instrumentation when Kover is enabled, preventing conflicts. --- 3. kotlin/internal/jvm/impl.bzl (+40 lines) Changes to kt_jvm_junit_test_impl: # Before (JaCoCo only) if ctx.configuration.coverage_enabled: jacocorunner = ctx.toolchains[_TOOLCHAIN_TYPE].jacocorunner coverage_runfiles = jacocorunner.files.to_list() # After (Kover + JaCoCo) if ctx.configuration.coverage_enabled: if _is_kover_enabled(ctx): # Setup Kover agent kover_agent_files = _get_kover_agent_files(ctx) kover_output_file, kover_args_file = _create_kover_agent_actions(...) kover_output_metadata_file = _create_kover_metadata_action(...) coverage_jvm_flags = [_get_kover_jvm_flags(...)] coverage_inputs = [depset(kover_agent_files)] coverage_runfiles = [kover_args_file, kover_output_metadata_file] else: # Existing JaCoCo path jacocorunner = ctx.toolchains[_TOOLCHAIN_TYPE].jacocorunner coverage_runfiles = jacocorunner.files.to_list() --- 4. kotlin/internal/jvm/kt_android_local_test_impl.bzl (+199 lines) Major changes: 1. _process_jvm function: Added Kover support mirroring the JVM test changes 2. _process_stub function (NEW): Custom stub processor that: - Skips merged instrumentation JAR creation when using Kover - Properly handles coverage environment variables for both Kover and JaCoCo 3. Helper functions (NEW): - _get_test_class(ctx) - Infers test class from sources - _create_stub(...) - Creates test launcher shell script - _get_classpath(s) - Formats classpath entries - _get_jvm_flags(...) - Assembles JVM flags for Robolectric --- 5. kotlin/internal/toolchains.bzl (+32 lines) New toolchain attributes: ┌───────────────────────────────────────────┬─────────────┬───────────────────────────────┐ │ Attribute │ Type │ Description │ ├───────────────────────────────────────────┼─────────────┼───────────────────────────────┤ │ experimental_kover_enabled │ bool │ Enable Kover (default: False) │ ├───────────────────────────────────────────┼─────────────┼───────────────────────────────┤ │ experimental_kover_agent │ label │ Kover agent JAR target │ ├───────────────────────────────────────────┼─────────────┼───────────────────────────────┤ │ experimental_kover_exclude │ string_list │ Class exclusion patterns │ ├───────────────────────────────────────────┼─────────────┼───────────────────────────────┤ │ experimental_kover_exclude_annotation │ string_list │ Annotation-based exclusions │ ├───────────────────────────────────────────┼─────────────┼───────────────────────────────┤ │ experimental_kover_exclude_inherited_from │ string_list │ Inherited class exclusions │ └───────────────────────────────────────────┴─────────────┴───────────────────────────────┘ Updated functions: - _kotlin_toolchain_impl: Adds Kover attributes to toolchain info - define_kt_toolchain: Adds Kover parameters to public API --- Architecture Diagram ┌─────────────────────────────────────────────────────────────────┐ │ bazel coverage //test │ └─────────────────────────────────────────────────────────────────┘ │ ┌───────────────┴───────────────┐ ▼ ▼ ┌──────────────┐ ┌──────────────┐ │ Kover Enabled │ │ JaCoCo Path │ └──────────────┘ └──────────────┘ │ │ ▼ ▼ ┌───────────────────┐ ┌─────────────────────┐ │ No recompilation │ │ Bytecode modified │ │ Agent instruments │ │ at compile time │ │ at runtime │ └─────────────────────┘ └───────────────────┘ │ ▼ ┌───────────────────┐ │ test-kover.args.txt│ ← Agent configuration │ test-kover_report.ic│ ← Binary coverage data │ test-kover_metadata.txt│ ← CLI report args └───────────────────┘ │ ▼ ┌───────────────────────────────────────┐ │ External: java -jar kover-cli.jar │ │ report @metadata.txt │ └───────────────────────────────────────┘ Usage To enable Kover in your project: define_kt_toolchain( name = "kotlin_toolchain", experimental_kover_enabled = True, experimental_kover_agent = "@maven//:org_jetbrains_kotlinx_kover_jvm_agent", experimental_kover_exclude = ["com.example.generated.*"], experimental_kover_exclude_annotation = ["Generated"], ) Then run: bazel coverage //your/kotlin/test_target Android Code Duplication kt_android_local_test_impl.bzl duplicates significant logic from rules_android. This needs to be kept in sync manually. There's ongoing work with Google to add better extensibility points to rules_android.

restingbull · 2026-03-24T15:33:19Z

General, LGTM, modulo the windows check.

agluszak · 2026-03-30T12:55:30Z

+#    rules_java and Bazel core. For now, we disabled JaCoCo instrumentation accross the board,
+#    you will need to cherry-pick this PR https://github.com/uber-common/bazel/commit/cb9f6f042c64af96bbd77e21fe6fb75936c74f47


Is that still true?

agluszak · 2026-03-30T12:58:05Z

+        ),
+    )
+
+# TODO: follow up with Google to have rules_android provide better extensibility points


Is there a specific ticket regarding this?

I asked them directly about this, they're open if we submit a PR that's reasonable.

restingbull · 2026-03-24T17:09:03Z

+        "-Xbootclasspath/a:%s" % (kover_agent_files[0].short_path),
+        "-javaagent:%s=file:%s" % (kover_agent_files[0].short_path, kover_args_file.short_path),
+    ]
+    return " ".join(jvm_args)


JVM argument parser might split this incorrectly -- return list?

restingbull · 2026-03-24T17:09:03Z

+    kover_output_file = ctx.actions.declare_file(binary_output_name)
+
+    # Hack: there is curently no way to indicate this file will be created Kover agent
+    ctx.actions.run_shell(


This can cause some issues with non *nix OSes. Use:

ctx.actions.write(kover_output_file, "")

restingbull · 2026-03-24T17:09:03Z

+    )
+    ctx.actions.write(
+        kover_args_file,
+        "report.file=../../%s" % binary_output_name,  # Kotlin compiler runs in runfiles folder, make sure file is created is correct location


Relationship between the runfiles root and bazel-bin is not guaranteed and can vary by OS, --sandbox_base setting, and execution strategy.

Try kover_output_file.short_path (workspace-relative path resolved at runtime) or construct the path via ctx.bin_dir.path. Might want to use ${RUNFILES_DIR} to anchor it.

restingbull · 2026-03-24T17:09:03Z

+    excludes = []
+
+    for dep in deps:
+        if dep.label.package != ctx.label.package:


Hm. dep.label.package == ctx.label.packagewhich will drop all cross-package deps. Example: test is in //tests/foo:bar_test and the library under test is in //core/baz:baz_lib, coverage will be empty.

What's the goal on this?

restingbull · 2026-03-24T17:09:03Z

+                        srcs.extend(["--src", path])
+
+        if JavaInfo in dep:
+            for classfile in dep[JavaInfo].transitive_runtime_jars.to_list():


Same comment above about reducing memory usage.

restingbull · 2026-03-24T17:09:03Z

@@ -1,6 +1,9 @@
 load(":jvm_deps_tests.bzl", "jvm_deps_test_suite")
+load(":kover_test.bzl", "kover_test_suite")


Missing kover_toolchain_test_suite, so the toolchain tests are never run.

restingbull · 2026-03-24T17:09:03Z

+
+    # Expected format includes both -Xbootclasspath/a and -javaagent flags
+    expected = "-Xbootclasspath/a:external/kover/kover-jvm-agent.jar -javaagent:external/kover/kover-jvm-agent.jar=file:bazel-out/k8-fastbuild/bin/test-kover.args.txt"
+    asserts.equals(env, expected, result)


Hm. Is this right? feels like it should be two strings for the jvm parse.

restingbull · 2026-03-24T17:09:03Z

+load(
+    "//kotlin/internal/jvm:kover.bzl",
+    _create_kover_agent_actions = "create_kover_agent_actions",
+    _create_kover_metadata_action = "create_kover_metadata_action",


nit: _get_kover_agent_files (plural) does not match the exported symbol get_kover_agent_file.

Also, single file, rather than list? Would clean up the ambiguous [0] bits below.

restingbull · 2026-03-31T17:07:50Z

+    metadata_output_name = "%s-kover_metadata.txt" % name
+    kover_output_metadata_file = ctx.actions.declare_file(metadata_output_name)
+
+    srcs = []


use Args for these collections.

restingbull · 2026-03-31T17:14:34Z

+            continue
+
+        if InstrumentedFilesInfo in dep:
+            for src in dep[InstrumentedFilesInfo].instrumented_files.to_list():


to_list is almost always a bad idea.

This can be accomplish this in a way that doesn't blow out starlark memory by using map and closure -- e.g.:

def filter(src): if src.short_path.startswith(ctx.label.package + "/"): path = _paths.dirname(src.short_path) return "--src\n" + path src.add_joined( dep[InstrumentedFilesInfo].instrumented_files, join_with="\n", map_each=lambda src: allow_closure = True, uniquify = True )

Summary This commit integrates Kover (JetBrains' Kotlin code coverage tool) into rules_kotlin as an alternative to JaCoCo. Kover uses a JVM agent for runtime instrumentation, eliminating the need for build-time bytecode modification. --- File-by-File Changes 1. kotlin/internal/jvm/kover.bzl (NEW - 167 lines) Purpose: Core Kover integration logic. ┌───────────────────────────────────┬─────────┬───────────────────────────────────────────────────┐ │ Function │ Lines │ Description │ ├───────────────────────────────────┼─────────┼───────────────────────────────────────────────────┤ │ is_kover_enabled(ctx) │ 49-50 │ Checks if Kover is enabled via toolchain │ ├───────────────────────────────────┼─────────┼───────────────────────────────────────────────────┤ │ get_kover_agent_file(ctx) │ 52-63 │ Retrieves Kover agent JAR from toolchain │ ├───────────────────────────────────┼─────────┼───────────────────────────────────────────────────┤ │ get_kover_jvm_flags(...) │ 65-72 │ Generates -javaagent:kover.jar=file:args.txt flag │ ├───────────────────────────────────┼─────────┼───────────────────────────────────────────────────┤ │ create_kover_agent_actions(...) │ 74-99 │ Creates .ic output file and args file for agent │ ├───────────────────────────────────┼─────────┼───────────────────────────────────────────────────┤ │ create_kover_metadata_action(...) │ 102-167 │ Generates metadata file for Kover CLI reports │ └───────────────────────────────────┴─────────┴───────────────────────────────────────────────────┘ Key implementation details: - Uses ctx.actions.run_shell hack to declare the .ic file (created by agent at runtime) - Filters source files and classfiles to include only same-package dependencies - Supports exclusions by class pattern, annotation, and inherited class --- 2. kotlin/internal/jvm/compile.bzl (+5 lines) Changes: # Added import load("//kotlin/internal/jvm:kover.bzl", _is_kover_enabled = "is_kover_enabled") # Modified instrumentation flag args.add("--instrument_coverage", ctx.coverage_instrumented() and not _is_kover_enabled(ctx)) Effect: Disables JaCoCo bytecode instrumentation when Kover is enabled, preventing conflicts. --- 3. kotlin/internal/jvm/impl.bzl (+40 lines) Changes to kt_jvm_junit_test_impl: # Before (JaCoCo only) if ctx.configuration.coverage_enabled: jacocorunner = ctx.toolchains[_TOOLCHAIN_TYPE].jacocorunner coverage_runfiles = jacocorunner.files.to_list() # After (Kover + JaCoCo) if ctx.configuration.coverage_enabled: if _is_kover_enabled(ctx): # Setup Kover agent kover_agent_files = _get_kover_agent_files(ctx) kover_output_file, kover_args_file = _create_kover_agent_actions(...) kover_output_metadata_file = _create_kover_metadata_action(...) coverage_jvm_flags = [_get_kover_jvm_flags(...)] coverage_inputs = [depset(kover_agent_files)] coverage_runfiles = [kover_args_file, kover_output_metadata_file] else: # Existing JaCoCo path jacocorunner = ctx.toolchains[_TOOLCHAIN_TYPE].jacocorunner coverage_runfiles = jacocorunner.files.to_list() --- 4. kotlin/internal/jvm/kt_android_local_test_impl.bzl (+199 lines) Major changes: 1. _process_jvm function: Added Kover support mirroring the JVM test changes 2. _process_stub function (NEW): Custom stub processor that: - Skips merged instrumentation JAR creation when using Kover - Properly handles coverage environment variables for both Kover and JaCoCo 3. Helper functions (NEW): - _get_test_class(ctx) - Infers test class from sources - _create_stub(...) - Creates test launcher shell script - _get_classpath(s) - Formats classpath entries - _get_jvm_flags(...) - Assembles JVM flags for Robolectric --- 5. kotlin/internal/toolchains.bzl (+32 lines) New toolchain attributes: ┌───────────────────────────────────────────┬─────────────┬───────────────────────────────┐ │ Attribute │ Type │ Description │ ├───────────────────────────────────────────┼─────────────┼───────────────────────────────┤ │ experimental_kover_enabled │ bool │ Enable Kover (default: False) │ ├───────────────────────────────────────────┼─────────────┼───────────────────────────────┤ │ experimental_kover_agent │ label │ Kover agent JAR target │ ├───────────────────────────────────────────┼─────────────┼───────────────────────────────┤ │ experimental_kover_exclude │ string_list │ Class exclusion patterns │ ├───────────────────────────────────────────┼─────────────┼───────────────────────────────┤ │ experimental_kover_exclude_annotation │ string_list │ Annotation-based exclusions │ ├───────────────────────────────────────────┼─────────────┼───────────────────────────────┤ │ experimental_kover_exclude_inherited_from │ string_list │ Inherited class exclusions │ └───────────────────────────────────────────┴─────────────┴───────────────────────────────┘ Updated functions: - _kotlin_toolchain_impl: Adds Kover attributes to toolchain info - define_kt_toolchain: Adds Kover parameters to public API --- Architecture Diagram ┌─────────────────────────────────────────────────────────────────┐ │ bazel coverage //test │ └─────────────────────────────────────────────────────────────────┘ │ ┌───────────────┴───────────────┐ ▼ ▼ ┌──────────────┐ ┌──────────────┐ │ Kover Enabled │ │ JaCoCo Path │ └──────────────┘ └──────────────┘ │ │ ▼ ▼ ┌───────────────────┐ ┌─────────────────────┐ │ No recompilation │ │ Bytecode modified │ │ Agent instruments │ │ at compile time │ │ at runtime │ └─────────────────────┘ └───────────────────┘ │ ▼ ┌───────────────────┐ │ test-kover.args.txt│ ← Agent configuration │ test-kover_report.ic│ ← Binary coverage data │ test-kover_metadata.txt│ ← CLI report args └───────────────────┘ │ ▼ ┌───────────────────────────────────────┐ │ External: java -jar kover-cli.jar │ │ report @metadata.txt │ └───────────────────────────────────────┘ Usage To enable Kover in your project: define_kt_toolchain( name = "kotlin_toolchain", experimental_kover_enabled = True, experimental_kover_agent = "@maven//:org_jetbrains_kotlinx_kover_jvm_agent", experimental_kover_exclude = ["com.example.generated.*"], experimental_kover_exclude_annotation = ["Generated"], ) Then run: bazel coverage //your/kotlin/test_target Android Code Duplication kt_android_local_test_impl.bzl duplicates significant logic from rules_android. This needs to be kept in sync manually. There's ongoing work with Google to add better extensibility points to rules_android.

oliviernotteghem requested review from Bencodes, ahumesky, cgruber, nkoroste and restingbull as code owners March 18, 2026 21:54

oliviernotteghem force-pushed the upstream-kover branch from 3c31758 to 99ac6ea Compare March 18, 2026 22:03

oliviernotteghem force-pushed the upstream-kover branch from 99ac6ea to b96fd43 Compare March 18, 2026 22:10

agluszak reviewed Mar 19, 2026

View reviewed changes

Comment thread kotlin/internal/jvm/kover.bzl

agluszak mentioned this pull request Mar 19, 2026

Code coverage meta-issue #1528

Open

oliviernotteghem added 3 commits March 26, 2026 21:22

Add examples/kover project

f69cb76

Fix formatting

883fe98

Skip Kover test on Windows

c4abbbf

agluszak reviewed Mar 30, 2026

View reviewed changes

restingbull reviewed Mar 31, 2026

View reviewed changes

		# rules_java and Bazel core. For now, we disabled JaCoCo instrumentation accross the board,
		# you will need to cherry-pick this PR https://github.com/uber-common/bazel/commit/cb9f6f042c64af96bbd77e21fe6fb75936c74f47

		@@ -1,6 +1,9 @@
		load(":jvm_deps_tests.bzl", "jvm_deps_test_suite")
		load(":kover_test.bzl", "kover_test_suite")

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oliviernotteghem commented Mar 18, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Summary

Why Kover?

New Toolchain Attributes

Usage

Changes

Notes

Test plan

Usage

Android Code Duplication

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restingbull commented Mar 24, 2026

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oliviernotteghem commented Mar 18, 2026 •

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