build_config.md

Introduction

This document describes the .build_config files that are used by the Chromium build system for Android-specific targets like APK, resources, and more.

[TOC]

I. Overview of .build_config files:

The Android build requires performing computations about dependencies in various targets, which are not possible with the GN build language. To address this, .build_config files are written during the build to store the needed per-target information as JSON files.

They are always written to $target_gen_dir/${target_name}.build_config.

Many scripts under build/android/gyp/, which are used during the build, can also accept parameter arguments using @FileArg references, which look like:

--some-param=@FileArg(<filename>:<key1>:<key2>:..<keyN>)

This placeholder will ensure that <filename> is read as a JSON file, then return the value at [key1][key2]...[keyN] for the --some-param option.

Apart from that, the scripts do not need to know anything about the structure of .build_config files (but the GN rules that invoke them do and select which @FileArg() references to use).

For a concrete example, consider the following GN fragment:

# From //ui/android/BUILD.gn:
android_resources("ui_java_resources") {
  custom_package = "org.chromium.ui"
  resource_dirs = [ "java/res" ]
  deps = [
    ":ui_strings_grd",
  ]
}

This will end up generating the following JSON file under $CHROMIUM_OUTPUT_DIR/gen/ui/android/ui_java_resources.build_config:

{
  "deps_info": {
    "deps_configs": [
      "gen/ui/android/ui_strings_grd.build_config"
    ],
    "name": "ui_java_resources.build_config",
    "package_name": "org.chromium.ui",
    "path": "gen/ui/android/ui_java_resources.build_config",
    "r_text": "gen/ui/android/ui_java_resources_R.txt",
    "resources_dirs": [
      "../../ui/android/java/res"
    ],
    "resources_zip": "resource_zips/ui/android/ui_java_resources.resources.zip",
    "srcjar": "gen/ui/android/ui_java_resources.srcjar",
    "type": "android_resources"
  },
  "gradle": {},
  "resources": {
    "dependency_zips": [
      "resource_zips/ui/android/ui_strings_grd.resources.zip"
    ],
    "extra_package_names": [],
    "extra_r_text_files": []
  }
}

NOTE: All path values in .build_config files are relative to your $CHROMIUM_OUTPUT_DIR.

II. Generation of .build_config files:

They are generated by the GN write_build_config() internal template, which ends up invoking write_build_config.py. For our example above, this is with the following parameters:

python ../../build/android/gyp/write_build_config.py \
    --type=android_resources \
    --depfile gen/ui/android/ui_java_resources__build_config_crbug_908819.d \
    --deps-configs=\[\"gen/ui/android/ui_strings_grd.build_config\"\] \
    --build-config gen/ui/android/ui_java_resources.build_config \
    --resources-zip resource_zips/ui/android/ui_java_resources.resources.zip \
    --package-name org.chromium.ui \
    --r-text gen/ui/android/ui_java_resources_R.txt \
    --resource-dirs=\[\"../../ui/android/java/res\"\] \
    --srcjar gen/ui/android/ui_java_resources.srcjar

Note that most of the content of the JSON file comes from command-line parameters, but not all of it.

In particular, the resources['dependency_zips'] entry was computed by inspecting the content of all dependencies (here, only ui_string_grd.build_config), and collecting their deps_configs['resources_zip'] values.

Because a target's .build_config file will always be generated after that of all of its dependencies, write_build_config.py can traverse the whole (transitive) set of direct and indirect dependencies for a given target and extract useful information out of it.

This is the kind of processing that cannot be done at the GN language level, and is very powerful for Android builds.

III. Usage of .build_config files:

In addition to being parsed by write_build_config.py, when they are listed in the --deps-configs of a given target, the .build_config files are used by other scripts under build/android/gyp/ to build stuff.

For example, the GN android_resources template uses it to invoke the [process_resources.py] script with the following command, in order to generate various related files (e.g. ui_java_resources_R.txt):

python ../../build/android/gyp/process_resources.py \
    --depfile gen/ui/android/ui_java_resources_1.d \
    --android-sdk-jar ../../third_party/android_sdk/public/platforms/android-29/android.jar \
    --aapt-path ../../third_party/android_sdk/public/build-tools/29.0.2/aapt \
    --dependencies-res-zips=@FileArg\(gen/ui/android/ui_java_resources.build_config:resources:dependency_zips\) \
    --extra-res-packages=@FileArg\(gen/ui/android/ui_java_resources.build_config:resources:extra_package_names\) \
    --extra-r-text-files=@FileArg\(gen/ui/android/ui_java_resources.build_config:resources:extra_r_text_files\) \
    --resource-dirs=\[\"../../ui/android/java/res\"\] \
    --debuggable \
    --resource-zip-out resource_zips/ui/android/ui_java_resources.resources.zip \
    --r-text-out gen/ui/android/ui_java_resources_R.txt \
    --srcjar-out gen/ui/android/ui_java_resources.srcjar \
    --non-constant-id \
    --custom-package org.chromium.ui \
    --shared-resources

Note the use of @FileArg() references here, to tell the script where to find the information it needs.

IV. Format of .build_config files:

Thanks to @FileArg() references, Python build scripts under build/android/gyp/ do not need to know anything about the internal format of .build_config files.

This format is decided between internal GN build rules and write_build_config.py. Since these changes rather often, the format documentation is kept inside the Python script itself, but can be extracted as a Markdown file and visualized with the following commands:

# Extract .build_config format documentation
build/android/gyp/write_build_config.py \
  --generate-markdown-format-doc > /tmp/format.md

# Launch a browser to visualize the format documentation.
python tools/md_browser/md_browser.py -d /tmp /tmp/format.md