How to Reference android_resources in Java (R.java)

Overview

This document has basic instructions on how to reference an android resource in java. It also attempts to explain what R.java is.

How to reference Android Resources in java

  1. Find the resources_package in your android_library target in BUILD.gn
android_library("java") {
  sources = [ "java/org/chromium/browser_ui/Widget.java" ]
  resources_package = "org.chromium.browser_ui"
  deps = [
    "//base:java",
    "//base:resources",
    "//third_party/android_deps:com_google_material_material_java",
    "//third_party/androidx:androidx_core_core_java",
  ]
}

  1. Add a dep on the android_resources target with your resource.

    • or android_aar_prebuilt or android_java_prebuilt for prebuilt targets such as those under //third_party/android_deps

  2. In your java file, import <resources_package>.R;

  3. You can now reference R.<type>.<name> in your java.

What not to do?

  • You should not reference multiple R classes.
    • All R classes will be the same in the final APK, you should never need to import multiple different R classes.
  • You should not import an R class from a different package than the one listed as the resources_package in you android_library target in BUILD.gn
  • You should not rely on transitive deps for resources you use directly.

Android Resources and R.java

Android resources are global for the whole app. While you import the R class from a package (eg: org.chromium.base.R or org.chromium.browser_ui.R) so you might assume that there are resources under the org.chromium.base package that are different from org.chromium.browser_ui, the actual resources from android's point of view are not divided into packages but form a global pool with unique consecutive ids.

The R class is a generated class that contains a mapping from the name of the resources R.string.hello to its numeric id 0x7f015d14. The build system generates this R class for you based on the resources in your dependencies.

In the final apk, all the R classes from all the packages will all inherit from one global R class that has the list of all resources with the final ids as generated by aapt2 (See Life of a Resource for more details on how resources are processed by our build system).

Resources and per library java compilation

Chrome does not compile all the java in an apk at once. But instead it builds each android_library separately and then brings them all together in the end. This means that when compiling a single android_library, the build system does not know the full list of resources that will end up in the apk. Thus, it can't actually generate the final R.class with real resource ids but instead creates a temporary one that only exists when compiling these library subtargets.

resources_package in your android_library target tells the build system which package to generate the R class in. During compile, this temporary R.class will contain only the resources you depend on in your build target.