22f47a4c5e
Bug: None Change-Id: If5f07b11df34bb0483da614c551630a2c51bf974 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6054210 Commit-Queue: Mohamed Heikal <mheikal@chromium.org> Auto-Submit: Mohamed Heikal <mheikal@chromium.org> Reviewed-by: Andrew Grieve <agrieve@chromium.org> Cr-Commit-Position: refs/heads/main@{#1389052}
73 lines
3.1 KiB
Markdown
73 lines
3.1 KiB
Markdown
# How to Reference android_resources in Java (R.java)
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## Overview
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This document has basic instructions on how to reference an android resource in
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java. It also attempts to explain what R.java is.
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## How to reference Android Resources in java
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1. Find the `resources_package` in your `android_library` target in BUILD.gn
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```gn
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android_library("java") {
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sources = [ "java/org/chromium/browser_ui/Widget.java" ]
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resources_package = "org.chromium.browser_ui"
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deps = [
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"//base:java",
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"//base:resources",
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"//third_party/android_deps:com_google_material_material_java",
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"//third_party/androidx:androidx_core_core_java",
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]
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}
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```
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2. Add a dep on the `android_resources` target with your resource.
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- or `android_aar_prebuilt` or `android_java_prebuilt` for prebuilt targets
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such as those under `//third_party/android_deps`
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3. In your java file, `import <resources_package>.R;`
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4. You can now reference `R.<type>.<name>` in your java.
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### What not to do?
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- **You should not** reference multiple `R` classes.
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- All R classes will be the same in the final APK, you should never need to
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import multiple different R classes.
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- **You should not** import an `R` class from a different package than the one
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listed as the `resources_package` in you `android_library` target in `BUILD.gn`
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- **You should not** rely on transitive deps for resources you use directly.
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## Android Resources and R.java
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Android resources are global for the whole app. While you import the R class
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from a package (eg: `org.chromium.base.R` or `org.chromium.browser_ui.R`) so you
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might assume that there are resources under the `org.chromium.base` package that
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are different from `org.chromium.browser_ui`, the actual resources from
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android's point of view are not divided into packages but form a global pool
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with unique consecutive ids.
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The R class is a generated class that contains a mapping from the name of the
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resources `R.string.hello` to its numeric id `0x7f015d14`. The build system
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generates this R class for you based on the resources in your dependencies.
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In the final apk, all the `R` classes from all the packages will all inherit
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from one global R class that has the list of all resources with the final ids as
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generated by `aapt2` (See [Life of a Resource](life_of_a_resources.md) for more
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details on how resources are processed by our build system).
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## Resources and per library java compilation
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Chrome does not compile all the java in an apk at once. But instead it builds
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each `android_library` separately and then brings them all together in the end.
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This means that when compiling a single `android_library`, the build system does
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not know the full list of resources that will end up in the apk. Thus, it can't
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actually generate the final `R.class` with real resource ids but instead
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creates a temporary one that only exists when compiling these library
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subtargets.
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`resources_package` in your android_library target tells the build system which
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package to generate the R class in. During compile, this temporary `R.class`
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will contain only the resources you depend on in your build target.
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