1# How to Reference android_resources in Java (R.java) 2 3## Overview 4 5This document has basic instructions on how to reference an android resource in 6java. It also attempts to explain what R.java is. 7 8## How to reference Android Resources in java 9 101. Find the `resources_package` in your `android_library` target in BUILD.gn 11 12```gn 13android_library("java") { 14 sources = [ "java/org/chromium/browser_ui/Widget.java" ] 15 resources_package = "org.chromium.browser_ui" 16 deps = [ 17 "//base:java", 18 "//base:resources", 19 "//third_party/android_deps:com_google_material_material_java", 20 "//third_party/androidx:androidx_core_core_java", 21 ] 22} 23``` 24 252. Add a dep on the `android_resources` target with your resource. 26 - or `android_aar_prebuilt` or `android_java_prebuilt` for prebuilt targets 27 such as those under `//third_party/android_deps` 28 293. In your java file, `import <resources_package>.R;` 30 314. You can now reference `R.<type>.<name>` in your java. 32 33### What not to do? 34 35 - **You should not** reference multiple `R` classes. 36 - All R classes will be the same in the final APK, you should never need to 37 import multiple different R classes. 38 - **You should not** import an `R` class from a different package than the one 39 listed as the `resources_package` in you `android_library` target in `BUILD.gn` 40 - **You should not** rely on transitive deps for resources you use directly. 41 42## Android Resources and R.java 43 44Android resources are global for the whole app. While you import the R class 45from a package (eg: `org.chromium.base.R` or `org.chromium.browser_ui.R`) so you 46might assume that there are resources under the `org.chromium.base` package that 47are different from `org.chromium.browser_ui`, the actual resources from 48android's point of view are not divided into packages but form a global pool 49with unique consecutive ids. 50 51The R class is a generated class that contains a mapping from the name of the 52resources `R.string.hello` to its numeric id `0x7f015d14`. The build system 53generates this R class for you based on the resources in your dependencies. 54 55In the final apk, all the `R` classes from all the packages will all inherit 56from one global R class that has the list of all resources with the final ids as 57generated by `aapt2` (See [Life of a Resource](life_of_a_resources.md) for more 58details on how resources are processed by our build system). 59 60## Resources and per library java compilation 61 62Chrome does not compile all the java in an apk at once. But instead it builds 63each `android_library` separately and then brings them all together in the end. 64This means that when compiling a single `android_library`, the build system does 65not know the full list of resources that will end up in the apk. Thus, it can't 66actually generate the final `R.class` with real resource ids but instead 67creates a temporary one that only exists when compiling these library 68subtargets. 69 70`resources_package` in your android_library target tells the build system which 71package to generate the R class in. During compile, this temporary `R.class` 72will contain only the resources you depend on in your build target. 73