How AGP 4.1.0 Eliminates Redundant R Files and Boosts Build Speed

Background

The static code analysis upgrade required a newer Android Gradle Plugin (AGP). The Cloud Music project originally used AGP 3.5.0, so the team compared it with AGP 3.6.0 and AGP 4.1.0.

AGP 3.6.0 – Simplified R class generation

AGP 3.6.0 generates a single R.class for each library module and shares it with dependent modules, removing the intermediate R.java → javac → R.class step. Each module must have a unique package name, and the visibility of the generated R class depends on the dependency configuration ( api vs implementation).

Each module directly produces R.class instead of the previous two‑step process.

Build artifact comparison:

AGP 3.5.0 output:

The reduction of the intermediate R generation step improves build efficiency.

AGP 4.1.0 – R class shrinking and inlining

Starting with AGP 4.1.0, fields of R classes are no longer kept by default. When code shrinking (e.g., R8) is enabled, references to R fields are replaced with their constant values, allowing the R class to be removed and reducing APK size dramatically—provided the app does not access R classes via reflection.

R file redundancy problem

Each library module historically generated its own static R class to avoid ID conflicts. During APK packaging the final R file is the accumulation of all library R files plus the app’s own R, leading to redundant entries that bloat the APK, especially in deep dependency trees.

R_lib1 = R_lib1;
R_lib2 = R_lib2;
R_lib3 = R_lib3;
R_biz1 = R_lib1 + R_lib2 + R_lib3 + R_biz1;
R_biz2 = R_lib2 + R_lib3 + R_biz2;
R_app   = R_lib1 + R_lib2 + R_lib3 + R_biz1 + R_biz2 + R_app;

R file inlining solution

A custom transform rewrites all library‑module R references to point to the top‑level R_app and then deletes the library R files, leaving only a single top‑level R in the APK. A more aggressive variant replaces every R reference with a constant and removes the top‑level R entirely.

Experimental comparison

Two builds were created: one with AGP 3.6.0 and another with AGP 4.1.0. Both enabled ProGuard shrinking ( minifyEnabled true) but disabled obfuscation ( -dontobfuscate) to keep class names readable.

buildTypes {
    release {
        minifyEnabled true // enable shrinking
        proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
    }
}

AGP 3.6.0 APK inspection shows the bizlib module still contains an R class, and SecondActivity bytecode references it.

AGP 4.1.0 APK inspection shows the bizlib module no longer contains an R class, and the bytecode of SecondActivity contains a constant instead of an R reference.

How R inlining is implemented

The inlining occurs in the minifyReleaseWithR8 task. A Gradle script was added to print task inputs and outputs, revealing that the whole app’s R JAR is an input to this task.

gradle.taskGraph.afterTask { task ->
    try {
        println("---- task name:" + task.name)
        println("-------- inputs:")
        task.inputs.files.each { println(it.absolutePath) }
        println("-------- outputs:")
        task.outputs.files.each { println(it.absolutePath) }
    } catch (Exception e) {}
}

The task is an instance of R8Task:

class CreationAction(...):
    override val type = R8Task::class.java
    // create minifyReleaseWithR8 task
    override val name = computeTaskName("minify", "WithR8")
    ...

During execution the task invokes the R8 tool:

fun runR8(
        inputClasses: Collection<Path>,
        ...
) {
    ClassFileProviderFactory(libraries).use { libraryClasses ->
        ClassFileProviderFactory(classpath).use { classpathClasses ->
            r8CommandBuilder.addLibraryResourceProvider(libraryClasses.orderedProvider)
            r8CommandBuilder.addClasspathResourceProvider(classpathClasses.orderedProvider)
            R8.run(r8CommandBuilder.build())
        }
    }
}

R8 performs shrinking, optimization, and dex conversion. While shrinking it replaces references to R fields with their constant values, after which the R class can be removed. The default keep rule for R fields (

-keepclassmembers class **.R$* { public static <fields>; }

) was removed in AGP 4.1.0, enabling this behavior.

R8 combines ProGuard and D8 capabilities, handling class‑to‑dex conversion, shrinking, and constant propagation.

Results

AGP 4.1.0 eliminates redundant R files by inlining them through R8.

In the test project the APK size decreased by approximately 7 MB.

Release build time was reduced by more than ten minutes due to task merging and the simplified R generation pipeline.

Test project used for the analysis: https://github.com/sunday1937/AgpTest2