Add support for R8-based minization

api/shadow.api

@@ -202,6 +202,28 @@ public abstract interface class com/github/jengelman/gradle/plugins/shadow/tasks
 	public abstract fun inheritFrom ([Ljava/lang/Object;Lorg/gradle/api/Action;)V
 }
 
+public abstract interface class com/github/jengelman/gradle/plugins/shadow/tasks/MinimizeSpec : com/github/jengelman/gradle/plugins/shadow/tasks/DependencyFilter {
+	public abstract fun getTool ()Lorg/gradle/api/provider/Property;
+	public abstract fun r8 (Lgroovy/lang/Closure;)V
+	public abstract fun r8 (Lorg/gradle/api/Action;)V
+}
+
+public final class com/github/jengelman/gradle/plugins/shadow/tasks/MinimizeTool : java/lang/Enum {
+	public static final field DEPENDENCY_ANALYZER Lcom/github/jengelman/gradle/plugins/shadow/tasks/MinimizeTool;
+	public static final field R8 Lcom/github/jengelman/gradle/plugins/shadow/tasks/MinimizeTool;
+	public static fun getEntries ()Lkotlin/enums/EnumEntries;
+	public static fun valueOf (Ljava/lang/String;)Lcom/github/jengelman/gradle/plugins/shadow/tasks/MinimizeTool;
+	public static fun values ()[Lcom/github/jengelman/gradle/plugins/shadow/tasks/MinimizeTool;
+}
+
+public abstract interface class com/github/jengelman/gradle/plugins/shadow/tasks/R8Spec {
+	public abstract fun enableObfuscation ()V
+	public abstract fun enableOptimization ()V
+	public abstract fun getArgs ()Lorg/gradle/api/provider/ListProperty;
+	public abstract fun getKeepRuleFiles ()Lorg/gradle/api/file/ConfigurableFileCollection;
+	public abstract fun getKeepRules ()Lorg/gradle/api/provider/ListProperty;
+}
+
 public class com/github/jengelman/gradle/plugins/shadow/tasks/ShadowCopyAction : org/gradle/api/internal/file/copy/CopyAction {
 	public static final field CONSTANT_TIME_FOR_ZIP_ENTRIES J
 	public static final field Companion Lcom/github/jengelman/gradle/plugins/shadow/tasks/ShadowCopyAction$Companion;
@@ -232,13 +254,16 @@ public abstract class com/github/jengelman/gradle/plugins/shadow/tasks/ShadowJar
 	public fun getEnableAutoRelocation ()Lorg/gradle/api/provider/Property;
 	public fun getEnableKotlinModuleRemapping ()Lorg/gradle/api/provider/Property;
 	public fun getExcludes ()Ljava/util/Set;
+	protected abstract fun getExecOperations ()Lorg/gradle/process/ExecOperations;
 	public fun getFailOnDuplicateEntries ()Lorg/gradle/api/provider/Property;
 	public fun getIncludedDependencies ()Lorg/gradle/api/file/ConfigurableFileCollection;
 	public fun getIncludes ()Ljava/util/Set;
 	public fun getMainClass ()Lorg/gradle/api/provider/Property;
 	public fun getManifest ()Lcom/github/jengelman/gradle/plugins/shadow/tasks/InheritManifest;
 	public synthetic fun getManifest ()Lorg/gradle/api/java/archives/Manifest;
 	public fun getMinimizeJar ()Lorg/gradle/api/provider/Property;
+	public fun getMinimizeSpec ()Lcom/github/jengelman/gradle/plugins/shadow/tasks/MinimizeSpec;
+	public fun getR8Classpath ()Lorg/gradle/api/file/ConfigurableFileCollection;
 	public fun getRelocationPrefix ()Lorg/gradle/api/provider/Property;
 	public fun getRelocators ()Lorg/gradle/api/provider/SetProperty;
 	public fun getSourceSetsClassesDirs ()Lorg/gradle/api/file/ConfigurableFileCollection;

docs/changes/README.md

@@ -7,6 +7,7 @@
 
 - Check `DuplicatesStrategy` for merging transformers. ([#2026](https://github.com/GradleUp/shadow/pull/2026))  
   This will log warnings when an incompatible `DuplicatesStrategy` (e.g., `EXCLUDE`) is applied in Gradle configuration for built-in `ResourceTransformer`s.
+- Add R8 as an opt-in `minimize { r8 { ... } }` tool for shrinking the final shadowed JAR.
 
 ### Changed
 

docs/configuration/minimizing/README.md

@@ -72,6 +72,209 @@ Similar to [`ShadowJar.dependencies`][ShadowJar.dependencies], projects can also
 > When excluding a `project`, all dependencies of the excluded `project` are automatically excluded from 
 > minimization as well.
 
+## Minimizing with R8
 
+Shadow can also run [R8](https://r8.googlesource.com/r8) over the final shadowed JAR. This is useful when you want
+whole-program shrinking instead of the default dependency analyzer. R8 runs after Shadow has merged, transformed, and
+relocated the JAR, so service descriptors in `META-INF/services` are used to keep service providers.
+
+The default R8 configuration only shrinks unused code. It disables name minification and optimization.
+
+=== "Kotlin"
+
+    ```kotlin
+    repositories {
+      google()
+    }
+
+    tasks.shadowJar {
+      minimize {
+        r8 {
+          // Optional extra configuration
+          keepRules.add("-keep class com.example.ReflectiveApi { *; }")
+          keepRuleFiles.from(layout.projectDirectory.file("r8-rules.pro"))
+        }
+      }
+    }
+    ```
+
+=== "Groovy"
+
+    ```groovy
+    repositories {
+      google()
+    }
+
+    tasks.named('shadowJar', com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar) {
+      minimize {
+        r8 {
+          // Optional extra configuration
+          keepRules.add('-keep class com.example.ReflectiveApi { *; }')
+          keepRuleFiles.from(layout.projectDirectory.file('r8-rules.pro'))
+        }
+      }
+    }
+    ```
+
+Shadow resolves R8 from the `shadowR8` configuration. The default dependency is `com.android.tools:r8`, which is
+published by Google Maven rather than Maven Central. Add `google()` to your repositories or override the dependency:
+
+=== "Kotlin"
+
+    ```kotlin
+    dependencies {
+      shadowR8("com.android.tools:r8:9.1.31")
+    }
+    ```
+
+=== "Groovy"
+
+    ```groovy
+    dependencies {
+      shadowR8 'com.android.tools:r8:9.1.31'
+    }
+    ```
+
+Advanced R8 command line arguments can be added with `args`. Replacing the default `args` value removes Shadow's
+default command line arguments, so prefer the helper functions for common obfuscation and optimization toggles. These
+helpers are independent and can be used together.
+
+For example, to downgrade R8 warnings to info:
+
+=== "Kotlin"
+
+    ```kotlin
+    repositories {
+      google()
+    }
+
+    tasks.shadowJar {
+      minimize {
+        r8 {
+          args.addAll(listOf("--map-diagnostics", "warning", "info"))
+        }
+      }
+    }
+    ```
+
+=== "Groovy"
+
+    ```groovy
+    repositories {
+      google()
+    }
+
+    tasks.named('shadowJar', com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar) {
+      minimize {
+        r8 {
+          args.addAll(['--map-diagnostics', 'warning', 'info'])
+        }
+      }
+    }
+    ```
+
+To enable name obfuscation:
+
+=== "Kotlin"
+
+    ```kotlin
+    repositories {
+      google()
+    }
+
+    tasks.shadowJar {
+      minimize {
+        r8 {
+          enableObfuscation()
+        }
+      }
+    }
+    ```
+
+=== "Groovy"
+
+    ```groovy
+    repositories {
+      google()
+    }
+
+    tasks.named('shadowJar', com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar) {
+      minimize {
+        r8 {
+          enableObfuscation()
+        }
+      }
+    }
+    ```
+
+To enable optimization:
+
+=== "Kotlin"
+
+    ```kotlin
+    repositories {
+      google()
+    }
+
+    tasks.shadowJar {
+      minimize {
+        r8 {
+          enableOptimization()
+        }
+      }
+    }
+    ```
+
+=== "Groovy"
+
+    ```groovy
+    repositories {
+      google()
+    }
+
+    tasks.named('shadowJar', com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar) {
+      minimize {
+        r8 {
+          enableOptimization()
+        }
+      }
+    }
+    ```
+
+To enable both:
+
+=== "Kotlin"
+
+    ```kotlin
+    repositories {
+      google()
+    }
+
+    tasks.shadowJar {
+      minimize {
+        r8 {
+          enableObfuscation()
+          enableOptimization()
+        }
+      }
+    }
+    ```
+
+=== "Groovy"
+
+    ```groovy
+    repositories {
+      google()
+    }
+
+    tasks.named('shadowJar', com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar) {
+      minimize {
+        r8 {
+          enableObfuscation()
+          enableOptimization()
+        }
+      }
+    }
+    ```
 
 [ShadowJar.dependencies]: ../../api/shadow/com.github.jengelman.gradle.plugins.shadow.tasks/-shadow-jar/dependencies.html