Switch to Google Java Format and configure Spotless auto-formatter

CONTRIBUTING.md

@@ -163,18 +163,15 @@ of the [Google Developer Documentation Style
 Guide](https://developers.google.com/style/).
 
 ## Code format
+We follow the [Google Java Code Style](https://google.github.io/styleguide/javaguide.html). The code should be
+auto-formatted before any pull request by running:
+  
+  ```
+  ./gradlew spotlessApply
+  ```
 
-We strive for code that is super-aligned. Ideally, the code should look like a
-set of tables, for variable declarations, parameter declarations, ternary
-operators, etc.
-
-- The basic indentation is 4 spaces (never tabs).
-
-- Curly braces are on separate lines.
-
-- The ideal maximum line length is 120 characters. Documentation and comments
-  should stick to this limit. Code that has a nice table-like structure can
-  exceed it.
+In particular some elements of the code style are not covered by Spotless and particular attention should be
+given to them:
 
 - Imports of multiple classes in the same package are declared explicitly, **no wildcard imports**.
 
@@ -183,10 +180,6 @@ operators, etc.
 
 - Whenever overriding a method, add the @Override annotation.
 
-In practice, an IDE can help to obtain a clean indentation and structure, but
-it is often not sufficient. Be careful to never upset a cleanly formatted
-source file by automatically reformatting it.
-
 ### Naming Conventions
 
 - Names (class names, field names, etc) should be descriptive.

android/build.gradle

@@ -9,6 +9,7 @@ plugins {
     id 'jacoco'
     id "org.jlleitschuh.gradle.ktlint"
     id "org.sonarqube"
+    id 'com.diffplug.spotless'
 }
 
 dependencies {
@@ -97,3 +98,8 @@ if (hasProperty('PROGUARD_SIGNING_KEY')) {
     }
 }
 
+spotless {
+    java {
+        googleJavaFormat()
+    }
+}

base/build.gradle

@@ -9,6 +9,7 @@ plugins {
     id 'jacoco'
     id "org.jlleitschuh.gradle.ktlint"
     id "java-test-fixtures"
+    id 'com.diffplug.spotless'
 }
 
 dependencies {
@@ -153,3 +154,9 @@ if (hasProperty('PROGUARD_SIGNING_KEY')) {
         sign publishing.publications
     }
 }
+
+spotless {
+    java {
+        googleJavaFormat()
+    }
+}

build.gradle

@@ -9,6 +9,7 @@ plugins {
     id "org.sonarqube" version "3.4.0.2513"
     id "io.freefair.aggregate-javadoc" version "6.5.0.3"
     id "org.jlleitschuh.gradle.ktlint" version "10.3.0" apply false
+    id "com.diffplug.spotless" version "6.13.0"
 }
 
 allprojects {

docs/md/releasenotes.md

@@ -16,6 +16,17 @@
 - Add `IdentifiedArrayReferenceValue.generalize()` to maintain `ID` when applied to two instances with same `ID`.
 - Improve `ExecutingInvocationUnit` support to methods that transform a `String` into an array of a primitive type. 
 
+## New Code Style
+
+We have changed our code style to follow the Google Java Style Guide. A new auto-formatter is now available to make contributing to 
+ProGuardCORE easier:
+
+```
+./gradlew spotlessApply
+```
+
+More information can be found in the `Code Style` section in `CONTRIBUTING.md`. 
+
 ## Version 9.1
 
 ### Improved

tools/build.gradle

@@ -4,6 +4,7 @@ plugins {
     id 'maven-publish'
     id 'signing'
     id "org.jlleitschuh.gradle.ktlint"
+    id 'com.diffplug.spotless'
 }
 
 dependencies {
@@ -78,3 +79,9 @@ if (hasProperty('PROGUARD_SIGNING_KEY')) {
         sign publishing.publications
     }
 }
+
+spotless {
+    java {
+        googleJavaFormat()
+    }
+}