ProGuard and R8 optimization in Android Kotlin

Introduction

ProGuard and R8 help make your Android app smaller and faster by removing unused code and optimizing what remains.

When you want to reduce the size of your app before publishing it.

When you want to protect your app code from easy copying or reverse engineering.

When you want to improve app performance by removing unnecessary parts.

When you want to shrink and optimize your app automatically during build.

When you want to keep your app secure and efficient for users.

Syntax

Android Kotlin

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

minifyEnabled true turns on code shrinking and optimization.

proguardFiles lists the configuration files for rules to keep or remove code.

Examples

Disables code shrinking and optimization, useful for debugging.

Android Kotlin

minifyEnabled false

Enables shrinking with custom ProGuard rules file.

Android Kotlin

minifyEnabled true
proguardFiles getDefaultProguardFile('proguard-android.txt'), 'my-proguard-rules.pro'

Typical setup for release build with R8 optimization and ProGuard rules.

Android Kotlin

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

Sample App

This example shows a simple Android app with ProGuard and R8 enabled for release builds. The ProGuard rules keep the main activity so it is not removed. When you build the release version, unused code is removed and the app size is smaller.

Android Kotlin

plugins {
    id 'com.android.application'
    id 'org.jetbrains.kotlin.android'
}

android {
    namespace 'com.example.proguarddemo'
    compileSdk 33

    defaultConfig {
        applicationId 'com.example.proguarddemo'
        minSdk 21
        targetSdk 33
        versionCode 1
        versionName '1.0'
    }

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

// proguard-rules.pro
# Keep MainActivity class
-keep class com.example.proguarddemo.MainActivity { *; }

// MainActivity.kt
package com.example.proguarddemo

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import android.widget.TextView

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val textView = TextView(this)
        textView.text = "Hello ProGuard and R8!"
        setContentView(textView)
    }
}

Important Notes

R8 is the default code shrinker and optimizer in Android Studio now, replacing ProGuard but still using ProGuard rules.

Always test your app after enabling minify to make sure important code is not removed.

You can add custom rules in proguard-rules.pro to keep code needed by reflection or libraries.

Summary

ProGuard and R8 help shrink and optimize your Android app code.

Enable minifyEnabled true in release build to activate them.

Use ProGuard rules files to control what code to keep or remove.

Practice

1. What is the main purpose of enabling minifyEnabled true in your Android app's release build?

easy

A. To automatically generate UI layouts

B. To increase the app's debug information for easier troubleshooting

C. To disable ProGuard and R8 optimizations

D. To shrink and optimize the app code by removing unused parts

Solution

1. Step 1: Understand the role of minifyEnabled

   Setting minifyEnabled true activates code shrinking and optimization tools like ProGuard and R8.

2. Step 2: Identify the effect on app code

   These tools remove unused code and optimize the remaining code to reduce app size and improve performance.

3. Final Answer:

   To shrink and optimize the app code by removing unused parts -> Option D

4. Quick Check:

   minifyEnabled true = shrink and optimize [OK]

Hint: minifyEnabled true means code shrinking is ON [OK]

Common Mistakes:

• Thinking it increases debug info instead of reducing code
• Confusing minifyEnabled with disabling optimizations
• Assuming it generates UI layouts automatically

2. Which of the following is the correct way to specify a ProGuard rules file in your build.gradle for release builds?

easy

A. proguardFiles 'proguard-rules.txt'

B. proguardFiles 'proguard-android-optimize.txt', getDefaultProguardFile('proguard-rules.pro')

C. proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'

D. minifyEnabled false

Solution

1. Step 1: Recall the standard syntax for ProGuard files

   The correct syntax uses getDefaultProguardFile for the default file and then adds custom rules.

2. Step 2: Match the correct order and file names

   proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro' correctly calls getDefaultProguardFile('proguard-android-optimize.txt') and then adds 'proguard-rules.pro'.

3. Final Answer:

   proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro' -> Option C

4. Quick Check:

   Correct ProGuard file syntax = proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro' [OK]

Hint: Default file uses getDefaultProguardFile() [OK]

Common Mistakes:

• Swapping file names or forgetting getDefaultProguardFile()
• Setting minifyEnabled false disables ProGuard
• Using wrong file extensions like .txt instead of .pro

3. Given this ProGuard rule:

-keep class com.example.MyClass { *; }

What will happen to MyClass during code shrinking?

medium

A. It will be kept intact with all its members

B. It will be removed if unused

C. Only its public methods will be kept

D. It will be renamed but removed if unused

Solution

1. Step 1: Understand the -keep rule

   The -keep directive tells ProGuard to keep the specified class and its members without removing or renaming them.

2. Step 2: Analyze the rule syntax

   The rule -keep class com.example.MyClass { *; } means keep the entire class and all its members intact.

3. Final Answer:

   It will be kept intact with all its members -> Option A

4. Quick Check:

   -keep class = keep all members [OK]

Hint: -keep means keep class and members intact [OK]

Common Mistakes:

• Thinking -keep removes unused classes
• Assuming only public methods are kept
• Confusing renaming with removal

4. You enabled minifyEnabled true but your app crashes at runtime due to missing classes. What is the most likely cause?

medium

A. You set minifyEnabled false by mistake

B. You forgot to add -keep rules for classes used via reflection

C. You used the wrong ProGuard file name in build.gradle

D. You disabled R8 in your project

Solution

1. Step 1: Identify the cause of missing classes

   When code shrinking removes classes used only by reflection, runtime crashes occur.

2. Step 2: Understand the role of -keep rules

   Adding -keep rules for such classes prevents their removal during shrinking.

3. Final Answer:

   You forgot to add -keep rules for classes used via reflection -> Option B

4. Quick Check:

   Missing classes after shrinking = missing -keep rules [OK]

Hint: Add -keep rules for reflection-used classes [OK]

Common Mistakes:

• Assuming minifyEnabled false causes missing classes
• Confusing file name errors with missing classes
• Thinking disabling R8 causes missing classes

5. You want to keep all classes in the package com.example.models but allow R8 to rename their members to reduce size. Which ProGuard rule should you use?

hard

A. -keepnames class com.example.models.**

B. -keepclassmembers class com.example.models.** { *; }

C. -keep class com.example.models.** { *; }

D. -keepclassmembernames class com.example.models.** { *; }

Solution

1. Step 1: Understand the difference between -keep and -keepnames

   -keep prevents removal and renaming; -keepnames prevents renaming but allows removal.

2. Step 2: Match the requirement

   You want to keep classes but allow member renaming, so -keepnames on classes is correct.

3. Final Answer:

   -keepnames class com.example.models.** -> Option A

4. Quick Check:

   Keep classes but rename members = -keepnames [OK]

Hint: -keepnames keeps class names but allows member renaming [OK]

Common Mistakes:

• Using -keep which prevents renaming
• Confusing -keepclassmembers with -keepnames
• Assuming -keepclassmembernames keeps classes