Enable the R8 optimizer's full potential

R8 provides two modes, compatibility mode and full mode. Full mode gives you powerful optimizations that improve your app performance.

This guide is for Android developers who want to use R8's most powerful optimizations. It explores the key differences between compatibility and full mode and provides the explicit configurations needed to migrate your project safely and avoid common runtime crashes.

Enable full mode

To enable full mode, remove the following line from your gradle.properties file:

android.enableR8.fullMode=false // Remove this line to enable full mode

Retain classes associated with attributes

Attributes are metadata stored within compiled class files that aren't part of the executable code. However, they can be needed for certain types of reflection. Common examples include Signature (which preserves generic type information after type erasure), InnerClasses and EnclosingMethod (for reflecting on class structure) and runtime-visible annotations.

The following code shows what a Signature attribute looks like for a field in bytecode. For a field:

List<User> users;

The compiled class file would contain the following bytecode:

.field public static final users:Ljava/util/List;
    .annotation system Ldalvik/annotation/Signature;
        value = {
            "Ljava/util/List<",
            "Lcom/example/package/User;",
            ">;"
        }
    .end annotation
.end field

Libraries that heavily use reflection (like Gson) often rely on these attributes to dynamically inspect and understand your code's structure. By default in R8's full mode, attributes are retained only if the associated class, field, or method is explicitly kept.

The following example demonstrates why attributes are necessary and what keep rules you need to add when migrating from compatibility to full mode.

Consider the following example where we deserialize a list of users using the Gson library.


import com.google.gson.Gson
import com.google.gson.reflect.TypeToken

data class User(
    @SerializedName("username")
    var username: String? = null,
    @SerializedName("age")
    var age: Int = 0
)

fun GsonRemoteJsonListExample() {
    val gson = Gson()

    // 1. The JSON string for a list of users returned from remote
    val jsonOutput = """[{"username":"alice","age":30}, {"username":"bob","age":25}]"""

    // 2. Deserialize the JSON string into a List<User>
    // We must use TypeToken for generic types like List
    val listType = object : TypeToken<List<User>>() {}.type
    val deserializedList: List<User> = gson.fromJson(jsonOutput, listType)

    // Print the list
    println("First user from list: ${deserializedList}")
}

During compilation, Java's type erasure removes generic type arguments. This means that at runtime, both List<String> and List<User> appear as a raw List. Therefore, libraries like Gson, which rely on reflection, cannot determine the specific object types the List was declared to contain when deserializing a JSON list, which can lead to runtime issues.

To preserve type information, Gson uses TypeToken. Wrapping TypeToken retains the necessary deserialization information.

The Kotlin expression object:TypeToken<List<User>>() {}.type creates an anonymous inner class that extends TypeToken and captures the generic type information. In this example, the anonymous class is named $GsonRemoteJsonListExample$listType$1.

The Java programming language saves the generic signature of a superclass as metadata, known as the Signature attribute, within the compiled class file. TypeToken then uses this Signature metadata to recover the type at runtime. This allows Gson to use reflection to read the Signature and successfully discover the full List<User> type it needs for deserialization.

When R8 is enabled in compatibility mode, it retains the Signature attribute for classes, including anonymous inner classes like $GsonRemoteJsonListExample$listType$1, even if specific keep rules are not explicitly defined. As a result, R8 compatibility mode does not require any further explicit keep rules for this example to work as expected.

// keep rule for compatibility mode
-keepattributes Signature

When R8 is enabled in full mode, the Signature attribute of the anonymous inner class $GsonRemoteJsonListExample$listType$1 is stripped. Without this type information in the Signature, Gson cannot find the correct application type, which results in an IllegalStateException. The keep rules necessary to prevent this are:

// keep rule required for full mode
-keepattributes Signature
-keep,allowobfuscation,allowshrinking,allowoptimization class com.google.gson.reflect.TypeToken
-keep,allowobfuscation,allowshrinking,allowoptimization class * extends com.google.gson.reflect.TypeToken

  • -keepattributes Signature: This rule instructs R8 to retain the attribute that Gson needs to read. In full mode, R8 only retains the Signature attribute for classes, fields, or methods that are explicitly matched by a keep rule.

  • -keep,allowobfuscation,allowshrinking,allowoptimization class com.google.gson.reflect.TypeToken: This rule is necessary because TypeToken wraps the type of the object being deserialized. After type erasure, an anonymous inner class is created to retain the generic type information. Without explicitly keeping com.google.gson.reflect.TypeToken, R8 in full mode won't include this class type in the Signature attribute needed for deserialization.

  • -keep,allowobfuscation,allowshrinking,allowoptimization class * extends com.google.gson.reflect.TypeToken: This rule retains the type information of anonymous classes that extend TypeToken, such as $GsonRemoteJsonListExample$listType$1 in this example. Without this rule, R8 in full mode strips the necessary type information, causing deserialization to fail.

Starting with Gson version 2.11.0, the library bundles necessary keep rules required for deserialization in full mode. When you build your app with R8 enabled, R8 automatically finds and applies these rules from the library. This provides the protection your app needs without you having to manually add or maintain these specific rules in your project.

It is important to understand that the rules shared earlier only solve the problem of discovering the generic type (e.g., List<User>). R8 also renames the fields of classes. If you don't use @SerializedName annotations on your data models, Gson will fail to deserialize JSON because the field names will no longer match the JSON keys.

However, if you are using a Gson version older than 2.11, or if your models don't use the @SerializedName annotation, you must add explicit keep rules for those models.

Retain the default constructor

In R8 full mode, the no-args/default constructor is not implicitly kept, even when the class itself is retained. If you are creating an instance of a class using class.getDeclaredConstructor().newInstance() or class.newInstance(), you must explicitly retain the no-args constructor in full mode. In contrast, compatibility mode always retains the no-args constructor.

Consider an example where an instance of PrecacheTask is created using reflection to dynamically call its run method. While this scenario doesn't require additional rules in compatibility mode, in full mode, the default constructor of PrecacheTask would be removed. Therefore, a specific keep rule is required.

// In library
interface StartupTask {
    fun run()
}
// The library object that loads and executes the task.
object TaskRunner {
    fun execute(taskClass: Class<out StartupTask>) {
        // The class isn't removed, but its constructor might be.
        val task = taskClass.getDeclaredConstructor().newInstance()
        task.run()
    }
}

// In app
class PreCacheTask : StartupTask {
    override fun run() {
        Log.d("Pre cache task", "Warming up the cache...")
    }
}

fun runTaskRunner() {
    // The library is given a direct reference to the app's task class.
    TaskRunner.execute(PreCacheTask::class.java)
}

# Full mode keep rule
# default constructor needs to be specified

-keep class com.example.fullmoder8.PreCacheTask {
    <init>();
}

Access modification is enabled by default

In compatibility mode, R8 does not alter the visibility of methods and fields within a class. However, in full mode, R8 enhances optimization by changing the visibility of your methods and fields, for example, from private to public. This enables more inlining.

This optimization can cause issues if your code uses reflection that specifically relies on members having particular visibility. R8 won't recognize this indirect usage, potentially leading to app crashes. To prevent this, you must add specific -keep rules to preserve the members, which will also preserve their original visibility.

For more information, see this example to understand why accessing private members using reflection is not advised and the keep rules to retain those fields/methods.