R8 compatibility FAQ

R8 uses the same configuration specification language as ProGuard, and tries to be compatible with ProGuard. However as R8 has different optimizations it can be necessary to change the configuration when switching to R8.

This FAQ collects some of the common issues.

GSON

Member in a data object is always null

For data classes used for serialization all fields that are used in the serialization must be kept by the configuration. R8 can decide to replace instances of types that are never instantiated with null. So if instances of a given class are only created through deserialization from JSON, R8 will not see that class as instantiated leaving it as always null.

If the @SerializedName annotation is used consistently for data classes the following keep rule can be used:

-keepclassmembers,allowobfuscation class * {
  @com.google.gson.annotations.SerializedName <fields>;
}

This will ensure that all fields annotated with SerializedName will be kept. These fields can still be renamed during obfuscation as the SerializedName annotation (not the source field name) controls the name in the JSON serialization.

If the @SerializedName annotation is not used the following conservative rule can be used for each data class:

-keepclassmembers class MyDataClass {
  !transient <fields>;
}

This will ensure that all fields are kept and not renamed for these classes. Fields with modifier transient are never serialized and therefore keeping these is not needed.

Error java.lang.IllegalArgumentException: class <class name> declares multiple JSON fields named <name>

This can be caused by obfuscation selecting the same name for private fields in several classes in a class hierachy. Consider the following example:

class A {
  private String fieldInA;
}

class B extends A {
  private String fieldInB;
}

Here R8 can choose to rename both fieldInA and fieldInB to the same name, e.g. a. This creates a conflict when GSON is used to either serialize an instance of class B to JSON or create an instance of class B from JSON. If the fields should not be serialized they should be marked transient so that they will be ignored by GSON:

class A {
  private transient String fieldInA;
}

class B extends A {
  private transient String fieldInB;
}

If the fields are to be serialized, the annotation SerializedName can be used to fix the IllegalArgumentException together the rule to keep fields annotated with SerializedName

class A {
  @SerializedName("fieldInA")
  private String fieldInA;
}

class B extends A {
  @SerializedName("fieldInB")
  private String fieldInB;
}

-keepclassmembers,allowobfuscation class * {
  @com.google.gson.annotations.SerializedName <fields>;
}

Both the use of transient and the use of the annotation SerializedName allow the fields to be renamed by R8 to the same name, but GSON serialization will work as expected.

R8 full mode

In full mode, R8 performs more aggressive optimizations, meaning that additional ProGuard configuration rules may be required. This section highlights some common issues that have been seen when using full mode.

Retrofit

Object instantiated with Retrofit's create() method is always replaced with null

This happens because Retrofit uses reflection to return an object that implements a given interface. The issue can be resolved by using the most recent keep rules from the Retrofit library.

See also https://github.com/square/retrofit/issues/3005 (“Insufficient keep rules for R8 in full mode”).