The annotation library described here is in development and considered to be in its prototype phase. As such it is not yet feature complete, but we are actively working on supporting all of the use cases we know of. Once the design exits the prototype phase, it is intended to move to an R8 independent library as part of androidx. All feedback: criticism, comments and suggestions are very welcome!
File new feature requests and bugs in the R8 component.
When using a Java/Kotlin shrinker such as R8 or Proguard, developers must inform the shrinker about parts of the program that are used either externally from the program itself or internally via reflection and therefore must be kept.
Traditionally these aspects would be kept by writing keep rules in a configuration file and passing that to the shrinker.
The keep annotations described in this document represent an alternative method using Java annotations. The motivation for using these annotations is foremost to place the description of what to keep closer to the program point using reflective behavior. Doing so more directly connects the reflective code with the keep specification and makes it easier to maintain as the code develops. Often the keep annotations are only in effect if the annotated method is used, allowing more precise shrinking. In addition, the annotations are defined independent from keep rules and have a hopefully more clear and direct meaning.
To use the keep annotations your build must include the library of annotations. It is currently built as part of each R8 build and if used with R8, you should use the matching version. You can find all archived builds at:
https://storage.googleapis.com/r8-releases/raw/<version>/keepanno-annotations.jar
Thus you may obtain version 8.2.34 by running:
wget https://storage.googleapis.com/r8-releases/raw/8.2.34/keepanno-annotations.jar
You will then need to set the system property com.android.tools.r8.enableKeepAnnotations to instruct R8 to make use of the annotations when shrinking:
java -Dcom.android.tools.r8.enableKeepAnnotations=1 \ -cp r8.jar com.android.tools.r8.R8 \ # ... the rest of your R8 compilation command here ...
The keep annotation library defines a family of annotations depending on your use case. You should generally prefer @UsesReflection where applicable. Common uses of reflection are to lookup fields and methods on classes. Examples of such use cases are detailed below.
For example, if your program is reflectively invoking a method, you should annotate the method that is doing the reflection. The annotation must describe the assumptions the reflective code makes.
In the following example, the method callHiddenMethod is looking up the method with the name hiddenMethod on objects that are instances of BaseClass. It is then invoking the method with no other arguments than the receiver.
The assumptions the code makes are that all methods with the name hiddenMethod and the empty list of parameters must remain valid for getDeclaredMethod if they are objects that are instances of the class BaseClass or subclasses thereof.
public class MyHiddenMethodCaller {
@UsesReflection({
@KeepTarget(
instanceOfClassConstant = BaseClass.class,
methodName = "hiddenMethod",
methodParameters = {})
})
public void callHiddenMethod(BaseClass base) throws Exception {
base.getClass().getDeclaredMethod("hiddenMethod").invoke(base);
}
}
For example, if your program is reflectively accessing the fields on a class, you should annotate the method that is doing the reflection.
In the following example, the printFieldValues method takes in an object of type PrintableFieldInterface and then looks for all the fields declared on the class of the object.
The @KeepTarget describes these field targets. Since the printing only cares about preserving the fields, the @KeepTarget.kind is set to KeepItemKind.ONLY_FIELDS. Also, since printing the field names and values only requires looking up the field, printing its name and getting its value the @KeepTarget.constraints are set to just KeepConstraint.LOOKUP, KeepConstraint.NAME and KeepConstraint.FIELD_GET.
public class MyFieldValuePrinter {
@UsesReflection({
@KeepTarget(
instanceOfClassConstant = PrintableFieldInterface.class,
kind = KeepItemKind.ONLY_FIELDS,
constraints = {KeepConstraint.LOOKUP, KeepConstraint.NAME, KeepConstraint.FIELD_GET})
})
public void printFieldValues(PrintableFieldInterface objectWithFields) throws Exception {
for (Field field : objectWithFields.getClass().getDeclaredFields()) {
System.out.println(field.getName() + " = " + field.get(objectWithFields));
}
}
}
If your program is reflectively inspecting annotations on classes, methods or fields, you will need to declare additional “annotation constraints” about what assumptions are made about the annotations.
In the following example, we have defined an annotation that will record the printing name we would like to use for fields instead of printing the concrete field name. That may be useful so that the field can be renamed to follow coding conventions for example.
We are only interested in matching objects that contain fields annotated by MyNameAnnotation, that is specified using @KeepTarget.fieldAnnotatedByClassConstant.
At runtime we need to be able to find the annotation too, so we add a constraint on the annotation using @KeepTarget.constrainAnnotations.
Finally, for the sake of example, we don't actually care about the name of the fields themselves, so we explicitly declare the smaller set of constraints to be KeepConstraint.LOOKUP since we must find the fields via Class.getDeclaredFields as well as KeepConstraint.VISIBILITY_RELAX and KeepConstraint.FIELD_GET in order to be able to get the actual field value without accessibility errors.
The effect is that the default constraint KeepConstraint.NAME is not specified which allows the shrinker to rename the fields at will.
public class MyAnnotationPrinter {
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface MyNameAnnotation {
String value();
}
public static class MyClass {
@MyNameAnnotation("fieldOne")
public int mFieldOne = 1;
@MyNameAnnotation("fieldTwo")
public int mFieldTwo = 2;
public int mFieldThree = 3;
}
@UsesReflection(
@KeepTarget(
fieldAnnotatedByClassConstant = MyNameAnnotation.class,
constrainAnnotations = @AnnotationPattern(constant = MyNameAnnotation.class),
constraints = {
KeepConstraint.LOOKUP,
KeepConstraint.VISIBILITY_RELAX,
KeepConstraint.FIELD_GET
}))
public void printMyNameAnnotatedFields(Object obj) throws Exception {
for (Field field : obj.getClass().getDeclaredFields()) {
if (field.isAnnotationPresent(MyNameAnnotation.class)) {
System.out.println(
field.getAnnotation(MyNameAnnotation.class).value() + " = " + field.get(obj));
}
}
}
}
If the annotations that need to be kept are not runtime visible annotations, then you must specify that by including the RetentionPolicy.CLASS value in the @AnnotationPattern.retention property. An annotation is runtime visible if its definition is explicitly annotated with Retention(RetentionPolicy.RUNTIME).
Sometimes reflecting code cannot be annotated. For example, the reflection can be done in native code or in a library outside your control. In such cases you can annotate the code that is being used by reflection with either @UsedByReflection or @UsedByNative. These two annotations are equivalent. Use the one that best matches why the annotation is needed.
Let's consider some code with reflection outside our control. For example, the same field printing as in the above example might be part of a library.
In this example, the MyClassWithFields is a class you are passing to the field-printing utility of the library. Since the library is reflectively accessing each field we annotate them with the @UsedByReflection annotation.
We could additionally add the @UsedByReflection.constraints property as we did previously. We elide it here for brevity.
public class MyClassWithFields implements PrintableFieldInterface {
@UsedByReflection final int intField = 42;
@UsedByReflection String stringField = "Hello!";
}
public static void run() throws Exception {
new FieldValuePrinterLibrary().printFieldValues(new MyClassWithFields());
}
Rather than annotate the individual fields we can annotate the holder and add a specification similar to the @KeepTarget. The @UsedByReflection.kind specifies that only the fields are used reflectively. In particular, the “field printer” example we are considering here does not make reflective assumptions about the holder class, so we should not constrain it.
To be more precise let's add the @UsedByReflection.constraints property now. This specifies that the fields are looked up, their names are used/assumed and their values are read.
@UsedByReflection(
kind = KeepItemKind.ONLY_FIELDS,
constraints = {KeepConstraint.LOOKUP, KeepConstraint.NAME, KeepConstraint.FIELD_GET})
public class MyClassWithFields implements PrintableFieldInterface {
final int intField = 42;
String stringField = "Hello!";
}
Our use of @UsedByReflection is still not as flexible as the original @UsesReflection. In particular, if we change our code to no longer have any call to the library method printFieldValues the shrinker will still keep all of the fields on our annotated class.
This is because the @UsesReflection implicitly encodes as a precondition that the annotated method is actually used in the program. If not, the @UsesReflection annotation is not “active”.
Luckily we can specify the same precondition using @UsedByReflection.preconditions.
@UsedByReflection(
preconditions = {
@KeepCondition(
classConstant = FieldValuePrinterLibrary.class,
methodName = "printFieldValues")
},
kind = KeepItemKind.ONLY_FIELDS,
constraints = {KeepConstraint.LOOKUP, KeepConstraint.NAME, KeepConstraint.FIELD_GET})
public class MyClassWithFields implements PrintableFieldInterface {
final int intField = 42;
String stringField = "Hello!";
}
If your code is being shrunk before release as a library, or if you have an API surface that is used via dynamic loading at runtime, then you need to keep the API surface. For that you should use the @KeepForApi annotation.
When annotating a class the default for @KeepForApi is to keep the class as well as all of its public and protected members:
@KeepForApi
public class MyApi {
public void thisPublicMethodIsKept() {
/* ... */
}
protected void thisProtectedMethodIsKept() {
/* ... */
}
void thisPackagePrivateMethodIsNotKept() {
/* ... */
}
private void thisPrivateMethodIsNotKept() {
/* ... */
}
}
The default can be changed using the @KeepForApi.memberAccess property:
@KeepForApi(
memberAccess = {
MemberAccessFlags.PUBLIC,
MemberAccessFlags.PROTECTED,
MemberAccessFlags.PACKAGE_PRIVATE
})
The @KeepForApi annotation can also be placed directly on members and avoid keeping unannotated members. The holder class is implicitly kept. When annotating the members directly, the access does not matter as illustrated here by annotating a package private method:
public class MyOtherApi {
public void notKept() {
/* ... */
}
@KeepForApi
void isKept() {
/* ... */
}
}
There is no automatic migration of keep rules. Keep annotations often invert the direction and rules have no indication of where the reflection is taking place or why. Thus, migrating existing keep rules requires user involvement. Keep rules also have a tendency to be very general, matching a large number of classes and members. Often the rules are much too broad and are keeping more than needed which will have a negative impact on the shrinkers ability to reduce size.
First step in converting a rule is to determine the purpose of the rule. Is it API surface or is it reflection? Note that a very general rule may be covering several use cases and even a mix of both reflection and API usage.
When migrating it is preferable to use @UsesReflection instead of @UsedByReflection. For very general rules it might not be easy or worth it to migrate without completely reevaluating the rule. If one still wants to replace it by annotations, the general @KeepEdge can be used to define a context independent keep annotation.
For example, to keep all main methods in the program one could use:
@KeepEdge(
consequences = {
@KeepTarget(
kind = KeepItemKind.CLASS_AND_MEMBERS,
methodName = "main",
methodReturnType = "void",
methodParameters = {"java.lang.String[]"},
methodAccess = {MethodAccessFlags.PUBLIC, MethodAccessFlags.STATIC})
})
public class SomeClass {
// ...
}
The annotation library is in active development and not all use cases are described here or supported. Reach out to the R8 team by filing a new issue in our tracker. Describe your use case and we will look at how best to support it.
If an annotation is not working as expected it may be helpful to inspect the rules that have been extracted for the annotation. This can be done by inspecting the configuration output of the shrinker. For R8 you can use the command line argument --pg-conf-output <path> to emit the full configuration used by R8.