Android Bytecode

We use ASM to rewrite Java bytecode in jar files during the build process, which usually happens after java compilation of the specific jar file and before the final assembly of apk file. Through bytecode rewrite, we can rewrite/redirect classes, methods, etc. without patching or manually overriding every occurance of them.

How to use

In order to use bytecode rewrite, you need to take the following steps:

1. Start by making a file and a class named {***}ClassAdapter under build/android/bytecode/java/org/brave/bytecode. The folder has a lot of examples of how the class shall be like. The most important part is it shall extends BraveClassVisitor and all redirections/rewrites happen in its constructor.
2. Go to build/android/bytecode/java/org/brave/bytecode/BraveClassAdapter.java and add your newly created class to the chain of invocation of the adapters.
3. Go to build/android/bytecode/BUILD.gn and add your newly created java file to sources.
4. If you are redirecting/rewriting any exisitng classes, methods or fields from upstream, you shall add tests for their existence in android/javatests/org/chromium/chrome/browser/BytecodeTest.java. In addition, if you are redirecting a class, also add tests for consturctors and supers.
5. If you are redirecting an existing method, add that method to android/java/proguard.flags following the examples of exising entries. The *** and ... part means the rule will match any method signature regardless of what types and how many parameters there are.
6. Check any references to the classes/methods/fields you want to rewrite is compiled to one of the jar files listed in build/android/bytecode/bytecode_rewriter.gni. If not, then add the output jar file to the list. The easiest way to find out which jar file is search for the name of the source java file you are trying to rewrite in gn/gni files and find the android_library target which compiles the file. The jar file shall be the one with the same target name under obj/{path to the gn file you are looking at}.

Supported redirection/rewrite

• changeSuperName(String className, String superName): change super of className to superName.

• deleteMethod(String className, String methodName): delete methodName from className.

• makePublicMethod(String className, String methodName): change the visibility of methodName in className to public.

• makePrivateMethod(String className, String methodName): change the visibility of methodName in className to private.

• changeMethodOwner(String currentOwner, String methodName, String newOwner): change any invocation of methodName from being from class currentOwner to class newOwner. This does not change method definition.

• deleteField(String className, String fieldName): delete fieldName from className.

• deleteInnerClass(String outerName, String innerName): delete and inner class innerName from class outerName.

• makeNonFinalClass(String className): remove keywork final from the definition of className.

• makePublicInnerClass(String outerName, String innerName): make the visibility of inner class innerName inside outerName to be public.

• makeProtectedField(String className, String fieldName): change the visibility of fieldName in className to protected.

• addMethodAnnotation(String className, String methodName, String annotationType): add annotation annotationType (e.g. Override) to the definition of methodName.

• redirectConstructor(String originalClassName, String newClassName): redirect any invocation of constructor originalClassName (i.e. new originalClassName()) to a new constructor newClassName.

• redirectTypeInMethod(String className, String methodName, String originalTypeName, String newTypeName): change any mention of type in definition of methodName in className from originalTypeName to newTypeName. This includes method signature, body and return type.