Question

The bytecode for system classes like android.os.Looper is simply a stub. For example, android.os.Looper.loop() from android.jar contains the following bytecode:

public static final void loop();
  Code:
   0:   new     #2; //class java/lang/RuntimeException
   3:   dup
   4:   ldc     #3; //String Stub!
   6:   invokespecial   #4; //Method java/lang/RuntimeException."<init>":(Ljava/lang/String;)V
   9:   athrow

But in AOSP, I can see the real source code that performs message dispatch (see AOSP_ROOT/frameworks/base/core/java/android/os/Looper.java). So how is this class handled exactly by the android system? Is the real code patched in when compiling the system android.jar into dex file or does it happen at run-time within the Dalvik VM?

Answer 1

The android.jar which you compile your code against contains only public classes with public (constant) fields and public methods, but all of those methods contain no implementation. All methods with a return type other that void throw the “Stub!” RuntimeException.

A runtime library with a real implementation is linked with your application only in a phone device or in an emulator.