Breaking the Android ClassLoader

ClassLoaders are not new. They've existed since Java 1.0 and are a core part of how the JVM works. Things work a bit different under the hood in Android, but the core principles are the same and we, as Android developers, largely take it for granted. This talk will be a deep dive into how class loading works in Android and later explore the implications of trying to manipulate the application's ClassLoader at runtime (for better or worse 🔥) in the context of logging, hot fixing, adding features, and interacting with 3rd party code.

Sample project: https://github.com/hzsweers/blackmirror

Transcript

1. Breaking the
   Android ClassLoader
   Zac Sweers
   @pandanomic
   Photo by Vitalis Hirschmann on Unsplash
   

   View Slide

2. ClassLoader
   

   View Slide

3. class ClassLoader
   

   View Slide

4. class ClassLoader {a
   Class> loadClass(String name);
   Class> findClass(String name);
   Class> defineClass(...);
   URL getResource(String name);
   // A few others
   }a
   

   View Slide

5. class ClassLoader {a
   Class> loadClass(String name);
   Class> findClass(String name);
   Class> defineClass(...);
   URL getResource(String name);
   // A few others
   }a
   

   View Slide

6. class URLClassLoader {a
   Class> loadClass(String name) {
   // Go fetch them from a jar
   }a
   }a
   

   View Slide

7. class URLClassLoader {a
   Class> loadClass(String name) {
   // Go fetch them from a jar
   }a
   }a
   

   View Slide

8. class URLClassLoader {a
   Class> loadClass(String name) {
   // Go fetch them from a jar
   }a
   }a
   

   View Slide

9. class URLClassLoader {a
   // ClassLoader parent
   Class> loadClass(String name)
   }a
   

   View Slide

10. class URLClassLoader {a
    // ClassLoader parent
    Class> loadClass(String name)
    }a
    

    View Slide

11. Class> loadClass(String name)
    

    View Slide

12. package example;
    class Foo {
    private Bar bar;
    }a
    

    View Slide

13. package example;
    class Foo {
    private Bar bar;
    }a
    loadClass("example.Foo")
    

    View Slide

14. package example;
    class Foo {
    private Bar bar;
    }a
    loadClass("example.Foo")
    Foo.class.getClassLoader().loadClass("example.Bar")
    

    View Slide

15. View Slide

16. class PathClassLoader
    

    View Slide

17. class PathClassLoader
    class DexClassLoader
    

    View Slide

18. class PathClassLoader extends BaseDexClassLoader
    class DexClassLoader extends BaseDexClassLoader
    

    View Slide

19. class BaseDexClassLoader {a
    DexPathList pathList
    }a
    

    View Slide

20. class BaseDexClassLoader {a
    DexPathList pathList
    // --> DexFile, etc
    }a
    

    View Slide

21. class BaseDexClassLoader {a
    DexPathList pathList
    // --> DexFile, etc
    }a
    

    View Slide

22. Story time!
    

    View Slide

23. View Slide

24. View Slide

25. View Slide

26. PathClassLoader classLoader = new PathClassLoader(
    context.getApplicationInfo().sourceDir,
    /* parent */ null
    );
    

    View Slide

27. class CustomClassLoader extends PathClassLoader
    

    View Slide

28. class CustomClassLoader extends PathClassLoader {
    @Override public Class> loadClass(String name) {
    Log.d("ClLoading", "Loading " + name);
    return super.loadClass(name);
    }
    }
    

    View Slide

29. CustomClassLoader cl = new CustomClassLoader(...);
    

    View Slide

30. CustomClassLoader cl = new CustomClassLoader(...);
    // set java.lang.ClassLoader$SystemClassLoader.loader
    

    View Slide

31. CustomClassLoader cl = new CustomClassLoader(...);
    // set java.lang.ClassLoader$SystemClassLoader.loader
    Thread.currentThread().setContextClassLoader(cl)
    

    View Slide

32. CustomClassLoader cl = new CustomClassLoader(...);
    // set java.lang.ClassLoader$SystemClassLoader.loader
    Thread.currentThread().setContextClassLoader(cl)
    ContextImpl
    

    View Slide

33. CustomClassLoader cl = new CustomClassLoader(...);
    // set java.lang.ClassLoader$SystemClassLoader.loader
    Thread.currentThread().setContextClassLoader(cl)
    ContextImpl.mClassLoader
    

    View Slide

34. CustomClassLoader cl = new CustomClassLoader(...);
    // set java.lang.ClassLoader$SystemClassLoader.loader
    Thread.currentThread().setContextClassLoader(cl)
    ContextImpl
    

    View Slide

35. CustomClassLoader cl = new CustomClassLoader(...);
    // set java.lang.ClassLoader$SystemClassLoader.loader
    Thread.currentThread().setContextClassLoader(cl)
    ContextImpl.mPackageInfo
    

    View Slide

36. CustomClassLoader cl = new CustomClassLoader(...);
    // set java.lang.ClassLoader$SystemClassLoader.loader
    Thread.currentThread().setContextClassLoader(cl)
    ContextImpl.mPackageInfo.mClassLoader
    

    View Slide

37. View Slide

38. BlackM
    irro
    r
    

    View Slide

39. Demo time!
    

    View Slide

40. github.com/hzsweers/
    blackmirror
    

    View Slide

41. Final notes
    

    View Slide

42. Harder in Pie
    g.co/dev/appcompat
    

    View Slide

43. Be mindful
    

    View Slide

44. github.com/hzsweers/
    blackmirror
    Zac Sweers
    @pandanomic
    

    View Slide