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
2. ClassLoader
3. class ClassLoader
4. class ClassLoader {a Class<?> loadClass(String name); Class<?> findClass(String name); Class<?> defineClass(...); URL getResource(String name); // A few others }a
5. class ClassLoader {a Class<?> loadClass(String name); Class<?> findClass(String name); Class<?> defineClass(...); URL getResource(String name); // A few others }a
6. class URLClassLoader {a Class<?> loadClass(String name) { // Go fetch them from a jar }a }a
7. class URLClassLoader {a Class<?> loadClass(String name) { // Go fetch them from a jar }a }a
8. class URLClassLoader {a Class<?> loadClass(String name) { // Go fetch them from a jar }a }a
9. class URLClassLoader {a // ClassLoader parent Class<?> loadClass(String name) }a
10. class URLClassLoader {a // ClassLoader parent Class<?> loadClass(String name) }a
11. Class<?> loadClass(String name)
12. package example; class Foo { private Bar bar; }a
13. package example; class Foo { private Bar bar; }a loadClass("example.Foo")
14. package example; class Foo { private Bar bar; }a loadClass("example.Foo") Foo.class.getClassLoader().loadClass("example.Bar")
16. class PathClassLoader
17. class PathClassLoader class DexClassLoader
18. class PathClassLoader extends BaseDexClassLoader class DexClassLoader extends BaseDexClassLoader
19. class BaseDexClassLoader {a DexPathList pathList }a
20. class BaseDexClassLoader {a DexPathList pathList // --> DexFile, etc }a
21. class BaseDexClassLoader {a DexPathList pathList // --> DexFile, etc }a
22. Story time!
26. PathClassLoader classLoader = new PathClassLoader( context.getApplicationInfo().sourceDir, /* parent */ null );
27. class CustomClassLoader extends PathClassLoader
28. class CustomClassLoader extends PathClassLoader { @Override public Class<?> loadClass(String name) { Log.d("ClLoading", "Loading " + name); return super.loadClass(name); } }
29. CustomClassLoader cl = new CustomClassLoader(...);
30. CustomClassLoader cl = new CustomClassLoader(...); // set java.lang.ClassLoader$SystemClassLoader.loader
31. CustomClassLoader cl = new CustomClassLoader(...); // set java.lang.ClassLoader$SystemClassLoader.loader Thread.currentThread().setContextClassLoader(cl)
32. CustomClassLoader cl = new CustomClassLoader(...); // set java.lang.ClassLoader$SystemClassLoader.loader Thread.currentThread().setContextClassLoader(cl) ContextImpl
33. CustomClassLoader cl = new CustomClassLoader(...); // set java.lang.ClassLoader$SystemClassLoader.loader Thread.currentThread().setContextClassLoader(cl) ContextImpl.mClassLoader
34. CustomClassLoader cl = new CustomClassLoader(...); // set java.lang.ClassLoader$SystemClassLoader.loader Thread.currentThread().setContextClassLoader(cl) ContextImpl
35. CustomClassLoader cl = new CustomClassLoader(...); // set java.lang.ClassLoader$SystemClassLoader.loader Thread.currentThread().setContextClassLoader(cl) ContextImpl.mPackageInfo
36. CustomClassLoader cl = new CustomClassLoader(...); // set java.lang.ClassLoader$SystemClassLoader.loader Thread.currentThread().setContextClassLoader(cl) ContextImpl.mPackageInfo.mClassLoader
38. BlackM irro r
39. Demo time!
40. github.com/hzsweers/ blackmirror
41. Final notes
42. Harder in Pie g.co/dev/appcompat
43. Be mindful
44. github.com/hzsweers/ blackmirror Zac Sweers @pandanomic