Developer patched closed-source Android app Smart Audiobook Player using apktool, jadx, and compiled Java injected via smali to add Audiobookshelf progress syncing.
Key Takeaways
APKs can be partially decompiled to readable Java via jadx; smali (from apktool) is the editable, recompilable bytecode layer that enables patching.
A single convergence point, PlayerService.u0(), handled all progress-save paths, making it the ideal smali hook for injecting an invoke-static call to new Java code.
New Java (~550 lines) was compiled with javac and packaged as classes2.dex via d8 (standard Android SDK), then dropped into the apktool tree, no smali logic required for the feature itself.
Book-to-server matching used a JSON config file per book directory, avoiding fragile name-lookup and supporting multiple books without UI changes to the closed-source app.
LLM dual-review (Claude + GPT) caught reflection code, a resource compression bug, and a race condition; human review caught a critical progress-overwrite issue LLMs missed entirely.
Hacker News Comment Review
Discussion is sparse; main technical thread focuses on how the injected Java accesses host app classes at compile time, a real gap the author acknowledged without fully resolving.
Commenters noted ReVanced as an existing framework for non-root Android app patching, relevant for builders wanting a more structured starting point than raw apktool workflows.
