Overload an APK and create custom Applications

Question

Is is possible to create a new APK by overloading/overriding and existing APK.

Say, we have WhatsApp messenger. Now if I want to show a notification like, "Person X is online now.", in notification area, it is not possible using the existing WhatsApp messenger. So, I'd like to develop a new custom messenger, which uses all the functionality of WhatsApp messenger, with some of my custom code. Just like importing JARs, can we import APK...??

This seems like using another persons work, but just from learning point of view, I'd like to know the possibilities. As of now, lets leave all the, security, vulnerability.

Answer 1

In a nutshell, no.

APKs are not like Jar files such that you can simply import them into your app. They also contain other resources like the XML and assets. The code is kept in a classes.dex file, which is generated from the Jar file of the app code.

Moreover, each app runs in its own DVM, and is sandboxed to prevent this kind of interaction between apps. You cannot simply use WhatsApp's code in your own app as a third party apps.

There are however two ways to achieve what you want.

1. If the app you are targeting provides an API by means of content providers, or even web services that you can use to access their data and events like users coming online. WhatsApp does not offer any such API
2. You decompile the target app, and insert your own code to do what you want. This can be very hard because most popular apps, like WhatsApp, obfuscate their code making it hard to decipher (but not impossible). Additionally, WhatsApp encrypts most of its data like messages, contacts, chat threads etc using AES, which adds an additional layer to bypass in some places. Oh, and it also violates several IP and copyright laws if you do this.