Introduction

Programming with Android NDK is essentially writing code in both Java and native languages such as C, C++, and assembly. Java code runs on Dalvik Virtual Machine (VM), while native code is compiled to binaries running directly on the operating system. Java Native Interface (JNI) acts like the bridge that brings both worlds together. This relationship between Java code, Dalvik VM, native code, and the Android system can be illustrated using the following diagram:

The arrow in the diagram indicates which party initiates the interaction. Both Dalvik VM and Native Code run on top of Android system (Android is a Linux-based OS). They require the system to provide the execution environment. JNI is part of Dalvik VM, which allows Native Code to access fields and invoke methods at Java Code. JNI also allows Java Code to invoke native methods implemented in Native Code. Therefore, JNI facilitates the two-way communication between Native Code and Java Code.

If you are familiar with Java...