This page will be getting more details about Hadoop development and testing using AspectJ based Injection Framework.
The idea of code injection is fairly simple: it is an infusion of new or modification of existing behavior into a code of a software application. Fault Injection, which will be discussed later, is a similar mechanism for adding errors and exceptions into an application's logic to achieve a higher coverage and fault tolerance of the system. Different implementations of this idea are available today. Hadoop's inject framework is built on top of Aspect Oriented Paradigm (AOP) implemented on top of AspectJ toolkit.
The document below relates to injection technique with regards to code/behavior modifications and to injection of programmatic faults (fault-injection). I'll refer to both injection and fault injection aspects of the framework unless narrower cases are explicitly mentioned...
You can read the rest of it from Apache Hadoop Wiki site