I was reading about code-injection at run-time and there are many tools/APIs available like Javassist, GluonJ and AspectJ which provide features to inject code. However, I did not understand purpose for injecting code at run-time while we can do override behaviors by sub-classing in Java. With Javassist and GluonJ, I can create classes at run-time but why does anyone do that, in the first place? Can anyone please help me to understand the difference and purpose of code injection ?
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Code injection is usually used into application that are used to modify/check/trace other software. In Java we usually reffer to this kind of library as Bytecode modification libraries so if you look on the internet you will probably find more information under this name.
Here I listed a couple of examples of big and famous projects that I now are using Bytecode modification into their cores:
- Evosuite: this project takes an application in input and generates unit test for it. Code injection is used to explore the desired project and dependencies and traceability
- JaCoCO: this project is a tool for Java project. It is supposed to be attached to your application and once you run your JUnit tests it is going to generate a report on the coverage achieved. Obviously here code injection is necessary to trace every method call made during test execution.
