How to implement a custom JavaCoveragerunner

I'm sorry, but this question is too generic. Please take a look at an existing implementation and try to understand it:
https://github.com/JetBrains/intellij-community/blob/master/plugins/coverage/src/com/intellij/coverage/JaCoCoCoverageRunner.java

After that try to implement your runner, and if you have problems, please ask specific questions.