I want to do something using Annotation processing in eclipse jdt.
Eclipse jdtapt help tells me I should configure things on the java/compiler/annotation processing pref
The annotation processing tab does not exist in the Preferences page that is accessible from the Eclipse menu bar. It is a project-specific menu that is accessible if you right-click on a project in the Project Explorer and select Preferences in the pop-up menu that will appear.
You may have to enable project-specific settings for the Java compiler tab and annotation processing explicitly before it will appear, as mentioned in this tutorial.