To convert a Java project to an AspectJ project right click on that project, then select Convert to AspectJ Project. This will add the AspectJ nature to that project and replace the Java builder with the AspectJ builder. It will also add the AspectJ runtime library, aspectjrt.jar, to the classpath.
To convert an AspectJ project back to a Java project right click on that project, then select AspectJ Tools > Remove AspectJ Nature. This will remove the AspectJ nature from that project and replace the AspectJ builder with the Java builder. It will also remove the AspectJ runtime from the classpath.
AspectJ projects
AspectJ builder
Creating AspectJ elements
Creating a new AspectJ project
Converting a plug-in project to an AspectJ project
Creating a new aspect