I just wanted to add my existing Java project to the current java web project and here is the simple step actually [of course from Eclipse 
]
1. Right click on the web project and select Properties
2. Select J2EE Module Dependencies
3. Here either you can select the project which resides in the eclipse workspace or you can import additional external jars.
4. Apply OK
5. Right click web project and select Build Path -> configure
6. On the projects tab, select the project tab
7. Click Add and select the project that you would like to add
8. OK.
You are done! It is advisable if you clean your project and build a new one
To to do this:
Go to project -> clean and select your project
Select start a build immediately
Select build only the selected projects
After this reboot your Application Server – and enjoy with your imported project.