This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
If you use RMI you do not have to do anything special for sending a HashMap across from one server to another.
However, the synchronization is a little tricky as the instance of HashMap will be different for the two JVMs you can not have a single lock monitor.
If you explain in detail by what do you mean by " keep it synchronized. ", we will be able to help more.