JRobin is a 100% pure java implementation of RRDTool's functionality. It follows the same logic and uses the same data sources, archive types and definitions as RRDTool does. JRobin supports all standard operations on Round Robin Database (RRD) files: CREATE, UPDATE, FETCH, LAST, DUMP, XPORT and GRAPH. JRobin's API is made for those who are familiar with RRDTool's concepts and logic, but prefer to work with pure java. If you provide the same data to RRDTool and JRobin, you will get exactly the same results and graphs. JRobin is made from the scratch and it uses very limited portions of RRDTool's original source code.
Details of JRobin can be found here http://www.jrobin.org/index.php/Main_Page