GraphHopper 
Java road routing engine
Purpose
Solving shortest path (related) problems is the main goal. GraphHopper is a routing engine which makes implementing shortest path problems in Java easier and more efficient (faster, less memory, etc) than a naive implementation. GraphHopper is tuned for road networks at the moment but can be useful for public transport problems in the future as well.
Features
- 100% Java and 100% Open Source (Apache 2 License)
- Memory efficient
- A small library (~3MB)
- Works on the desktop, from the web and even offline on Android
- Well tested