NoSQL DB (directed graphs) for Java/GWT?

I'm looking for no-sql directed graphs DB engine for GWT application. Requirements are:

1. SQLite-like simplicity.
2. Has a tool to explore/modify its structure.
3. Doesn't require being installed (or very easy to install without fixin开发者_开发问答g configuration files and other stuff).
4. No special requirements for performance.

What should I use?

Neo4j looks suitable. It is simple to use, has an eclipse-based tool for exploring the structure visually (although it is not perfect), and can be run embedded (no installation).