Welcome to Neo4j. For a Java or JVM-language developer, Neo4j offers a rich set of integration possibilities. Make sure to have looked at our information on graph databases and Neo4j in general. You should also check out our graph query language Cypher which makes it very easy to get going regardless of the development platform.
Neo4j comes in two flavors. The standalone Neo4j-Server can be installed on any machine and then accessed via its REST API. You can then use whatever REST-library you choose for any JVM language. The dedicated Neo4j drivers go beyond that by offering comprehensive APIs for integrating with graph based applications.
But you can also run Neo4j embedded in your JVM process, much like HSQL or Derby. This is great for unit testing, but also for high performance / no-network setups. If you use Neo4j embedded you can use the Neo4j Core-API directly. Besides an object oriented approach to the graph database, working with Nodes, Relationships and Paths, it also offers highly customizable high-speed traversal- and graph-algorithm implementations. You can also choose from any useful wrapping drivers that exist either for specific programming languages or that add interesting functionality.
One example is Spring Data Neo4j which integrates tightly with the SpringFramework and offers Object-Graph mapping on top of Neo4j. Just to tease your interest - there are hundreds of open source Neo4j-related ecosystem projects, example applications and tutorials that can spark new ideas and possibilities.
As a JVM developer you are also on the bright side when it comes to pouring large amounts of data into Neo4j. With its non-transactional batch-insertion facilities it can ingest millions of nodes and relationships in just seconds.