This is a pure-Java implementation of Berkeley DB. From the

WWW: http://www.sleepycat.com/products/je.shtml

"Berkeley DB JE was designed from the ground up in Java. It takes full
 advantage of the Java environment. The Berkeley DB JE API provides a
 Java Collections-style interface, as well as a programmatic interface
 similar to the Berkeley DB API.

 Berkeley DB JE is different from all other Java databases available
 today. Berkeley DB JE is not a relational engine built in Java. It
 is a Berkeley DB-style embedded store, with an interface designed
 for programmers, not DBAs. Berkeley DB JE's architecture employs a
 log-based, no-overwrite storage system, enabling high concurrency and
 speed while providing ACID transactions and record-level locking.
 Berkeley DB JE efficiently caches most commonly used data in memory,
 without exceeding application-specified limits. In this way Berkeley
 DB JE works with an application to use available JVM resources while
 providing access to very large data sets.

 The Berkeley DB JE architecture provides an underlying storage layer
 for any Java application requiring high performance, transactional
 integrity and recoverability."