From the webpage:
JDBM is a transactional persistence engine for Java. It aims to be for Java what GDBM is for other languages (C/C++, Python, Perl, etc.): a fast, simple persistence engine. You can use it to store a mix of objects and BLOBs, and all updates are done in a transactionally safe manner. JDBM also provides scalable data structures, such as HTree and B+Tree, to support persistence of large object collections.
This came up in a discussion of data structures very close to the metal and their influence on “accepted” operating characteristics. Would not hurt to run down links and information on GDBM as well. Something for later this week.