The Java Engine for Testing (JET) is a system for running distributed automated tests. It has been used by Sun for testing the High Availability DataBase (HADB), JavaDB, PostgreSQL, Memcached, and the MySQL Cluster Manager. JET should be a good way to write your tests if the product you want to test is distributed or has a client/server architecture, if it is natural for you to write the tests in Java, and if you want to invest time in developing tests and abstractions for later reuse.
CODERU is a Java code quality tool to enforce good design in respect to package dependencies. Its rules rely on reserved package names, and the allowed dependency rules between them are expressed in a general way. Unlike other tools that force you to define allowed or disallowed individual package dependencies, CODERU is based on a fixed set of general rules. The dependencies between packages need not be defined explicitly. These general dependency rules are based on common design practices which are reflected in preserved package names: "api", "common", "impl", and "factory". It includes a tool for checking your code for rule conformance.
Zynaptic Reaction is a flexible asynchronous programming framework for Java which may be used to implement complex event-driven applications. It is heavily influenced by the Twisted programming framework developed by TwistedMatrix Labs for the Python programming language. The focus of the Reaction library is on the concurrency and callback model and as such it is application neutral. It can be used to manage lots of concurrent I/O or to farm out compute intensive tasks to multicore processors. As well as being usable as a basic Java library, Reaction can also run as an independent OSGi service and integrate into any GUI framework you choose.
SimpleDBM is a multi-threaded transactional database engine. It has a very small footprint and can be embedded in the address space of an application, or used in network client server mode. It provides a simple programming API that can be learned very quickly. SimpleDBM supports ACID transactions, steal/no-force buffer management, lock-based concurrency, row level locks, b-tree indexes, tables, write ahead log, crash recovery, and deadlock detection.