ObjectDB is a powerful yet compact Object Database Management System.
EditByBSEditor: ObjectDB is a powerful yet Compact Object Database Management System (ODBMS), written entirely in Java, and compliant with the new Java Data Objects (JDO) standard developed by Sun (JSR12). ObjectDB is designed to efficiently handle databases of various sizes, ranging from a few KBs to hundreds of GBs. Among its features are small footprint, high performance, rapid development, easy deployment and flexible architecture (1-Tier, 2-Tier or N-Tier).

ObjectDB Architecture
ObjectDB for Java/JDO can operate in two modes: embedded database mode and client server mode.
Client-Server Mode

In client-server mode, an ObjectDB server is running on a separate JVM (in a separate process). Applications running on different JVMs communicate with the database server using TCP/IP. Many client processes (running on the same machine or on different machines) can safely access the database simultaneously, because the database server manages an automatic Lock mechanism.
Embedded Database Mode

In embedded mode, the application uses ObjectDB as a class library, so no server process is needed in another JVM. The database file is accessed directly by the applications process, using JDO and ObjectDB jar files that are added to the applications classpath. In this mode, the database file is locked for a single process, and multi user support may be achieved only by multithreading (several threads) in a single process.
Client-Server Mode vs. Embedded Database Mode

The embedded database mode is easier to use and FASTER because no server has to run in the background and no TCP/IP communication is needed. Adding JDO and ObjectDB jar files to the classpath is the only requirement. Client-Server mode, however, is useful for accessing remote databases and for database access of several processes simultaneously. Moving from one mode to the other can be done simply by changing a single String (the connection URL), because the JDO API is the same for both modes. you can free download ObjectDB for Java/JDO - Free Edition 1.00 now.