Apache ObJectRelationalBridge (OJB) is an Object/Relational mapping tool that allows transparent persistence for Java Objects against relational databases. It was released on April 6, 2005.

As of January 16, 2011 Apache ObJectRelationalBridge has been retired.

OJB is an open source project. It is lightweight and easy to use, requiring simply configure two files to implement a persistence layer. It is easy to integrate into an existing application because it does not generate code. It allows the use of different patterns of persistence: owner (PersistenceBroker API), JDO and Object Data Management Group (ODMG).

OJB uses an XML based Object/Relational mapping. The mapping resides in a dynamic MetaData layer, which can be manipulated at runtime through a simple Meta-Object-Protocol (MOP) to change the behaviour of the persistence kernel.

At least two files are required to configure OJB: OJB.properties and repository.xml

For mapping a 1-1 relationship, for example, you have two tables: person and account. In this case, a person has an account and vice versa.