db4o

Summary

• useful alternative to MySQL for small and embedded environments
• overcomes object-relational impedance mismatch problems
• object primary key is its OID (Object Identifier) which is auto-generated
• uses Query by Example (QBE) to select by comparison against a prototype object
• use of SODA (Simple Object Database Access) and native queries for efficiency
• copes with object refactoring e.g. adding/removing fields, primitive type conversion, class rename
• data is typically stored in files which can be encrypted using eXtended Tiny Encryption Algorithm (XTEA) or 3rd party hook-ins
• duplicate objects can be stored in the same table
• data is unaccessible from unsupported languages e.g. non Java or .NET code

Key Classes

• ObjectContainer - interface to db4o database
• ObjectSet - query resultset, list of items
• Db4o - factory class for database engine utilities
• ExtObjectContainer - volatile experimental methods

Key Methods

• Db4o.openFile(String fileName) - open the database
• objectContainer.set(Object obj) - INSERT the object into the database
• objectContainer.get(Object obj) - SELECT the matching objects from the database
• objectContainer.delete(Object obj) - DELETE the object from the database
• objectContainer.commit() - commit the transaction
• objectSet.next() - iterate through the result set
• objectContainer.ext().getID(Object obj) - get the OID for the object
• objectContainer.ext().getByID(long ID) - get the object by the ID
• Db4o.configure().objectClass("com.gth.org.uk.Person").rename("com.org.uk.Individual") - refactor class name
• Db4o.configure().activationDepth(int depth) - change the default activation depth to avoid infinite recursion

Alternative OODBMS

• InterSystems Cache
• Versant
• NeoDatis ODB

Source: The busy Java developer's guide to db4o