This documents track potential backward compatibility issues for Joomla 3.0 and Joomla Platform 12.1 which is included in Joomla 3.0. Listed are issues which potentially break extensions. Please help making this document complete.
The base of this comparison is Joomla 2.5/Platform 11.4.
Backwards compatibility issues were also tracked for Joomla 2.5: Potential backward compatibility issues in Joomla 2.5 and Joomla Platform 11.4
The platform removed a larger number of deprecated classes and methods. For most extensions written for Joomla 2.5 the required changes should be reasonably small. Note that there are still deprecated classes in the platform that will be removed in future versions, developers should stop using them where possible.
All classes listed here should now be autoloadable. Explicit requires and includes have to be modified or removed.
JDatabase had been renamed JDatabaseDriver. JDatabase is still around (but deprecated) and is the base of JDatabaseDriver, this way type hints and instance of checks keep working.
Due to lazy-connecting a connection is only opened when running a query or explicitly calling JDatabseDriver::connected(). Beware of that when checking JDatabaseDriver::connected().