The DBAccessBase class is being deprecated and removed. Instead of extending it, inject a load balancer (factory) and optionally a wiki into the constructor.
Original description:
DBAccessBase is difficult to use with dependency injection
DBAccessBase is a base class for objects that need access to other wiki’s databases. Its constructor calls MediaWikiServices::getInstance()->getDBLoadBalancerFactory(), which means that this class, and any class extending it, is impossible to use in a unit test (where the service wiring is not set up). The lb factory should instead be injected into the constructor (optionally falling back to MediaWikiServices).
An added complication is that, although DBAccessBase is only extended in Wikibase, the class itself is defined in MediaWiki, and marked @stable to extend.