If a SAX application needs to implement customized handling for external entities, it must implement this interface and register an instance with the SAX parser using the parser’s setEntityResolver method.
The parser then allows the application to intercept any external entities (including the external DTD subset and external parameter entities, if any) before including them.
Many SAX applications will not need to implement this interface, but it will be especially useful for applications that build XML documents from databases or other specialized input sources, or for applications that use URI types other than URLs.