Allow the application to resolve external entities.
The parser will call this method before opening any external entity except the top-level document entity. Such entities include the external DTD subset and external parameter entities referenced within the DTD (in either case, only if the parser reads external parameter entities), and external general entities referenced within the document element (if the parser reads external general entities). The application may request that the parser locate the entity itself, that it use an alternative URI, or that it use data provided by the application (as a character or byte input stream).
Application writers can use this method to redirect external system identifiers to secure and/or local URIs, to look up public identifiers in a catalogue, or to read an entity from a database or other input source (including, for example, a dialog box). Neither XML nor SAX specifies a preferred policy for using public or system IDs to resolve resources. However, SAX specifies how to interpret any InputSource returned by this method, and that if none is returned, then the system ID will be dereferenced as a URL.
If the system identifier is a URL, the SAX parser must resolve it fully before reporting it to the application.
@param publicId The public identifier of the external entitybeing referenced, or null if none was supplied.
@param systemId The system identifier of the external entitybeing referenced.
@return An InputSource object describing the new input source,or null to request that the parser open a regular URI connection to the system identifier.
@exception org.xml.sax.SAXException Any SAX exception, possiblywrapping another exception.
@exception java.io.IOException A Java-specific IO exception,possibly the result of creating a new InputStream or Reader for the InputSource.
@see org.xml.sax.InputSource