DTDHandler

DTDHandler is an interface in the Java API for XML Processing (JAXP) that provides a mechanism for handling DTD (Document Type Definition) related events during XML parsing. When an XML parser encounters a DTD, it can notify an application that implements the DTDHandler interface about certain events. These events typically include the declaration of an unparsed entity, a notation declaration, and attribute default declarations.

The DTDHandler interface defines three methods: unparsedEntityDecl, notationDecl, and attributeDecl. The unparsedEntityDecl method is called when the parser encounters an unparsed entity declaration in the DTD. An unparsed entity is typically an external resource such as an image or a script that is not intended to be parsed as XML. The notationDecl method is invoked when a notation declaration is found. Notations are used to define the data type of an unparsed entity. Finally, the attributeDecl method is called when an attribute default declaration is encountered within the DTD. This declaration specifies a default value for an attribute if it is not explicitly provided in the XML document.

Applications can implement DTDHandler to gain awareness of these DTD constructs and potentially take specific actions based on them. For instance, an application might log information about notations or unparsed entities, or it might use attribute default declarations to populate missing attribute values. To utilize DTDHandler, an application typically obtains an XMLReader, sets the DTDHandler on the reader, and then starts the parsing process. The parser will then call the appropriate methods of the DTDHandler implementation as it encounters corresponding declarations in the DTD.