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The Simple API for XML (SAX) APIs

The basic outline of the SAX parsing APIs are shown at right. To start the process, an instance of the SAXParserFactory classed is used to generate an instance of the parser.

Figure 1 SAX APIs

The parser wraps a SAXReader object. When the parser's parse() method is invoked, the reader invokes one of several callback methods implemented in the application. Those methods are defined by the interfaces ContentHandler, ErrorHandler, DTDHandler, and EntityResolver.

Here is a summary of the key SAX APIs:

SAXParserFactory

A SAXParserFactory object creates an instance of the parser determined by the system property, javax.xml.parsers.SAXParserFactory.

SAXParser

The SAXParser interface defines several kinds of parse() methods. In general, you pass an XML data source and a DefaultHandler object to the parser, which processes the XML and invokes the appropriate methods in the handler object.

SAXReader

The SAXParser wraps a SAXReader. Typically, you don't care about that, but every once in a while you need to get hold of it using SAXParser's getXMLReader(), so you can configure it. It is the SAXReader which carries on the conversation with the SAX event handlers you define.

DefaultHandler

Not shown in the diagram, a DefaultHandler implements the ContentHandler, ErrorHandler, DTDHandler, and EntityResolver interfaces (with null methods), so you can override only the ones you're interested in.

ContentHandler

Methods like startDocument, endDocument, startElement, and endElement are invoked when an XML tag is recognized. This interface also defines methods characters and processingInstruction, which are invoked when the parser encounters the text in an XML element or an inline processing instruction, respectively.

ErrorHandler

Methods error, fatalError, and warning are invoked in response to various parsing errors. The default error handler throws an exception for fatal errors and ignores other errors (including validation errors). That's one reason you need to know something about the SAX parser, even if you are using the DOM. Sometimes, the application may be able to recover from a validation error. Other times, it may need to generate an exception. To ensure the correct handling, you'll need to supply your own error handler to the parser.

DTDHandler

Defines methods you will generally never be called upon to use. Used when processing a DTD to recognize and act on declarations for an unparsed entity.

EntityResolver

The resolveEntity method is invoked when the parser must identify data identified by a URI. In most cases, a URI is simply a URL, which specifies the location of a document, but in some cases the document may be identified by a URN--a public identifier, or name, that is unique in the Web space. The public identifier may be specified in addition to the URL. The EntityResolver can then use the public identifier instead of the URL to find the document, for example to access a local copy of the document if one exists.

A typical application implements most of the ContentHandler methods, at a minimum. Since the default implementations of the interfaces ignore all inputs except for fatal errors, a robust implementation may want to implement the ErrorHandler methods, as well.

The SAX Packages

The SAX parser is defined in the following packages listed in Table 1.

Table 1 SAX Packages

Package	Description
org.xml.sax	Defines the SAX interfaces. The name org.xml is the package prefix that was settled on by the group that defined the SAX API.
org.xml.sax.ext	Defines SAX extensions that are used when doing more sophisticated SAX processing, for example, to process a document type definitions (DTD) or to see the detailed syntax for a file.
org.xml.sax.helpers	Contains helper classes that make it easier to use SAX--for example, by defining a default handler that has null-methods for all of the interfaces, so you only need to override the ones you actually want to implement.
javax.xml.parsers	Defines the SAXParserFactory class which returns the SAXParser. Also defines exception classes for reporting errors.

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