SAX2 Programming Guide


	Using the SAX2 API

The SAX2 API for XML parsers was originally developed for Java. Please be aware that there is no standard SAX2 API for C++, and that use of the Xerces-C++ SAX2 API does not guarantee client code compatibility with other C++ XML parsers.

The SAX2 API presents a callback based API to the parser. An application that uses SAX2 provides an instance of a handler class to the parser. When the parser detects XML constructs, it calls the methods of the handler class, passing them information about the construct that was detected. The most commonly used handler classes are ContentHandler which is called when XML constructs are recognized, and ErrorHandler which is called when an error occurs. The header files for the various SAX2 handler classes are in '<xerces-c2_5_0>/include/xercesc/sax2'

As a convenience, Xerces-C++ provides the class DefaultHandler, which is a single class which is publicly derived from all the Handler classes. DefaultHandler's default implementation of the handler callback methods is to do nothing. A convenient way to get started with Xerces-C++ is to derive your own handler class from DefaultHandler and override just those methods in HandlerBase which you are interested in customizing. This simple example shows how to create a handler which will print element names, and print fatal error messages. The source code for the sample applications show additional examples of how to write handler classes.

This is the header file MySAX2Handler.hpp:

#include <xercesc/sax2/DefaultHandler.hpp>

class MySAX2Handler : public DefaultHandler {
public:
    void startElement(
        const   XMLCh* const    uri,
        const   XMLCh* const    localname,
        const   XMLCh* const    qname,
        const   Attributes&     attrs
    );
    void fatalError(const SAXParseException&);
};

This is the implementation file MySAX2Handler.cpp:

#include "MySAX2Handler.hpp"
#include <iostream.h>

MySAX2Handler::MySAX2Handler()
{
}

MySAX2Handler::startElement(const   XMLCh* const    uri,
                            const   XMLCh* const    localname,
                            const   XMLCh* const    qname,
                            const   Attributes&     attrs)
{
    char* message = XMLString::transcode(name);
    cout << "I saw element: "<< message << endl;
    XMLString::release(&message);
}

MySAX2Handler::fatalError(const SAXParseException& exception)
{
    char* message = XMLString::transcode(exception.getMessage());
    cout << "Fatal Error: " << message
         << " at line: " << exception.getLineNumber()
         << endl;
}

The XMLCh and Attributes types are supplied by Xerces-C++ and are documented in the include files. Examples of their usage appear in the source code to the sample applications.


	SAX2XMLReader


	Constructing an XML Reader

In order to use Xerces-C++ to parse XML files, you will need to create an instance of the SAX2XMLReader class. The example below shows the code you need in order to create an instance of SAX2XMLReader. The ContentHandler and ErrorHandler instances required by the SAX API are provided using the DefaultHandler class supplied with Xerces-C++.

    #include <xercesc/sax2/SAX2XMLReader.hpp>
    #include <xercesc/sax2/XMLReaderFactory.hpp>
    #include <xercesc/sax2/DefaultHandler.hpp>
    #include <xercesc/util/XMLString.hpp>

    int main (int argc, char* args[]) {

        try {
            XMLPlatformUtils::Initialize();
        }
        catch (const XMLException& toCatch) {
            char* message = XMLString::transcode(toCatch.getMessage());
            cout << "Error during initialization! :\n"
            cout << "Exception message is: \n"
                 << message << "\n";
            XMLString::release(&message);
            return 1;
        }

        char* xmlFile = "x1.xml";
        SAX2XMLReader* parser = XMLReaderFactory::createXMLReader();
        parser->setFeature(XMLUni::fgSAX2CoreValidation, true)   // optional
        parser->setFeature(XMLUni::fgSAX2CoreNameSpaces, true)   // optional

        DefaultHandler* defaultHandler = new DefaultHandler();
        parser->setContentHandler(defaultHandler);
        parser->setErrorHandler(defaultHandler);

        try {
            parser->parse(xmlFile);
        }
        catch (const XMLException& toCatch) {
            char* message = XMLString::transcode(toCatch.getMessage());
            cout << "Exception message is: \n"
                 << message << "\n";
            XMLString::release(&message);
            return -1;
        }
        catch (const SAXParseException& toCatch) {
            char* message = XMLString::transcode(toCatch.getMessage());
            cout << "Exception message is: \n"
                 << message << "\n";
            XMLString::release(&message);
            return -1;
        }
        catch (...) {
            cout << "Unexpected Exception \n" ;
            return -1;
        }

        delete parser;
        delete defaultHandler;
        return 0;
    }


	Supported Features in SAX2XMLReader

The behavior of the SAX2XMLReader is dependant on the values of the following features. All of the features below can be set using the function SAX2XMLReader::setFeature(cons XMLCh* const, const bool). And can be queried using the function bool SAX2XMLReader::getFeature(const XMLCh* const).

None of these features can be modified in the middle of a parse, or an exception will be thrown.


	SAX2 Features

http://xml.org/sax/features/namespaces
true:	Perform Namespace processing.
false:	Do not perform Namespace processing.
default:	true
XMLUni Predefined Constant:	fgSAX2CoreNameSpaces
note:	If the validation feature is set to true, then the document must contain a grammar that supports the use of namespaces.
see:	http://xml.org/sax/features/namespace-prefixes
see:	http://xml.org/sax/features/validation

http://xml.org/sax/features/namespace-prefixes
true:	Report the original prefixed names and attributes used for Namespace declarations.
false:	Do not report attributes used for Namespace declarations, and optionally do not report original prefixed names.
default:	false
XMLUni Predefined Constant:	fgSAX2CoreNameSpacePrefixes

http://xml.org/sax/features/validation
true:	Report all validation errors.
false:	Do not report validation errors.
default:	true
XMLUni Predefined Constant:	fgSAX2CoreValidation
note:	If this feature is set to true, the document must specify a grammar. If this feature is set to false and document specifies a grammar, that grammar might be parsed but no validation of the document contents will be performed.
see:	http://apache.org/xml/features/validation/dynamic
see:	http://apache.org/xml/features/nonvalidating/load-external-dtd


	Xerces Features

http://apache.org/xml/features/validation/dynamic
true:	The parser will validate the document only if a grammar is specified. (http://xml.org/sax/features/validation must be true).
false:	Validation is determined by the state of the http://xml.org/sax/features/validation feature.
default:	false
XMLUni Predefined Constant:	fgXercesDynamic
see:	http://xml.org/sax/features/validation

http://apache.org/xml/features/validation/schema
true:	Enable the parser's schema support.
false:	Disable the parser's schema support.
default:	true
XMLUni Predefined Constant:	fgXercesSchema
note	If set to true, namespace processing must also be turned on.
see:	http://xml.org/sax/features/namespaces

http://apache.org/xml/features/validation/schema-full-checking
true:	Enable full schema constraint checking, including checking which may be time-consuming or memory intensive. Currently, particle unique attribution constraint checking and particle derivation restriction checking are controlled by this option.
false:	Disable full schema constraint checking.
default:	false
XMLUni Predefined Constant:	fgXercesSchemaFullChecking
note:	This feature checks the schema grammar itself for additional errors that are time-consuming or memory intensive. It does not affect the level of checking performed on document instances that use schema grammars.
see:	http://apache.org/xml/features/validation/schema

http://apache.org/xml/features/nonvalidating/load-external-dtd
true:	Load the external DTD.
false:	Ignore the external DTD completely.
default:	true
XMLUni Predefined Constant:	fgXercesLoadExternalDTD
note	This feature is ignored and DTD is always loaded when validation is on.
see:	http://xml.org/sax/features/validation

http://apache.org/xml/features/continue-after-fatal-error
true:	Attempt to continue parsing after a fatal error.
false:	Stops parse on first fatal error.
default:	false
XMLUni Predefined Constant:	fgXercesContinueAfterFatalError
note:	The behavior of the parser when this feature is set to true is undetermined! Therefore use this feature with extreme caution because the parser may get stuck in an infinite loop or worse.

http://apache.org/xml/features/validation-error-as-fatal
true:	The parser will treat validation error as fatal and will exit depends on the state of http://apache.org/xml/features/continue-after-fatal-error.
false:	The parser will report the error and continue processing.
default:	false
XMLUni Predefined Constant:	fgXercesValidationErrorAsFatal
note:	Setting this true does not mean the validation error will be printed with the word "Fatal Error". It is still printed as "Error", but the parser will exit if http://apache.org/xml/features/continue-after-fatal-error is set to false.
see:	http://apache.org/xml/features/continue-after-fatal-error

http://apache.org/xml/features/validation/use-cachedGrammarInParse
true:	Use cached grammar if it exists in the pool.
false:	Parse the schema grammar.
default:	false
XMLUni Predefined Constant:	fgXercesUseCachedGrammarInParse
note:	If http://apache.org/xml/features/validation/cache-grammarFromParse is enabled, this feature is set to true automatically. Any setting to this feature by the users is a no-op.
see:	http://apache.org/xml/features/validation/cache-grammarFromParse

http://apache.org/xml/features/validation/cache-grammarFromParse
true:	Cache the grammar in the pool for re-use in subsequent parses.
false:	Do not cache the grammar in the pool
default:	false
XMLUni Predefined Constant:	fgXercesCacheGrammarFromParse
note:	If set to true, the http://apache.org/xml/features/validation/use-cachedGrammarInParse is also set to true automatically.
see:	http://apache.org/xml/features/validation/use-cachedGrammarInParse

http://apache.org/xml/features/standard-uri-conformant
true:	Force standard uri conformance.
false:	Do not force standard uri conformance.
default:	false
XMLUni Predefined Constant:	fgXercesStandardUriConformant
note:	If set to true, malformed uri will be rejected and fatal error will be issued.

http://apache.org/xml/features/calculate-src-ofs
true:	Enable src offset calculation.
false:	Disable src offset calculation.
default:	false
XMLUni Predefined Constant:	fgXercesCalculateSrcOfs
note:	If set to true, the user can inquire about the current src offset within the input source. Setting it to false (default) improves the performance.


	Supported Properties in SAX2XMLReader

The behavior of the SAX2XMLReader is dependant on the values of the following properties. All of the properties below can be set using the function SAX2XMLReader::setProperty(const XMLCh* const, void*). It takes a void pointer as the property value. Application is required to initialize this void pointer to a correct type. Please check the column "Value Type" below to learn exactly what type of property value each property expects for processing. Passing a void pointer that was initialized with a wrong type will lead to unexpected result. If the same property is set more than once, the last one takes effect.

Property values can be queried using the function void* SAX2XMLReader::getProperty(const XMLCh* const). The parser owns the returned pointer, and the memory allocated for the returned pointer will be destroyed when the parser is deleted. To ensure accessibility of the returned information after the parser is deleted, callers need to copy and store the returned information somewhere else. Since the returned pointer is a generic void pointer, check the column "Value Type" below to learn exactly what type of object each property returns for replication.

None of these properties can be modified in the middle of a parse, or an exception will be thrown.


	Xerces Properties

http://apache.org/xml/properties/schema/external-schemaLocation
Description	The XML Schema Recommendation explicitly states that the inclusion of schemaLocation/ noNamespaceSchemaLocation attributes in the instance document is only a hint; it does not mandate that these attributes must be used to locate schemas. Similar situation happens to <import> element in schema documents. This property allows the user to specify a list of schemas to use. If the targetNamespace of a schema specified using this method matches the targetNamespace of a schema occurring in the instance document in schemaLocation attribute, or if the targetNamespace matches the namespace attribute of <import> element, the schema specified by the user using this property will be used (i.e., the schemaLocation attribute in the instance document or on the <import> element will be effectively ignored).
Value	The syntax is the same as for schemaLocation attributes in instance documents: e.g, "http://www.example.com file_name.xsd". The user can specify more than one XML Schema in the list.
Value Type	XMLCh*
XMLUni Predefined Constant:	fgXercesSchemaExternalSchemaLocation

http://apache.org/xml/properties/schema/external-noNamespaceSchemaLocation
Description	The XML Schema Recommendation explicitly states that the inclusion of schemaLocation/ noNamespaceSchemaLocation attributes in the instance document is only a hint; it does not mandate that these attributes must be used to locate schemas. This property allows the user to specify the no target namespace XML Schema Location externally. If specified, the instance document's noNamespaceSchemaLocation attribute will be effectively ignored.
Value	The syntax is the same as for the noNamespaceSchemaLocation attribute that may occur in an instance document: e.g."file_name.xsd".
Value Type	XMLCh*
XMLUni Predefined Constant:	fgXercesSchemaExternalNoNameSpaceSchemaLocation

http://apache.org/xml/properties/scannerName
Description	This property allows the user to specify the name of the XMLScanner to use for scanning XML documents. If not specified, the default scanner "IGXMLScanner" is used.
Value	The recognized scanner names are: 1."WFXMLScanner" - scanner that performs well-formedness checking only. 2. "DGXMLScanner" - scanner that handles XML documents with DTD grammar information. 3. "SGXMLScanner" - scanner that handles XML documents with XML schema grammar information. 4. "IGXMLScanner" - scanner that handles XML documents with DTD or/and XML schema grammar information. Users can use the predefined constants defined in XMLUni directly (fgWFXMLScanner, fgDGXMLScanner, fgSGXMLScanner, or fgIGXMLScanner) or a string that matches the value of one of those constants.
Value Type	XMLCh*
XMLUni Predefined Constant:	fgXercesScannerName
note:	See Use Specific Scanner for more programming details.

http://apache.org/xml/properties/security-manager
Description	Certain valid XML and XML Schema constructs can force a processor to consume more system resources than an application may wish. In fact, certain features could be exploited by malicious document writers to produce a denial-of-service attack. This property allows applications to impose limits on the amount of resources the processor will consume while processing these constructs.
Value	An instance of the SecurityManager class (see `xercesc/util/SecurityManager`). This class's documentation describes the particular limits that may be set. Note that, when instantiated, default values for limits that should be appropriate in most settings are provided. The default implementation is not thread-safe; if thread-safety is required, the application should extend this class, overriding methods appropriately. The parser will not adopt the SecurityManager instance; the application is responsible for deleting it when it is finished with it. If no SecurityManager instance has been provided to the parser (the default) then processing strictly conforming to the relevant specifications will be performed.
Value Type	SecurityManager*
XMLUni Predefined Constant:	fgXercesSecurityManager