SAX Programming Guide


	Using the SAX API

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

The SAX API presents a callback based API to the parser. An application that uses SAX 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 DocumentHandler which is called when XML constructs are recognized, and ErrorHandler which is called when an error occurs. The header files for the various SAX handler classes are in the xercesc/sax/ directory.

As a convenience, Xerces-C++ provides HandlerBase, a single class which is publicly derived from all the Handler classes. HandlerBase'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 HandlerBase 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 MySAXHandler.hpp:


	#include <xercesc/sax/HandlerBase.hpp> class MySAXHandler : public HandlerBase { public: void startElement(const XMLCh* const, AttributeList&); void fatalError(const SAXParseException&); };

This is the implementation file MySAXHandler.cpp:

#include "MySAXHandler.hpp"
#include <iostream>

using namespace std;

MySAXHandler::MySAXHandler()
{
}

void MySAXHandler::startElement(const XMLCh* const name,
                           AttributeList& attributes)
{
    char* message = XMLString::transcode(name);
    cout << "I saw element: "<< message << endl;
    XMLString::release(&message);
}

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

The XMLCh and AttributeList types are supplied by Xerces-C++ and are documented in the API reference. Examples of their usage appear in the source code for the sample applications.


	SAXParser


	Constructing a SAXParser

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

    #include <xercesc/parsers/SAXParser.hpp>
    #include <xercesc/sax/HandlerBase.hpp>
    #include <xercesc/util/XMLString.hpp>

    #include <iostream>

    using namespace std;
    using namespace xercesc;

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

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

        char* xmlFile = "x1.xml";
        SAXParser* parser = new SAXParser();
        parser->setDoValidation(true);
        parser->setDoNamespaces(true);    // optional

        DocumentHandler* docHandler = new HandlerBase();
        ErrorHandler* errHandler = (ErrorHandler*) docHandler;
        parser->setDocumentHandler(docHandler);
        parser->setErrorHandler(errHandler);

        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 docHandler;
        return 0;
    }


	SAXParser Supported Features

The behavior of the SAXParser is dependant on the values of the following features. All of the features below are set using the "setter" methods (e.g. setDoNamespaces), and are queried using the corresponding "getter" methods (e.g. getDoNamespaces). The following only gives you a quick summary of supported features. Please refer to API Documentation for complete detail.

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

void setDoNamespaces(const bool)
true:	Perform Namespace processing.
false:	Do not perform Namespace processing.
default:	false
note:	If the validation scheme is set to Val_Always or Val_Auto, then the document must contain a grammar that supports the use of namespaces.
see:	setValidationScheme

void setValidationScheme(const ValSchemes)
Val_Auto:	The parser will report validation errors only if a grammar is specified.
Val_Always:	The parser will always report validation errors.
Val_Never:	Do not report validation errors.
default:	Val_Never
note:	If set to Val_Always, the document must specify a grammar. If this feature is set to Val_Never and document specifies a grammar, that grammar might be parsed but no validation of the document contents will be performed.
see:	setLoadExternalDTD

void setDoSchema(const bool)
true:	Enable the parser's schema support.
false:	Disable the parser's schema support.
default:	false
note	If set to true, namespace processing must also be turned on.
see:	setDoNamespaces

void setValidationSchemaFullChecking(const bool)
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
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:	setDoSchema

void setLoadSchema(const bool)
true:	Load the schema.
false:	Don't load the schema if it wasn't found in the grammar pool.
default:	true
note:	This feature is ignored and no schemas are loaded if schema processing is disabled.
see:	setDoSchema

void setLoadExternalDTD(const bool)
true:	Load the External DTD.
false:	Ignore the external DTD completely.
default:	true
note	This feature is ignored and DTD is always loaded if the validation scheme is set to Val_Always or Val_Auto.
see:	setValidationScheme

void setExitOnFirstFatalError(const bool)
true:	Stops parse on first fatal error.
false:	Attempt to continue parsing after a fatal error.
default:	true
note:	The behavior of the parser when this feature is set to false is undetermined! Therefore use this feature with extreme caution because the parser may get stuck in an infinite loop or worse.

void setValidationConstraintFatal(const bool)
true:	The parser will treat validation error as fatal and will exit depends on the state of setExitOnFirstFatalError.
false:	The parser will report the error and continue processing.
default:	false
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 setExitOnFirstFatalError is set to true.
see:	setExitOnFirstFatalError

void useCachedGrammarInParse(const bool)
true:	Use cached grammar if it exists in the pool.
false:	Parse the schema grammar.
default:	false
note:	The getter function for this method is called isUsingCachedGrammarInParse.
note:	If the grammar caching option is enabled, this option is set to true automatically and any setting to this option by the user is a no-op.
see:	cacheGrammarFromParse

void cacheGrammarFromParse(const bool)
true:	Cache the grammar in the pool for re-use in subsequent parses.
false:	Do not cache the grammar in the pool
default:	false
note:	The getter function for this method is called isCachingGrammarFromParse
note:	If set to true, the useCachedGrammarInParse is also set to true automatically.
see:	useCachedGrammarInParse

void setStandardUriConformant(const bool)
true:	Force standard uri conformance.
false:	Do not force standard uri conformance.
default:	false
note:	If set to true, malformed uri will be rejected and fatal error will be issued.

void setCalculateSrcOfs(const bool)
true:	Enable src offset calculation.
false:	Disable src offset calculation.
default:	false
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.

void setIdentityConstraintChecking(const bool);
true:	Enable identity constraint checking.
false:	Disable identity constraint checking.
default:	true

void setGenerateSyntheticAnnotations(const bool);
true:	Enable generation of synthetic annotations. A synthetic annotation will be generated when a schema component has non-schema attributes but no child annotation.
false:	Disable generation of synthetic annotations.
default:	false

setValidateAnnotation
true:	Enable validation of annotations.
false:	Disable validation of annotations.
default:	false
note:	Each annotation is validated independently.

setIgnoreAnnotations
true:	Do not generate XSAnnotations when traversing a schema.
false:	Generate XSAnnotations when traversing a schema.
default:	false

setDisableDefaultEntityResolution
true:	The parser will not attempt to resolve the entity when the resolveEntity method returns NULL.
false:	The parser will attempt to resolve the entity when the resolveEntity method returns NULL.
default:	false

setSkipDTDValidation
true:	When schema validation is on the parser will ignore the DTD, except for entities.
false:	The parser will not ignore DTDs when validating.
default:	false
see:	DoSchema

setIgnoreCachedDTD
true:	Ignore a cached DTD when an XML document contains both an internal and external DTD, and the use cached grammar from parse option is enabled. Currently, we do not allow using cached DTD grammar when an internal subset is present in the document. This option will only affect the behavior of the parser when an internal and external DTD both exist in a document (i.e. no effect if document has no internal subset).
false:	Don't ignore cached DTD.
default:	false
see:	useCachedGrammarInParse

setHandleMultipleImports
true:	During schema validation allow multiple schemas with the same namespace to be imported.
false:	Don't import multiple schemas with the same namespace.
default:	false

*void setExternalSchemaLocation(const XMLCh const)**
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*

*void setExternalNoNamespaceSchemaLocation(const XMLCh const)**
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*

*void useScanner(const XMLCh const)**
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*
note:	See Use Specific Scanner for more programming details.

*setSecurityManager(Security Manager const)**
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*

setLowWaterMark(XMLSize_t)
Description	If the number of available bytes in the raw buffer is less than the low water mark the parser will attempt to read more data before continuing parsing. By default the value for this parameter is 100 bytes. You may want to set this parameter to 0 if you would like the parser to parse the available data immediately without potentially blocking while waiting for more date.
Value	New low water mark.
Value Type	XMLSize_t

setInputBufferSize(const size_t bufferSize)
Description	Set maximum input buffer size. This method allows users to limit the size of buffers used in parsing XML character data. The effect of setting this size is to limit the size of a ContentHandler::characters() call. The parser's default input buffer size is 1 megabyte.
Value	The maximum input buffer size
Value Type	XMLCh*