Loading a document when I Don't know what's in it

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Problem

Depending on how you set your system up, it is possible that you receive an XML document, not knowing what it contains. For Example if your writing a service that can process a number of different requests. Each different request may be represented by a different XML element. So the service handling the requests would not know what they contained until its opened them.

Simple Example

<xsd:element name = "StatementRequest"> ... <xsd:element name = "BalanceRequest"> ...

Sample Document

<?xml version = "1.0" encoding = "UTF-8"?> <StatementRequest> ...

Resolution

Load the Document Dynamically

Because we don't know the type of document we are dealing with, we have to load it and have a 'peek' at it. We can then create the appropriate type of object, and load the data into it. When we re-load the data we take care not to re-parse the document (saving processing and memory overhead).

C++

Sample Document try { // load the document using the XML parser that comes with Liquid XML LtXmlLib20::CXmlDocument XmlDoc; XmlDoc.Load(strFullPath.c_str()); // Now use the class factory to create the object using the root element in the docuemnt as a guide LtXmlLib20::CXmlObjectBasePtr spObj = tns::CClassFactory::IDocumentCreateObject(XmlDoc.GetDocumentElement()).Ptr(); // Now read the document into the new object spObj->FromXmlFile(strFullPath.c_str()); // Check the actual type of the root element std::tstring typeName = XmlDoc.GetDocumentElement()->GetAttribute(_T("type"), _T("http://www.w3.org/2001/XMLSchema-instance")); if (typeName == _T("RequestTradeConfirmation")) { // cast the spObj to specific type tns::CRequestTradeConfirmationPtr spRequestTradeConfirmation = dynamic_cast<tns::CRequestTradeConfirmation*>(spObj.Ptr()); std::tstring strXML = spRequestTradeConfirmation->ToXml(); _tprintf(strXML.c_str()); } } catch (CLtException& ex) { _tprintf(_T("FAILED - to validate %s\nError %s\n"), lpctFilename, ex.GetFullMessage().c_str()); }

C++

Sample Document

try { // load the document using the XML parser that comes with Liquid XML LtXmlLib20::CXmlDocument XmlDoc; XmlDoc.Load(strFullPath.c_str()); // Now use the class factory to create the object using the root element in the docuemnt as a guide LtXmlLib20::CXmlObjectBasePtr spObj = tns::CClassFactory::IDocumentCreateObject(XmlDoc.GetDocumentElement()).Ptr(); // Now read the document into the new object spObj->FromXmlFile(strFullPath.c_str()); // Check the actual type of the root element std::tstring typeName = XmlDoc.GetDocumentElement()->GetAttribute(_T("type"), _T("http://www.w3.org/2001/XMLSchema-instance")); if (typeName == _T("RequestTradeConfirmation")) { // cast the spObj to specific type tns::CRequestTradeConfirmationPtr spRequestTradeConfirmation = dynamic_cast<tns::CRequestTradeConfirmation*>(spObj.Ptr()); std::tstring strXML = spRequestTradeConfirmation->ToXml(); _tprintf(strXML.c_str()); } } catch (CLtException& ex) { _tprintf(_T("FAILED - to validate %s\nError %s\n"), lpctFilename, ex.GetFullMessage().c_str()); }

C#

Sample Document try { // use the standard .Net XML document to load the XML System.Xml.XmlDocument xmlDoc = new System.Xml.XmlDocument(); xmlDoc.Load(filename); // Now examine the root element and create the correct type of object if (xmlDoc.DocumentElement.LocalName == "StatementRequest") { StatementRequest stmtRq = new StatementRequest(); stmtRq.FromXmlElement(xmlDoc.DocumentElement); MyStatementRequestHandler(stmtRq); } else if (xmlDoc.DocumentElement.LocalName == "BalanceRequest") { BalanceRequest balRq = new BalanceRequest(); balRq.FromXmlElement(xmlDoc.DocumentElement); MyBalanceRequestHandler(balRq); } else { // Error Unknown XML document } } catch (Exception e) { // Note : exceptions are likely to contain inner exceptions // that provide further detail about the error. while (e != null) { Console.WriteLine("Error - " + e.Message); e = e.InnerException; } }

Sample Document

try { // use the standard .Net XML document to load the XML System.Xml.XmlDocument xmlDoc = new System.Xml.XmlDocument(); xmlDoc.Load(filename); // Now examine the root element and create the correct type of object if (xmlDoc.DocumentElement.LocalName == "StatementRequest") { StatementRequest stmtRq = new StatementRequest(); stmtRq.FromXmlElement(xmlDoc.DocumentElement); MyStatementRequestHandler(stmtRq); } else if (xmlDoc.DocumentElement.LocalName == "BalanceRequest") { BalanceRequest balRq = new BalanceRequest(); balRq.FromXmlElement(xmlDoc.DocumentElement); MyBalanceRequestHandler(balRq); } else { // Error Unknown XML document } } catch (Exception e) { // Note : exceptions are likely to contain inner exceptions // that provide further detail about the error. while (e != null) { Console.WriteLine("Error - " + e.Message); e = e.InnerException; } }

Java

Sample Document try { // load the document using the XML parser that comes with Liquid XML com.liquid_technologies.ltxmllib20.dom.XmlDocument parser = new com.liquid_technologies.ltxmllib20.dom.XmlDocument(); parser.parse(filename); // Now examine the root element and create the correct type of object if (xmlDoc.getDocumentElement().getLocalName().equals("StatementRequest")) { StatementRequest stmtRq = new StatementRequest(); stmtRq.fromXmlElement(xmlDoc.getDocumentElement()); MyStatementRequestHandler(stmtRq); } else if (xmlDoc.getDocumentElement().getLocalName().equals("BalanceRequest")) { BalanceRequest balRq = new BalanceRequest(); balRq.fromXmlElement(xmlDoc.getDocumentElement()); MyBalanceRequestHandler(balRq); } else { // Error Unknown XML document } } catch (Throwable e) { // Note : exceptions are likely to contain inner exceptions // that provide further detail about the error. System.out.println("OK - Successfully failed to load an invalid file " + filename); while (e != null) { System.out.println("Error " + e.getMessage()); e = e.getCause(); } }

Java

Sample Document

try { // load the document using the XML parser that comes with Liquid XML com.liquid_technologies.ltxmllib20.dom.XmlDocument parser = new com.liquid_technologies.ltxmllib20.dom.XmlDocument(); parser.parse(filename); // Now examine the root element and create the correct type of object if (xmlDoc.getDocumentElement().getLocalName().equals("StatementRequest")) { StatementRequest stmtRq = new StatementRequest(); stmtRq.fromXmlElement(xmlDoc.getDocumentElement()); MyStatementRequestHandler(stmtRq); } else if (xmlDoc.getDocumentElement().getLocalName().equals("BalanceRequest")) { BalanceRequest balRq = new BalanceRequest(); balRq.fromXmlElement(xmlDoc.getDocumentElement()); MyBalanceRequestHandler(balRq); } else { // Error Unknown XML document } } catch (Throwable e) { // Note : exceptions are likely to contain inner exceptions // that provide further detail about the error. System.out.println("OK - Successfully failed to load an invalid file " + filename); while (e != null) { System.out.println("Error " + e.getMessage()); e = e.getCause(); } }

Visual Basic

Sample Document Dim elm As IDocument Dim oCF As New Sample.ClassFactory Dim oMsXML As New MSXML2.DOMDocument ' Because the initial element is abstract ' we need to create it using the ClassFactory method IRequestTypeCreateObject oMsXML.Load strFilename ' Now examine the root element and create the correct type of object if xmlDoc.DocumentElement.LocalName = "StatementRequest" Then Dim oStmtRq As new StatementRequest stmtRq.FromXmlFile strFilename MyStatementRequestHandler(stmtRq); ElseIf xmlDoc.DocumentElement.LocalName = "BalanceRequest" Then Dim oBalRq As new BalanceRequest oBalRq.FromXmlFile strFilename MyBalanceRequestHandler oBalRq Else ' Error Unknown XML document End If

Visual Basic

Sample Document

Dim elm As IDocument Dim oCF As New Sample.ClassFactory Dim oMsXML As New MSXML2.DOMDocument ' Because the initial element is abstract ' we need to create it using the ClassFactory method IRequestTypeCreateObject oMsXML.Load strFilename ' Now examine the root element and create the correct type of object if xmlDoc.DocumentElement.LocalName = "StatementRequest" Then Dim oStmtRq As new StatementRequest stmtRq.FromXmlFile strFilename MyStatementRequestHandler(stmtRq); ElseIf xmlDoc.DocumentElement.LocalName = "BalanceRequest" Then Dim oBalRq As new BalanceRequest oBalRq.FromXmlFile strFilename MyBalanceRequestHandler oBalRq Else ' Error Unknown XML document End If

Notes

Description	Value
Article Created	6/2/2006
Versions	Liquid XML 2005 (4.2.0) and greater

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