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(Lianja) Efficiently Process a Huge XML File
Demonstrates a technique for processing a huge XML file (can be any size, even many gigabytes).
Note: This example requires Chilkat v220.127.116.11 or greater.
// This example shows a way to efficiently process a gigantic XML file -- one that may be too large // to fit in memory. // // Two types of XML parsers exist: DOM parsers and SAX parsers. // A DOM parser is a Document Object Model parser, where the entire XML is loaded into memory // and the application has the luxury of interacting with the XML in a convenient, random-access // way. The Chilkat Xml class is a DOM parser. Because the entire XML is loaded into memory, // huge XML files (on the order of gigabytes) are usually not loadable for memory constraints. // A SAX parser is such that the XML file is parsed as an input stream. No DOM exists. // Using a SAX parser is generally less palatable than using a DOM parser, for many reasons. // // The technique described here is a hybrid. It streams the XML file as unstructured text // to extract fragments that are individually treated as separate XML documents loaded into // the Chilkat Xml parser. // // For example, imagine your XML file is several GBs in size, but has a relatively simple structure, such as: // // <Transactions> // <Transaction id="1"> // ... // </Transaction> // <Transaction id="2"> // ... // </Transaction> // <Transaction id="3"> // ... // </Transaction> // ... // </Transactions> // In the following code, each <Transaction ...> ... </Transaction> // is extracted and loaded separately into an Xml object, where it can be manipulated // independently. The entire XML file is never entirely loaded into memory. loFac = createobject("CkFileAccess") llSuccess = loFac.OpenForRead("qa_data/xml/transactions.xml") if (llSuccess = .F.) then ? loFac.LastErrorText release loFac return endif loXml = createobject("CkXml") loSb = createobject("CkStringBuilder") llFirstIteration = .T. lnRetval = 1 lnNumTransactions = 0 // The begin marker is "XML tag aware". If the begin marker begins with "<" // and ends with ">", then it is assumed to be an XML tag and it will also match // substrings where the ">" can be a whitespace char. lcBeginMarker = "<Transaction>" lcEndMarker = "</Transaction>" do while lnRetval = 1 loSb.Clear() // The retval can have the following values: // 0: No more fragments exist. // 1: Captured the next fragment. The text from beginMarker to endMarker, including the markers, are returned in sb. // -1: Error. lnRetval = loFac.ReadNextFragment(llFirstIteration,lcBeginMarker,lcEndMarker,"utf-8",loSb) llFirstIteration = .F. if (lnRetval = 1) then lnNumTransactions = lnNumTransactions + 1 llSuccess = loXml.LoadSb(loSb,.T.) // Your application may now do what it needs with this particular XML fragment... endif enddo if (lnRetval < 0) then ? loFac.LastErrorText endif ? "numTransactions: " + str(lnNumTransactions) release loFac release loXml release loSb
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