Java：从xml文件中读取LAST条目

Question

So I have a project where I need to pull an xml file that is updated every 5 minutes. However I'm designing my program to pull this file every time it updates.

The data structure of the xml file is like this...

<m:REPORT_DATA>
    <m:DATA_ITEM>ENC</m:DATA_ITEM>
    <m:RESOURCE_NAME>DLAP</m:RESOURCE_NAME>
    <m:OPR_DATE>2012-06-02</m:OPR_DATE>
    <m:INTERVAL_NUM>1</m:INTERVAL_NUM>
    <m:VALUE>16.77734</m:VALUE>
</m:REPORT_DATA>
<m:REPORT_DATA>
    <m:DATA_ITEM>ENC</m:DATA_ITEM>
    <m:RESOURCE_NAME>DLAP</m:RESOURCE_NAME>
    <m:DATE>2012-06-02</m:OPR_DATE>
    <m:INTERVAL_NUM>2</m:INTERVAL_NUM>
    <m:VALUE>16.77739</m:VALUE>
</m:REPORT_DATA>
....

Assuming that I pull it for the 200th time that day, how would I grab just the last value

"<m:VALUE>16.77739</m:VALUE>"

And get that value for my database?

I'm torn about using Sax, Xpath, or DOM. Some help would be amazing.

Answer 1

If you had a root, lets say <m:REPORTS> , finding the last VALUE using XPath would be rather simple:

    XPathFactory f = XPathFactory.newInstance() ;
    XPath x = f.newXPath() ;
    try {
        InputSource source = new InputSource(new FileInputStream("logfile.xml")) ;
        XPathExpression expr = x.compile("//REPORT_DATA[DATA_ITEM='ENC'][last()]/VALUE/text()") ;
        String s = expr.evaluate(source) ;
        System.out.println("Last value: " + s ) ;
    }
    catch(Throwable t) {
        System.err.println("Error: " + t) ;
    }

Answer 2

This isn't a well formed xml, You can use xpath to find last node, for example //REPORT_DATA[position() = last()] returns last REPORT_DATA node and for reading xml using xpath see How to read XML using XPath in Java

//REPORT_DATA[last()]/DATA_ITEM[text()="ENC"]

and this returns node that it's DATA_ITEM equals to "ENC"

or //REPORT_DATA[last()]/VALUE[text()="ENC"]

Answer 3

This is not an "XML File" in the sense that it is not well-formed, since it has no root element (or it has multiple root elements). As such it cannot be loaded directly by an XML library, so you cannot use DOM, XPath, or XSLT.

You are better off using some simple pattern matching to detect the start of each segment, find the last segment, and then load only that segment into a DOM for extraction.

Answer 4

Use SAX.

With either xpath or DOM, you have to build a DOM which is slow and memory expensive, especially for 1 lookup.

SAX is faster, but is going to require you to keep track of your place and state, which in your case should be easy. Just look for your REPORT_DATA element, gather up its encapsulated data and if it is the last one (end document reached), you have your output.

Answer 5

    //filePath the path to the file you want to parse, tag  the tag of the node you want to search.    
public static String getLastNode(String filePath, String tag) throws             ParserConfigurationException, SAXException, IOException{
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    DocumentBuilder builder = factory.newDocumentBuilder();
    Document doc = docBuilder.parse(filePath);

    return doc.getElementsByTagName(tag).item(doc.getElementsByTagName(tag).getLength()-1).getTextContent();
//if you don't care about specific tag name just use :
//return doc.getLastChild().getTextContent;


}