Java - SAXParser 和 XMLReader 获取 null 属性值

Question

我有一个 XML 文件，我想使用 SAX 从中获取数据。它一直持续到 startElement() 为止，出现错误，因为当我尝试获取属性值时，这些属性值为 null。为什么？

首先，我称这个方法为:load()

public RealEstate load() {
        RealEstate data = null; 
        try {     
            //Create a "parser factory" for creating SAX parsers
            SAXParserFactory spfac = SAXParserFactory.newInstance();

            //Now use the parser factory to create a SAXParser object
            SAXParser sp = spfac.newSAXParser();
            XMLReader xmlReader = sp.getXMLReader(); 

            //Create an instance of this class; it defines all the handler methods
            SaxParserRealEstateHandler handler = new SaxParserRealEstateHandler();

            //assign our handler
            xmlReader.setContentHandler(handler);

            // Convert file to URL.
            URL url = handler.fileToURL(new File(xmlFilename));

            // Parse file.
            xmlReader.parse(url.toString()); 

            data = handler.getData();

        } catch (ParserConfigurationException ex) {
            Logger.getLogger(RealStatePersistXML.class.getName()).log(Level.SEVERE, null, ex);
        } 
        catch (SAXException ex) {
            Logger.getLogger(RealStatePersistXML.class.getName()).log(Level.SEVERE, null, ex);
        }
        catch (IOException ex) {
           Logger.getLogger(RealStatePersistXML.class.getName()).log(Level.SEVERE, null, ex);
       }
        return data;
    }

这里有方法startElement和endElement。

    class SaxParserRealEstateHandler extends DefaultHandler
    {

        private String temp;
        private RealEstate data;
        private Estate estate;
        private estateAddress address;

        /**
         * CONSTRUCTOR
         */
        public SaxParserRealEstateHandler() 
        {
        }


        /**
         * getData()
         * This method gets the list of estates from RealEstate class.
         * @return data list of states. Null in case of error or list not found.
         */
        public RealEstate getData()
        {
        //    RealEstate data = new RealEstate();
           // data.addEstate(estate);
           // data.getEstates();
        //    return data;
        }

        /*
        * When the parser encounters plain text (not XML elements),
        * it calls(this method, which accumulates them in a string buffer
        */
       public void characters(char[] buffer, int start, int length) {
            temp = new String(buffer, start, length);
       }


        /*
        * Every time the parser encounters the beginning of a new element,
        * it calls this method, which resets the string buffer
        */ 
        public void startElement(String uri, String localName, String qName, Attributes attributes){ 

            switch (qName){     
                case"realState":
                    data = new RealEstate();
                    break;
                case"estate":
                    estate = new Estate();
                    estate.setType(attributes.getValue("type"));
                    estate.setSurface(Double.parseDouble(attributes.getValue("surface")));
                    //estate.setAddress(attributes.getValue("address"));
                    estate.setPrice(Integer.parseInt(attributes.getValue("price"))); 
                    break;     
                case"address":
                    address = new estateAddress();
                    address.setStreet(attributes.getValue("type"));
                    address.setNumber(Integer.parseInt(attributes.getValue("surface")));
                    address.setFloor(Integer.parseInt(attributes.getValue("type")));
                    address.setDoor(Integer.parseInt(attributes.getValue("surface")));
                    break;
                default:
                    break;
            }
        }



    public void endElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {

    switch (qName){
        case"estate": 
            data.getEstates().add(estate);//quant ha llegit la etiqueta (</estate>) l'agreguem
            break; 
        case"surface":
            double surface = Double.parseDouble(temp);
            estate.setSurface(surface);
            break;  
        case"price":
            double price = Double.parseDouble(temp);//tractar tema errors del parseDouble!
            estate.setPrice(price);
            break;    
        case"address":                    
            estate.setAddress(address);
            break;      
        case"street": 
            address.setStreet(temp);
            break;
        case"number":
            int number = Integer.parseInt(temp);
            address.setNumber(number);
            break;
        case"floor":
            address.setStreet(temp);
            break;  
        case"door":
            address.setStreet(temp);
            break;   
        default:
            break;
    }
} 



/** fileToURL()
 * Convenience method to convert a file to a url
 * @throws Error if malformed url exception is generated
 */

    public URL fileToURL(File file) 
{
    String path = file.getAbsolutePath();
    String fSep = System.getProperty("file.separator");
    if (fSep != null && fSep.length() == 1)
        path = path.replace(fSep.charAt(0), '/');
    if (path.length() > 0 && path.charAt(0) != '/')
        path = '/' + path;
    try  
    {
        return new URL("file", null, path);
    }
    catch (java.net.MalformedURLException e) 
    {
        throw new Error("Unexpected MalformedURLException");//no he pogut muntar una url com deu mana
        //retornar null
    }
}
    }

XML 代码

<?xml version="1.0" encoding="UTF-8"?>
<realState xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="realState.xsd">
    <estate>
        <type>House</type>
        <surface>250,0</surface>
        <address>
            <street>C/Tarragona</street>
            <number>54</number>
            <floor>2</floor>
            <door>1</door>
        </address>
        <price>140000,0</price>
    </estate>
</realState>

XSD 代码:http://pastebin.com/nzEDsdLg

类 RealEstate 和 Estate 工作正常!

谢谢!

Answer 1

type , surface ，和price不是 <estate> 的属性元素。它们是子元素。

作为属性，您的 XML 应该是:

<estate type="House" surface="250,0" price="140000,0">
    <address>
        <street>C/Tarragona</street>
        <number>54</number>
        <floor>2</floor>
        <door>1</door>
    </address>
</estate>

使用 StAX 解析器比 SAX 解析器容易得多，并且具有非常相似的性能特征，因此我建议您改用它。