docs/faq-write.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE faqs SYSTEM 'dtd/faqs.dtd'>
<faqs title='Writing Application FAQs'>
 <faq title='Creating a DOM Parser'>
  <q>How do I create a DOM parser?</q>
  <a>
   <p>
    You can create a DOM parser by using the Java APIs for
    XML Processing (JAXP). The following source code shows
    how:
   </p>
   <source>import java.io.IOException;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.FactoryConfigurationError;
import javax.xml.parsers.ParserConfigurationException;
import org.w3c.dom.Document;
import org.xml.sax.SAXException;

  ...

String xmlFile = &quot;file:///&parserdir;/data/personal.xml&quot;; 
try {
    DocumentBuilderFactory factory = 
        DocumentBuilderFactory.newInstance();
    DocumentBuilder builder = factory.newDocumentBuilder();
    Document document = builder.parse(xmlFile);
}
catch (FactoryConfigurationError e) {
    // unable to get a document builder factory
} 
catch (ParserConfigurationException e) {
    // parser was unable to be configured
catch (SAXException e) {
    // parsing error
} 
catch (IOException e) {
    // i/o error
}</source>
  </a>
 </faq>
 <faq title="Creating a SAX Parser">
  <q>How do I create a SAX parser?</q>
  <a>
   <p>
    You can create a SAX parser by using the Java APIs for
    XML Processing (JAXP). The following source code shows
    how:
   </p>
   <source>
import java.io.IOException; 
import javax.xml.parsers.FactoryConfigurationError;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

  ...

String xmlFile = &quot;file:///&parserdir;/data/personal.xml&quot;; 
try {
    SAXParserFactory factory = SAXParserFactory.newInstance();
    SAXParser parser = factory.newSAXParser();
    DefaultHandler handler = /* custom handler class */;
    parser.parse(xmlFile, handler);
} 
catch (FactoryConfigurationError e) {
    // unable to get a document builder factory
} 
catch (ParserConfigurationException e) {
    // parser was unable to be configured
catch (SAXException e) {
    // parsing error
} 
catch (IOException e) {
    // i/o error
}</source>
  </a>
 </faq>
 <!--
   - REVISIT: make sure that JAXP implementation can handle
   -          passing features and properties through to the
   -          parser implementation. Then complete this section.
 <faq title='Controlling parser options'>
  <q>How do I control the various parser options?</q>
  <a>TBD</a>
 </faq>
 -->
 <faq title='Handling Errors'>
  <q>How do handle errors?</q>
  <a>
   <p>
    You should register an error handler with the parser by supplying
    a class which implements the <code>org.xml.sax.ErrorHandler</code>
    interface. This is true regardless of whether your parser is a
    DOM based or SAX based parser.
   </p>
   <p>
    You can register an error handler on a <code>DocumentBuilder</code>
    created using JAXP like this:
   </p>
   <source>import javax.xml.parsers.DocumentBuilder;
import org.xml.sax.ErrorHandler;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;

ErrorHandler handler = new ErrorHandler() {
    public void warning(SAXParseException e) throws SAXException {
        System.err.println("[warning] "+e.getMessage());
    }
    public void error(SAXParseException e) throws SAXException {
        System.err.println("[error] "+e.getMessage());
    }
    public void fatalError(SAXParseException e) throws SAXException {
        System.err.println("[fatal error] "+e.getMessage());
	throw e;
    }
};

DocumentBuilder builder = /* builder instance */;
builder.setErrorHandler(handler);</source>
   <p>
    You can also register an error handler on a SAXParser using JAXP
    like this:
   </p>
   <source>import javax.xml.parsers.SAXParser;
import org.xml.sax.ErrorHandler;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;

ErrorHandler handler = new ErrorHandler() {
    public void warning(SAXParseException e) throws SAXException {
        System.err.println("[warning] "+e.getMessage());
    }
    public void error(SAXParseException e) throws SAXException {
        System.err.println("[error] "+e.getMessage());
    }
    public void fatalError(SAXParseException e) throws SAXException {
        System.err.println("[fatal error] "+e.getMessage());
	throw e;
    }
};

SAXParser parser = /* parser instance */;
parser.getXMLReader().setErrorHandler(handler);</source>
  </a>
 </faq>
 <faq title='Controlling Entity Representation'>
  <q>
   How can I control the way that entities are represented in the DOM?
  </q>
  <a>
   <p>
    The feature 
    <code>http://apache.org/xml/features/dom/create-entity-ref-nodes</code>
    controls how entities appear in the DOM tree. When this feature
    is set to true (the default), an occurance of an entity reference
    in the XML document will be represented by a subtree with an 
    EntityReference node at the root whose children represent the 
    entity expansion.
   </p>
   <p>
    If the property is false, an entity reference in the XML document
    is represented by only the nodes that represent the entity 
    expansion.
   </p> 
   <p>
    In either case, the entity expansion will be a DOM tree 
    representing the structure of the entity expansion, not a text
    node containing the entity expansion as text.
   </p>
  </a>
 </faq>
 <faq title='What does &quot;non-validating&quot; mean?'>  
  <q>
   Why does &quot;non-validating&quot; not mean &quot;well-formedness 
   checking only&quot;?
  </q>
  <a>
   <p>
    Using a &quot;non-validating&quot; parser does not mean that
    only well-formedness checking is done! There are still many 
    things that the XML specification requires of the parser, 
    including entity substitution, defaulting of attribute values, 
    and attribute normalization.
   </p>
   <p>
    This table describes what &quot;non-validating&quot; really
    means for &ParserName; parsers. In this table, "no DTD" 
    means no internal or external DTD subset is present.
   </p>
   <table>
    <tr>
     <tn/>
     <th colspan="2">non-validating parsers</th>
     <th colspan="2">validating parsers</th>
    </tr>
    <tr>
     <tn/>
     <th>DTD present</th>
     <th>no DTD</th>
     <th>DTD present</th>
     <th>no DTD</th>
    </tr>
    <tr>
     <th>DTD is read</th>
     <td>Yes</td>
     <td>No</td>
     <td>Yes</td>
     <td>Error</td>
    </tr>
    <tr>
     <th>entity substitution</th>
     <td>Yes</td>
     <td>No</td>
     <td>Yes</td>
     <td>Error</td>
    </tr>
    <tr>
     <th>defaulting of attributes</th>
     <td>Yes</td>
     <td>No</td>
     <td>Yes</td>
     <td>Error</td>
    </tr>
    <tr>
     <th>attribute normalization</th>
     <td>Yes</td>
     <td>No</td>
     <td>Yes</td>
     <td>Error</td>
    </tr>
    <tr>
     <th>checking against model</th>
     <td>No</td>
     <td>No</td>
     <td>Yes</td>
     <td>Error</td>
    </tr>
   </table>
  </a>
 </faq>
 <faq title='Associating Data with a Node'>
  <q>How do I associate my own data with a node in the DOM tree?</q>
  <a>
   <p>
    The class <code>org.apache.xerces.dom.NodeImpl</code> provides a 
    <code>void setUserData(Object o)</code> and an <code>Object 
    getUserData()</code> method that you can use to attach any object
    to a node in the DOM tree.
   </p>
   <p>
    Beware that you should try and remove references to your data on
    nodes you no longer use (by calling <code>setUserData(null)</code>,
    or these nodes will not be garbage collected until the entire
    document is garbage collected.
   </p>
  </a>
 </faq>
 <faq title='Parsing Several Documents'>
  <q>
   How do I more efficiently parse several documents sharing a 
   common DTD?
  </q>
  <a>
   <p>
    DTDs are not currently cached by the parser. The common DTD, 
    since it is specified in each XML document, will be re-parsed 
    once for each document.
   </p>
   <p>
    However, there are things that you can do now, to make the 
    process of reading DTD's more efficient:
   </p>
   <ul>
    <li>keep your DTD and DTD references local</li>
    <li>use internal DTD subsets, if possible</li>
    <li>load files from server to local client before parsing</li>
    <li>
     Cache document files into a local client cache. You should do an 
     HTTP header request to check whether the document has changed, 
     before accessing it over the network.
    </li>
    <li>
     Do not reference an external DTD or internal DTD subset at all. 
     In this case, no DTD will be read.
    </li>
    <li>
     Use a custom <code>EntityResolver</code> and keep common 
     DTDs in a memory buffer.
    </li>
   </ul>
  </a>
 </faq>
 <!--
   - REVISIT: Rewrite this section with better information.
 <faq title='How do I read data from a stream as it arrives?'>
  <q>How do I read data from a stream as it arrives?</q>
  <a>
   <p>There are 2 problems you have to deal with:</p>
   <ol>
    <li>
     The Apache parsers terminate when they reach end-of-file; with 
     a data stream, unless the sender drops the socket, you have no
     end-of-file, so you need to terminate in some other way
    </li>
    <li>
     The Apache parsers close the input stream on termination, and 
     this closes the socket; you normally don't want this, because 
     you'll want to send an ack to the data stream source, and you
     may want to have further exchanges on the socket anyway.
    </li>
   </ol>
   <p>Terminating the parse</p>
   <p>
    One way that works forSAX is to throw an exception when you
    detect the logical end-of-document.
   </p>
   <p>
    For instance, in your class extending DefaultHandler, you can 
    have:
   </p>
   <source>public class DocProcessor extends DefaultHandler {
    private int level;
    .
    .
    public void startElement(String uri,
                             String localName,
                             String raw,
                             Attributes attrs) throws SAXException
    {
        ++level;
    }

    public void endElement (String namespaceURI,
                            String localName,
                            String qName) throws SAXException
    {
        level = level - 1;
        if (level == 0) {
            throw new SAXException ("Finished");
        }
    }</source>
  <p>Preventing the parser from closing the socket</p>
  <p>
   One way is to subclass BufferedReader to provide an empty close 
   method. So, invoke the parser as follows:
  </p>
  <source>Socket socket;

    // code to set the socket

    parser.parse(new InputSource(new MyBufferedReader(new InputStreamReader(socket.getInputStream()))));
    .
    .
    class MyBufferedReader extends BufferedReader
    {
        public MyBufferedReader(InputStreamReader i) {
            super(i);
        }

        public void close() {
        }
    }</source>
  </a>
 </faq>
 -->
</faqs>