docs/faq-write.xml

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<!DOCTYPE faqs SYSTEM 'dtd/faqs.dtd'>
<faqs title='Writing Application FAQs'>

 <!--
   - 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 I 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>If you are using <jump href="http://www.w3.org/DOM/DOMTR#DOML3">DOM Level 3</jump> 
    you can register an error handler with the <code>DOMBuilder</code> by supplying
    a class which implements the <code>org.w3c.dom.DOMErrorHandler</code>
    interface. For more information, refer to <link idref="faq-dom">FAQ</link></p>
   <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='What does &quot;non-validating&quot; mean?'>  
  <q>Why does &quot;non-validating&quot; not mean &quot;well-formedness checking&quot; only?</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='Parsing Several Documents'>
  <q>
   How do I more efficiently parse several documents sharing a 
   common DTD?
  </q>
  <a>
   <p>
    By default, the parser does not cache DTD's. 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 to make the 
    process of reading DTD's more efficient:
   </p>
   <ul>
    <li>First, have a look at 
        <link idref="faq-grammars">the grammar caching/preparsing FAQ</link>
    </li>
    <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>
 <faq title='Pull-parsing documents'>
  <q>
   How can I parse documents in a pull-parsing fashion?
  </q>
  <a>
   <p>
    Since the pull-parsing API is specific to Xerces, you have to use
    a Xerces-specific method to create parsers, and parse documents.
   </p>
   <p>
    First, you need to create a parser configuration that implements the
    <code>XMLPullParserConfiguration</code> interface. Then you need to
    create a parser from this configuration. To parse documents, method
    <code>parse(boolean)</code> should be called.
   </p>
   <source>import &DefaultConfigLong;;
import org.apache.xerces.parsers.SAXParser;
import org.apache.xerces.xni.parser.XMLInputSource;

  ...

boolean continueParse = true;
void pullParse(XMLInputSource input) throws Exception {
    &DefaultConfig; config = new &DefaultConfig;();
    SAXParser parser = new SAXParser(config);
    config.setInputSource(input);
    parser.reset();
    while (continueParse) {
        continueParse = continueParse &amp;&amp; config.parse(false);
    }
}</source>
    <p>
     In the above example, a SAXParser is used to pull-parse an
     XMLInputSource. DOMParser can be used in a similar way. A flag
     <code>continueParse</code> is used to indicate whether to continue
     parsing the document. The application can stop the parsing by
     setting this flag to false.
    </p>
  </a>
 </faq>
 <faq title='Getting More Information for Your Entity Resolver'>
  <q>
    I would like to know more about the kind of entity my XMLEntityResolver's
    been asked to resolve.  How can I go about convincing Xerces to tell me more?
  </q>
  <a>
    <p>
        XNI only guarantees that you'll receive an XMLResourceIdentifier object 
        during an XMLEntityResolver#resolveEntity callback.
        Nonetheless, the xni.grammars package has a number of
        interfaces which extend XMLResourceIdentifier that can
        provide considerably more information.
    </p>
    <p>
        To take advantage of this, you'll first need to see
        whether the object you've been passed is an instance of
        the
        <code>org.apache.xerces.xni.grammars.XMLGrammarDescription</code>
        interface.  This interface contains a method called
        <code>getGrammarType</code> which can tell you what kind
        of grammar is involved (for the moment, XML Schema and
        DTD's are all that's supported).  Once you know the type
        of grammar, you can cast once again to either
        <code>org.apache.xerces.xni.grammars.XMLDTDDescription</code>
        or
        <code>org.apache.xerces.xni.grammars.XMLSchemaDescription</code>
        which contain a wealth of information specific to these
        types of grammars.  The javadocs for these interfaces
        should provide sufficient information for you to know
        what their various methods return.
    </p> 
  </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>