1_read.html

XML_JAVA指南 书籍语言： 简体中文 书籍类型： 程序设计 授权方式： 免费软件 书籍大小： 377 KB

<blockquote>
  <p> </p>
</blockquote>
Now, add the code highlighted below to get a instance of a builder, and use it 
to parse the specified file: 
<blockquote> 
  <p></p>
  <pre>try {
   <b>DocumentBuilder builder = factory.newDocumentBuilder();</b>
   <b>document = builder.parse( new File(argv[0]) );</b>
<br>} catch (SAXParseException spe) {
</pre>
</blockquote>
<h3><b><a name="write"></a></b>Write out the XML</h3>
<p>At this point, the code has the ability to read in and parse an XML document. 
  To write out the document for inspection, you'll need to step outside the DOM 
  level 1 standard that makes up the DOM section of JAXP. DOM write operations 
  are not specified until DOM level 3. To get by in the meantime, you'll cast 
  the Document object returned by DocumentBuilder to the real object that the 
  reference implementation returns: XmlDocument.</p>
<blockquote> 
  <p><img src="../images/sun.gif" width="62" height="29"> <b>Note:</b><br>
    This material is specific to Project X, Sun's reference implementation for 
    the JAXP standard. The material in this section is <i>not</i> part of the 
    standard. Instead, it represents helpful functionality that you may need to 
    take advantage of until some equivalent mechanism is standardized. Because 
    it is not part of the JAXP standard, the functionality described here may 
    very well <i>not</i> exist in other JAXP-standard parsers. In fact, as standards 
    evolve, future versions of the JAXP reference implementation could employ 
    different mechanisms to achieve the same goals.</p>
</blockquote>
<h4><b><a name="XmlDocument"></a></b>Use XmlDocument</h4>
<p>Start by adding the import statement that defines the class:</p>
<blockquote> 
  <p></p>
  <pre>import org.w3c.dom.Document;
import org.w3c.dom.DOMException;<br>
<b>import com.sun.xml.tree.XmlDocument;</b><br>
public class DomEcho
{  </pre>
</blockquote>
<p>(The <tt>com.sun.</tt> prefix on that class is your tipoff to the fact that 
  you are moving outside the JAXP standard, and making use of a feature in Sun's 
  reference implementation.) <br>
</p>
<p>Next, add the code highlighted below to cast the document object to XmlDocument 
  and write it out:</p>
<blockquote> 
  <pre>try {
  DocumentBuilder builder = factory.newDocumentBuilder();
  document = builder.parse( new File(argv[0]) ); 
           
  <b>XmlDocument xdoc = (XmlDocument) document;
  xdoc.write (System.out);</b>

} catch (SAXParseException spe) {
</pre>
</blockquote>
<h4><b><a name="run"></a>Run the Program</b></h4>
<p>Throughout most of the DOM tutorial, you'll be using the sample slideshows 
  you created in the SAX section. In particular, you'll use <a href="samples/slideSample01.xml"><code>slideSample01.xml</code></a>, 
  a simple XML file without nothing much in it, and <a href="samples/slideSample10.xml"><code>slideSample10.xml</code></a>, 
  a more complex example that includes a DTD, processing instructions, entity 
  references, and a CDATA section.</p>
<p>For instructions on how to compile and run your program, see <a href="../sax/2a_echo.html#compiling">Compiling 
  the Program</a> and <a href="../sax/2a_echo.html#running">Running the Program</a>, 
  from the SAX tutorial. Substitute &quot;DomEcho&quot; for &quot;Echo&quot; as 
  the name of the program, and you're ready to roll. When you run the program 
  on slideSample01.xml, this is the output you see:</p>
<blockquote> 
  <p></p>
  <pre>&lt;?xml version="1.0" encoding="UTF-8"?>

&lt;!--  A SAMPLE set of slides  -->
&lt;slideshow title="Sample Slide Show" date="Date of publication" author="Yours Truly">

    
  &lt;!-- TITLE SLIDE -->
    
  &lt;slide type="all">
      
    &lt;title>Wake up to WonderWidgets!&lt;/title>
    
  &lt;/slide>

    
  &lt;!-- OVERVIEW -->
    
  &lt;slide type="all">
      
    &lt;title>Overview&lt;/title>
      
    &lt;item>Why 
      &lt;em>WonderWidgets&lt;/em> are great
    &lt;/item>
      
    &lt;item />
      
    &lt;item>Who 
      &lt;em>buys&lt;/em> WonderWidgets
    &lt;/item>
    
  &lt;/slide>


&lt;/slideshow>
</pre>
</blockquote>
<p>When you run the program on slideSample10.xml, the result is pretty similar. 
  In particular, note that the entity reference stayed as it was originally written 
  (it was not replace with the entity text): </p>
<blockquote> 
  <p></p>
  <pre>&lt;item>
 <b>&amp;copyright;</b>
&lt;/item>
</pre>
 </blockquote>
Also, notice that the CDATA section has been preserved: 
<blockquote> 
  <p></p>
  <pre>
      
    &lt;item>
      <b>&lt;![CDATA[</b>Diagram:
                         
    frobmorten <------------ fuznaten
        |            <3>        ^
        | <1>                   |   <1> = fozzle
        V                       |   <2> = framboze    
      staten--------------------+   <3> = frenzle
                     <2>
      <b>]]&gt;</b>
    &lt;/item>
</pre>
</blockquote>
<h3><a name="addl"></a>Additional Information</h3>
<p>Now that you have successfully read in a DOM, there are one or two more things 
  you need to know in order to use DocumentBuilder effectively. Namely, you need 
  to know about:</p>
<ul>
  <li>Configuring the Factory</li>
  <li>Handling Validation Errors</li>
</ul>
<h4><a name="config"></a>Configuring the Factory</h4>
<p>By default, the factory returns a nonvalidating parser that knows nothing about 
  namespaces. To get a <a href="../glossary.html#validatingParser">validating 
  parser</a>, and/or one that understands <a href="../glossary.html#namespace">namespace</a>s, 
  you configure the factory to set either or both of those options using the command(s) 
  highlighted below: </p>
<blockquote> 
  <pre>public static void main (String argv [])
{
    if (argv.length != 1) {
        ...
    }
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    <b>factory.setValidating(true);</b>
    <b>factory.setNamespaceAware(true);</b>
    try {<br>        ...
</pre>
  <p><b>Note: </b><br>
    JAXP-conformant parsers are not required to support all combinations of those 
    options, even though the reference parser does. If you specify an invalid 
    combination of options, the factory generates a ParserConfigurationException 
    when you attempt to obtain a parser instance.</p>
</blockquote>
<p>You'll be learning more about how to use namespaces in the last section of 
  the DOM tutorial, <a href="6_ns.html">Using Namespaces</a>. To complete this 
  section, though, you'll want to learn something about...</p>
<h4><a name="val"></a>Handling Validation Errors</h4>
<p>Remember when you were wading through the SAX tutorial, and all you really 
  wanted to do was construct a DOM? Well, here's when that information begins 
  to pay off.</p>
<p>Recall that the default response to a validation error, as dictated by the 
  SAX standard, is to do nothing. The JAXP standard requires throwing SAX exceptions, 
  so you exactly the same error handling mechanisms as you used for a SAX app. 
  In particular, you need to use the DocumentBuilder's <code>setErrorHandler</code> 
  method to supply it with an object that implements the SAX ErrorHandler interface.</p>
<blockquote> 
  <p>Note:<br>
    DocumentBuilder also has a <tt>setEntityResolver</tt> method you can use </p>
</blockquote>
<p>The code below uses an anonymous inner class adapter to provide that ErrorHandler. 
  The highlighted code is the part that makes sure validation errors generate 
  an exception.</p>
<blockquote> 
  <pre>builder.setErrorHandler(
  new org.xml.sax.ErrorHandler() {<br>      // ignore fatal errors (an exception is guaranteed)
      public void fatalError(SAXParseException exception)
      throws SAXException {
      }

<new>      <b>// treat validation errors as fatal</b></new>
<new>      <b>public void error (SAXParseException e)</b></new>
<b><new>      throws SAXParseException</new>
<new>      {</new>
        <new>throw e;</new><new></new>
<new>      }</new></b><new></new>

<new>      // dump warnings too</new>
<new>      public void warning (SAXParseException err)</new>
<new>      throws SAXParseException</new>
<new>      {</new>
        <new>System.out.println ("** Warning"</new>
           <new>+ ", line " + err.getLineNumber ()</new>
           <new>+ ", uri " + err.getSystemId ());</new>
        <new>System.out.println("   " + err.getMessage ());</new>
<new>      }
</new>  }
); </pre>
</blockquote>
<p>This code uses an anonymous inner class to generate an instance of an object 
  that implements the ErrorHandler interface. Since it has no class name, it's 
  &quot;anonymous&quot;. You can think of it as an &quot;ErrorHandler&quot; instance, 
  although technically it's a no-name instance that implements the specified interface. 
  The code is substantially the same as that described the <a href="../sax/3_error.html">Handling 
  Errors</a> section of the SAX tutorial. For a more background on validation 
  issues, refer to <a href="../sax/6_val.html">Using the Validating Parser</a> 
  in that part of the tutorial.</p>
<blockquote>
  <p><b>Note:</b><br>
    Inner classes are supported in version 1.2 and later versions of the Java 
    Platform. If you are coding for version 1.1, create an external class that 
    implements ErrorHandler as shown above, and use that.</p>
</blockquote>
<h3><a name="look"></a>Looking Ahead</h3>
<p>At this point, you have successfully parsed an XML document and written it 
  out. To do anything useful with the DOM, though, you will to need to know more 
  about it's structure. For example, how do the entity references and CDATA sections 
  appear in the DOM?</p>
<p>Another interesting question is: How can you convert an existing data structure 
  into an XML document? You'll get answers to those questions in the sections 
  ahead. </p>
<p>In the next section, you'll take a quick look at a mechanism you can use in 
  the JAXP reference implementation (Project X) to convert an arbitrary data structure 
  into XML (assuming that you already have a program capable of reading that structure). 
  In the section after that, you'll display the DOM in a JTree so you can begin 
  to understand its internal structure.<br>
</p>
<blockquote> 
  <p> 
  <hr size=4>
  <p></p>
</blockquote>
<p> 
<p> 
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