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<TITLE>Intercepting All Accesses to External Entities During XML SAX Parsing (Java Developers Almanac Example)
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    <B><A HREF="../org.xml.sax/pkg.html">org.xml.sax</A></B><font color="#666666" class="xsmall-font">
        &nbsp;[3 examples]
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  <h3>e548. Intercepting All Accesses to External Entities During XML SAX Parsing</h3>

This capability is useful in situations where the public or system id
in an XML file do not refer to actual resources and must be
redirected.  An <code>EntityResolver</code> object must be installed on the parser
in order to intercept the accesses. If a mapping is made, the entity
resolver must return an <code>InputSource</code> object to the resource.


<pre>    try {
        // Create an XML parser
        DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
    
        // Install the entity resolver
        builder.setEntityResolver(new MyResolver());
    
        // Parse the XML file
        Document doc = builder.parse(new File(<font color="#0066ff"><i>"infilename.xml"</i></font>));
    } catch (SAXException e) {
        // A parsing error occurred; the xml input is not valid
    } catch (ParserConfigurationException e) {
    } catch (IOException e) {
    }
    
    public class MyResolver implements EntityResolver {
        // This method is called whenever an external entity is accessed
        // for the first time.
        public InputSource resolveEntity (String publicId, String systemId) {
            try {
                // Wrap the systemId in a URI object to make it convenient
                // to extract the components of the systemId
                URI uri = new URI(systemId);
    
                // Check if external source is a file
                if ("file".equals(uri.getScheme())) {
                    String filename = uri.getSchemeSpecificPart();
                    return new InputSource(new FileReader(filename));
                }
            } catch (URISyntaxException e) {
            } catch (IOException e) {
            }
    
            // Returning null causes the caller to try accessing the systemid
            return null;
        }
    }
</pre>
This is the sample input for the example:

<pre>    &lt;?xml version="1.0" encoding="UTF-8"?&gt;
    &lt;!DOCTYPE root SYSTEM "System.dtd" [
        &lt;!ENTITY entity1 SYSTEM "External.xml"&gt;
        &lt;!ENTITY entity2 SYSTEM "http://hostname.com/my.dtd"&gt;
        &lt;!ENTITY % entity3 SYSTEM "More.dtd"&gt;
        %entity3;
    ]&gt;
    &lt;root&gt;
        &amp;entity1;
        &amp;entity2;
    &lt;/root&gt;
</pre>
The resulting system ids passed into <code>MyResolve.resolveEntity()</code> are:

<pre>    file:d:/almanac/1.4/egs/org.xml.sax/More.dtd
    file:d:/almanac/1.4/egs/org.xml.sax/Systemid.dtd
    file:d:/almanac/1.4/egs/org.xml.sax/External.dtd
    http://hostname.com/my.dtd
</pre>
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            &nbsp;<b>Related Examples</b></font></td>
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e549. <a class="eglink" href="GetAttr.html?l=rel">
    Getting the Attributes of an Element During XML SAX Parsing
</a>
<br>

e550. <a class="eglink" href="SetLocator.html?l=rel">
    Determining the Parsing Location of an XML SAX Parser
</a>
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