The Java Developers Almanac 1.4

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e548. Intercepting All Accesses to External Entities During XML SAX Parsing

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 EntityResolver object must be installed on the parser in order to intercept the accesses. If a mapping is made, the entity resolver must return an InputSource object to the resource.

    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("infilename.xml"));
    } 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;
        }
    }

This is the sample input for the example:

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE root SYSTEM "System.dtd" [
        <!ENTITY entity1 SYSTEM "External.xml">
        <!ENTITY entity2 SYSTEM "http://hostname.com/my.dtd">
        <!ENTITY % entity3 SYSTEM "More.dtd">
        %entity3;
    ]>
    <root>
        &entity1;
        &entity2;
    </root>

The resulting system ids passed into MyResolve.resolveEntity() are:

    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

Related Examples

e549. Getting the Attributes of an Element During XML SAX Parsing
e550. Determining the Parsing Location of an XML SAX Parser

© 2002 Addison-Wesley.