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javaxmlsaxxml-namespacesxerces

Get xml namespace (without triggering UnknownHostException)

I have some Java code that determines the namespace of the root-level element of an xml document using SAX. If the namespace is "http://sbgn.org/libsbgn/pd/0.1", it should return version 1. If the namespace is "http://sbgn.org/libsbgn/0.2", the version should be 2. So all the code does is read the first element, and set a variable based on the namespace. Here is the code:

private static class VersionHandler extends DefaultHandler
{
    private int version = -1;

    @Override
    public void startElement (String uri, String localName, String qName, Attributes attributes) throws SAXException
    {
        if ("sbgn".equals (qName))
        {
            System.out.println (uri);
            if ("http://sbgn.org/libsbgn/0.2".equals(uri))
            {
                version = 2;
            } 
            else if ("http://sbgn.org/libsbgn/pd/0.1".equals(uri))
            {
                version = 1;
            } 
            else
            {
                version = -1;
            }
        }
    }

    public int getVersion() { return version; }
};

public static int getVersion(File file) throws SAXException, FileNotFoundException, IOException
{
    XMLReader xr;   
    xr = XMLReaderFactory.createXMLReader();

    VersionHandler versionHandler = new VersionHandler();

    xr.setContentHandler(versionHandler);
    xr.setErrorHandler(versionHandler);
    xr.parse(new InputSource(
        InputStreamToReader.inputStreamToReader(
            new FileInputStream (file))));

    return versionHandler.getVersion();
}

This works, but has two problems:

  1. It is inefficient, because the whole document will be parsed even though only the first element is needed.
  2. More importantly, this code sometimes (apparently depending on firewall configuration) triggers a UnknownHostException like so:
    java.net.UnknownHostException: www.w3.org 
    at java.net.PlainSocketImpl.connect(Unknown Source)
    at java.net.SocksSocketImpl.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at sun.net.NetworkClient.doConnect(Unknown Source)
    at sun.net.www.http.HttpClient.openServer(Unknown Source)
    at sun.net.www.http.HttpClient.openServer(Unknown Source)
    at sun.net.www.http.HttpClient.(Unknown Source)
    at sun.net.www.http.HttpClient.New(Unknown Source)
    at sun.net.www.http.HttpClient.New(Unknown Source)
    at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(Unknown
    Source)
    at sun.net.www.protocol.http.HttpURLConnection.plainConnect(Unknown Source)
    at sun.net.www.protocol.http.HttpURLConnection.connect(Unknown Source)
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.impl.XMLEntityManager.setupCurrentEntity(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.impl.XMLEntityManager.startEntity(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.impl.XMLEntityManager.startDTDEntity(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.impl.XMLDTDScannerImpl.setInputSource(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$DTDDriver.dispatch(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$DTDDriver.next(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$PrologDriver.next(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown
    Source)
    at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown
    Source)
    at
    com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown
    Source)
    at org.sbgn.SbgnVersionFinder.getVersion(SbgnVersionFinder.java:57)

So my questions are:

  1. Apparently this bit of code is connecting to the internet. How can I avoid that? Besides leading to problems with firewalls, it is also needlessly slow.
  2. Why is it connecting to the internet? Please help me understand the logic here, there should be absolutely no need for it.
  3. Is there a more efficient way to determine the namespace of the root element of an xml document?

Edit: here is a link to a sample document that I'm trying to parse this way: https://libsbgn.svn.sourceforge.net/svnroot/libsbgn/trunk/test-files/PD/adh.sbgn

Edit2: A note regarding to the solution of this bug: In fact the problem was triggered because the wrong document was being parsed, instead of the intended document, I was parsing an XHMTML document that does in fact refer to www.w3.org. Of course the solution is to use the correct document. Nevertheless, I found it useful to add this line:

 xr.setEntityResolver(null);

To prevent xerces from going over the internet when it's really completely unnecessary.

Solution

  • I believe you need to set the entity resolver. See the javadoc. Also, this article seems relevant.