How can I force a SAX parser to use a DTD if one is not specified in the input file?

Question

How can I force a SAX parser (specifically, Xerces in Java) to use a DTD when parsing a document without having any doctype in the input document? Is this even possible?

Here are some more details of my scenario:

We have a bunch of XML documents that conform to the same DTD that are generated by multiple different systems (none of which I can change). Some of these systems add a doctype to their output documents, others do not. Some use named character entities, some do not. Some use named character entities without declaring a doctype. I know that's not kosher, but it's what I have to work with.

I'm working on system that needs to parse these files in Java. Currently, it's handling the above cases by first reading in the XML document as a stream, attempting to detect if it has a doctype defined, and adding a doctype declaration if one isn't already present. The problem is that this code is buggy, and I'd like to replace it with something cleaner.

The files are large, so I can't use a DOM-based solution. I'm also trying get character entities resolved, so it doesn't help to use an XML Schema.

If you have a solution, could you please post it directly instead of linking to it? It doesn't do Stack Overflow much good if in a the future there's a correct solution with a dead link.

Answer 1

I think it is no sane way to set DOCTYPE, if document hasn't one. Possible solution is write fake one, as you already do. If you're using SAX, you can use this fake InputStream and fake DefaultHandler implementation. (will work only for latin1 one-byte encoding)

I know this solution also ugly, but it only one works well with big data streams.

Here is some code.

private enum State {readXmlDec, readXmlDecEnd, writeFakeDoctipe,  writeEnd};

private class MyInputStream extends InputStream{

    private final InputStream is;
    private StringBuilder sb = new StringBuilder();
    private int pos = 0;
    private String doctype = "<!DOCTYPE register SYSTEM \"fake.dtd\">";
    private State state = State.readXmlDec;

    private MyInputStream(InputStream source) {
        is = source;
    }
    @Override
    public int read() throws IOException {
        int bit;

        switch (state){ 
            case readXmlDec:
                bit = is.read();
                sb.append(Character.toChars(bit));
                if(sb.toString().equals("<?xml")){
                    state = State.readXmlDecEnd;
                }
                break;
            case readXmlDecEnd:
                bit = is.read();
                if(Character.toChars(bit)[0] == '>'){
                    state = State.writeFakeDoctipe;
                }
                break;
            case writeFakeDoctipe:
                bit =  doctype.charAt(pos++);
                if(doctype.length() == pos){
                    state = State.writeEnd;
                }
                break;
            default:
                bit = is.read();
                break;
        }
        return bit;
    }

    @Override
    public void close() throws IOException {
        super.close();
        is.close();
    }
}

private static class MyHandler extends DefaultHandler {

    @Override
    public InputSource resolveEntity(String publicId, String systemId) throws IOException, SAXException {
        System.out.println("resolve "+ systemId);
        // get real dtd
        InputStream is = ClassLoader.class.getResourceAsStream("/register.dtd");
        return new InputSource(is);
    }

 ... // rest of code
}