Dynamic Conversion of XML to JSON

Question

Is there any way (with Java code examples, if possible) to convert, on the fly, an XML input to JSON, without any knowledge of the actual contents and the structure of the XML source (file, string, etc.)?

Say, for instance, that one has a very large XML dataset with unknown structure and multiple nesting levels, stored to a big text file. Reading everything into memory is not possible (for lack of space) and they want to convert this into JSON directly, i.e., without having to write any code to detect and handle the StAX tags (e.g., START_ELEMENT, CHARACTERS, END_ELEMENT).

The ideal solution would be to get a Reader or InputStream from the converter, so that, for instance, one supplies the XML file and the Reader or InputStream produces JSON, to be fed to a FileOutputStream, or even directly to a JSON parser like Jackson. If that is not possible, at least a way of progressively reading an XML file, converting to JSON and writing to another file would be an acceptable compromise.

Tools that can be used for converting from/to XML/JSON (e.g., StaxON, JSON-lib, Jettison, XStream) do not seem to do that but only conversion of a known structure.

EDIT: Getting a Reader or InputStream from an OutputStream or a Writer (which would also cover the "conversion" I spoke of above), can be done in a number of ways, although for best results and "infinite" input sizes multithreading is involved. Solutions are described in this article in Ostermiller.org and a similar implementation can be found in the IO-Tools library.

Answer 1

Here's a trivial example using Java's built-in StAX implementation to parse XML and Jettison to produce JSON from it. The XMLEventWriter has a convenient add(XMLEventWriter) method for bridging a reader to a writer, making this super-simple:

import org.codehaus.jettison.mapped.MappedXMLOutputFactory;

import javax.xml.stream.XMLEventReader;
import javax.xml.stream.XMLEventWriter;
import javax.xml.stream.XMLInputFactory;
import java.io.StringReader;
import java.util.HashMap;

public class Main {
    public static void main(String[] args) throws Exception {
        String xml =
            "<root><foo>foo string</foo><bar><x>1</x><y>5</y></bar></root>";
        XMLEventReader reader = XMLInputFactory.newInstance()
            .createXMLEventReader(new StringReader(xml));
        XMLEventWriter writer = new MappedXMLOutputFactory(new HashMap())
            .createXMLEventWriter(System.out);
        writer.add(reader);
        writer.close();
        reader.close();
    }
}

I've created a self-contained Maven project demonstrating this on Github.

Answer 2

You can do this with StAXON, see https://github.com/beckchr/staxon/wiki/Converting-XML-to-JSON for sample code.