StreamingUnmarshaller.java

package org.caudexorigo.xml;

import static javax.xml.stream.XMLStreamConstants.CHARACTERS;
import static javax.xml.stream.XMLStreamConstants.DTD;
import static javax.xml.stream.XMLStreamConstants.END_ELEMENT;
import static javax.xml.stream.XMLStreamConstants.START_DOCUMENT;

import java.io.Closeable;
import java.io.InputStream;
import java.util.Iterator;
import java.util.NoSuchElementException;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Unmarshaller;
import javax.xml.stream.FactoryConfigurationError;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.XMLStreamReader;

public class StreamingUnmarshaller<T> implements Iterable<T>, Iterator<T>, Closeable
{
	XMLStreamReader reader;
	Class<T> clazz;
	Unmarshaller unmarshaller;

	public StreamingUnmarshaller(InputStream stream, Class<T> clazz) throws XMLStreamException, FactoryConfigurationError, JAXBException
	{
		this.clazz = clazz;
		this.unmarshaller = JAXBContext.newInstance(clazz).createUnmarshaller();
		this.reader = XMLInputFactory.newInstance().createXMLStreamReader(stream);

		/* ignore headers */
		skipElements(START_DOCUMENT, DTD);
		/* ignore root element */
		reader.nextTag();
		/* if there's no tag, ignore root element's end */
		skipElements(END_ELEMENT);
	}

	@Override
	public T next()
	{
		if (!hasNext())
			throw new NoSuchElementException();

		try
		{
			T value = unmarshaller.unmarshal(reader, clazz).getValue();
			skipElements(CHARACTERS, END_ELEMENT);
			return value;
		}
		catch (XMLStreamException | JAXBException e)
		{
			throw new RuntimeException(e);
		}
	}

	@Override
	public boolean hasNext()
	{
		try
		{
			return reader.hasNext();
		}
		catch (XMLStreamException e)
		{
			throw new RuntimeException(e);
		}
	}

	@Override
	public void close()
	{
		try
		{
			reader.close();
		}
		catch (XMLStreamException e)
		{
			// ignore since we are closing the reader
		}
	}

	private void skipElements(int... elements) throws XMLStreamException
	{
		int eventType = reader.getEventType();

		while (arrayContains(elements, eventType))
		{
			eventType = reader.next();
		}
	}

	private boolean arrayContains(int[] searchArray, int value)
	{
		for (int ix : searchArray)
		{
			if (ix == value)
			{
				return true;
			}
		}

		return false;
	}

	@Override
	public Iterator<T> iterator()
	{
		return this;
	}
}