How to stream large Files using JAXB Marshaller?

Question

The Problem I'm facing is how to marshall a large list of objects into a single XML File, so large I can not marshall the complete list in one step. I have a method that returns these objects in chunks, but then I marshall these using JAXB, the marshaller returns with an exception that these objects are no root elements. This is ok for the normal case there you want to marshall the complete document in one step, but it also happens if I set the JAXB_FRAGMENT Property to true.

This is the desired XML output:

<rootElem>       <startDescription></startDescription>       <repeatingElem></repeatingElem>     <repeatingElem></repeatingElem>... </rootElem> 

So I assume I need some kind of listener that dynamically loads the next chunk of repeatingElements to feed it to the marshaller before he would write the closing tag of the rootElement. But how to do that? Up until now I only used JAXB to marshall small files and the JAXB documentation does not give much hints for that use case.

Answer 1

I'm aware that this is an old question but I came across it while searching for duplicates of another similar question.

As @skaffman suggests, you want to Marshal with JAXB_FRAGMENT enabled and your objects wrapped in JAXBElement. You then repeatedly marshal each individual instance of the repeated element. Basically it sounds like you want something roughly like this:

public class StreamingMarshal<T> {     private XMLStreamWriter xmlOut;     private Marshaller marshaller;     private final Class<T> type;      public StreamingMarshal(Class<T> type) throws JAXBException     {         this.type = type;         JAXBContext context = JAXBContext.newInstance(type);         Marshaller m = context.createMarshaller();         m.setProperty(Marshaller.JAXB_FRAGMENT, Boolean.TRUE);     }      public void open(String filename) throws XMLStreamException, IOException     {         xmlOut = XMLOutputFactory.newFactory().createXMLStreamWriter(new FileOutputStream(filename));         xmlOut.writeStartDocument();         xmlOut.writeStartElement("rootElement");     }      public void write(T t) throws JAXBException     {         JAXBElement<T> element = new JAXBElement<T>(QName.valueOf(type.getSimpleName()), type, t);         marshaller.marshal(element, xmlOut);     }      public void close() throws XMLStreamException     {         xmlOut.writeEndDocument();         xmlOut.close();     } } 

Answer 2

As you've discovered, if a class does not have the @XmlRootElement annotation, then you can't pass an instance of that class to the marshaller. However, there is an easy way around this - wrap the object in a JAXBElement, and pass that to the marshaller instead.

Now JAXBElement is a rather clumsy beast, but what it does is contains the element name and namespace of the object that you want to marshal, information which would normally be contained in the @XmlRootElement annotation. As long as you have the name and namespace, you can construct a JAXBElement to wrap your POJO, and marshal that.

If your POJOs were generated by XJC, then it will also have generated an ObjectFactory class which contains factory methods for building JAXBElement wrappers for you, making things a bit easier.

You'll still have to use the JAXB_FRAGMENT property for the repeating inner elements, otherwise JAXB will generate stuff like the XML prolog each time, which you don't want.