Question

I use the JAXBContext.newInstance operation in my JBoss based web application. This operation, as I understand, is very heavyweight. I only require two unique instances of the Marshaller class.

My initial proposal is to have a static initializer block that will initialize these two instances only once upon the class loading:

public class MyWebApp {
    private static Marshaller requestMarshaller;
    private static Marshaller responseMarshaller;

    static {
        try {
            // one time instance creation
            requestMarshaller = JAXBContext.newInstance(Request.class).createMarshaller();
            responseMarshaller = JAXBContext.newInstance(Response.class).createMarshaller();
        } catch (JAXBException e) {
            e.printStackTrace();
        }
    }

    private void doSomething() {
            requestMarshaller.marshall(...);
            responseMarshaller.marshall(...);
            ...
    }

}

If this is a reasonable solution then I guess I’ll have answered my own question, but I would like to know if this is the correct way to do this?

Answer 1

A JAXB implementation (Metro, EclipseLink MOXy, Apache JaxMe, etc) typically initializes its metadata during the JAXBContext.newInstance call. All OXM tools need to initialize mapping metadata at some point and try to minimize the cost of this operation. Since it is impossible to do it with zero cost, it is best to only do it once. Instances of JAXBContext are thread safe, so yes you only need to create it once.

From the JAXB 2.2 Specification, Section 4.2 JAXB Context:

To avoid the overhead involved in
creating a JAXBContext instance, a
JAXB application is encouraged to
reuse a JAXBContext instance. An
implementation of abstract class
JAXBContext is required to be
thread-safe, thus, multiple threads in
an application can share the same
JAXBContext instance.

Instances of Marshaller and Unmarshaller are not thread safe and must not be shared among threads, they are lightweight to create.