JAXB creating context and marshallers cost

The question is a bit theoretical, what is the cost of creating JAXB context, marshaller and unmarshaller?

I've found that my code could benefit from keeping the same JAXB context and possibly the same marshaller for all marshaling operations rather than creating context and marshaller on each marshaling.

So what is the cost of creating JAXB context and marshaller/unmarshaller? Is it okay to create context+marshaller for each marshaling operation or it's better to avoid it?

Note: I'm the EclipseLink JAXB (MOXy) lead and a member of the JAXB 2 (JSR-222) expert group.

JAXBContext is thread safe and should only be created once and reused to avoid the cost of initializing the metadata multiple times. Marshaller and Unmarshaller are not thread safe, but are lightweight to create and could be created per operation.

Ideally, you should have a singleton JAXBContext and local instances of Marshaller and Unmarshaller.

JAXBContext instances are thread-safe while Marshaller and Unmarshaller instances are not thread-safe and should never be shared across threads.

It's a pity that this isn't specifically described in the javadoc. What I can tell is that Spring uses a global JAXBContext, shared between threads, whereas it creates a new marshaller for each marshalling operation, with a javadoc comment in the code saying that JAXB marshallers are not necessarily thread-safe.

The same is said on this page:https://javaee.github.io/jaxb-v2/doc/user-guide/ch03.html#other-miscellaneous-topics-performance-and-thread-safety.

I would guess that creating a JAXBContext is a costly operation, because it involves scanning classes and packages for annotations. But measuring it is the best way to know.

JAXB 2.2 (JSR-222) has this to say, in section "4.2 JAXBContext":

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.

[..]

JAXBContext class is designed to be immutable and thus threadsafe. Given the amount of dynamic processing that potentially could take place when creating a new instance of JAXBContxt, it is recommended that a JAXBContext instance be shared across threads and reused as much as possible to improve application performance.

Unfortunately, the specification does not make any claims regarding thread-safety of Unmarshaller and Marshaller. So it is best to assume they are not.

I solved this problem using:

• shared thread safe JAXBContext and thread local un/marschallers
• (so theoretically, there will be as many un/marshaller instances as there are threads which accessed them)
• with synchronization only on un/marshaller's initialization.

public class MyClassConstructor {
    private final ThreadLocal<Unmarshaller> unmarshallerThreadLocal = new ThreadLocal<Unmarshaller>() {
        protected synchronized Unmarshaller initialValue() {
            try {
                return jaxbContext.createUnmarshaller();
            } catch (JAXBException e) {
                throw new IllegalStateException("Unable to create unmarshaller");
            }
        }
    };
    private final ThreadLocal<Marshaller> marshallerThreadLocal = new ThreadLocal<Marshaller>() {
        protected synchronized Marshaller initialValue() {
            try {
                return jaxbContext.createMarshaller();
            } catch (JAXBException e) {
                throw new IllegalStateException("Unable to create marshaller");
            }
        }
    };

    private final JAXBContext jaxbContext;

    private MyClassConstructor(){
        try {
            jaxbContext = JAXBContext.newInstance(Entity.class);
        } catch (JAXBException e) {
            throw new IllegalStateException("Unable to initialize");
        }
    }
}