JAXB creating context and marshallers cost

问题:

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?

回答1:

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.

回答2:

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.

回答3:

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: http://jaxb.java.net/guide/Performance_and_thread_safety.html.

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.

回答4:

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.

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");
    }
}

回答5:

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.

回答6:

Even better!! Based on the good solution from the post above, create the context just-once in the constructor, and save it instead of the class.

Replace the line:

  private Class clazz;

with this one:

  private JAXBContext jc;

And the main constructor with this one:

  private Jaxb(Class clazz)
  {
     this.jc = JAXBContext.newInstance(clazz);
  }

so in the getMarshaller/getUnmarshaller you can remove this line:

  JAXBContext jc = JAXBContext.newInstance(clazz);

This improvement makes, in my case, that processing times drops from 60~70ms to just 5~10ms

回答7:

I usually solve problems like this with a ThreadLocal class pattern. Given the fact that you need a different marshaller for each Class, you can combine it with a singleton-map pattern.

To save you 15 minutes, of work. Here follows my implementation of a thread-safe Factory for Jaxb Marshallers and Unmarshallers.

It allows you to access the instances as follows ...

Marshaller m = Jaxb.get(SomeClass.class).getMarshaller();
Unmarshaller um = Jaxb.get(SomeClass.class).getUnmarshaller();

And the code you will need is a little Jaxb class that looks as follows:

public class Jaxb
{
  // singleton pattern: one instance per class.
  private static Map<Class,Jaxb> singletonMap = new HashMap<>();
  private Class clazz;

  // thread-local pattern: one marshaller/unmarshaller instance per thread
  private ThreadLocal<Marshaller> marshallerThreadLocal = new ThreadLocal<>();
  private ThreadLocal<Unmarshaller> unmarshallerThreadLocal = new ThreadLocal<>();

  // The static singleton getter needs to be thread-safe too, 
  // so this method is marked as synchronized.
  public static synchronized Jaxb get(Class clazz)
  {
    Jaxb jaxb =  singletonMap.get(clazz);
    if (jaxb == null)
    {
      jaxb = new Jaxb(clazz);
      singletonMap.put(clazz, jaxb);
    }
    return jaxb;
  }

  // the constructor needs to be private, 
  // because all instances need to be created with the get method.
  private Jaxb(Class clazz)
  {
     this.clazz = clazz;
  }

  /**
   * Gets/Creates a marshaller (thread-safe)
   * @throws JAXBException
   */
  public Marshaller getMarshaller() throws JAXBException
  {
    Marshaller m = marshallerThreadLocal.get();
    if (m == null)
    {
      JAXBContext jc = JAXBContext.newInstance(clazz);
      m = jc.createMarshaller();
      marshallerThreadLocal.set(m);
    }
    return m;
  }

  /**
   * Gets/Creates an unmarshaller (thread-safe)
   * @throws JAXBException
   */
  public Unmarshaller getUnmarshaller() throws JAXBException
  {
    Unmarshaller um = unmarshallerThreadLocal.get();
    if (um == null)
    {
      JAXBContext jc = JAXBContext.newInstance(clazz);
      um = jc.createUnmarshaller();
      unmarshallerThreadLocal.set(um);
    }
    return um;
  }
}