Question

According to the documentation JAXB factory methods do not have arguments. Is there a JAXB implementation that allow me to create a factory method that receives as a parameter the class of the object I need to create ?
It happens that all my JAXB objects follow the same creation pattern (a particular byte code instrumentation), therefore I would like to encapsulate this in one single factory method having as a parameter the class of the JAXB object to create, avoiding in this way the creation of different factory methods for each JAXB class that basically do exactly the same thing.

I found someone asking the same question in an OTN forum: https://forums.oracle.com/forums/thread.jspa?messageID=9969927#9969927, but not a real answer has been proposed yet.

Thanks for any help

Answer 1

This is currently not possible using the standard JAXB APIs. I have entered the following enhancement request to have this behaviour added to EclipseLink JAXB (MOXy):

• https://bugs.eclipse.org/363192

MOXy Specific Solution

You could leverage the @XmlCustomizer extension in EclipseLink JAXB (MOXy) to customize how the objects are instantiated. This mechanism is leveraged to tweak MOXy’s underlying metadata.

CommonFactory

import java.util.Date;

public class CommonFactory {

    public static Object create(Class<?> clazz) {
        if(Foo.class == clazz) {
            return new Foo(new Date());
        } else if(Bar.class == clazz) {
            return new Bar(new Date());
        }
        return null;
    }

}

Foo.class

The Foo class is annotated normally except that we will use the @XmlCustomizer annotation to specify a DescriptorCustomizer that we are going to use to tweak MOXy’s metadata.

import java.util.Date;
import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.XmlCustomizer;

@XmlRootElement
@XmlType(factoryClass=CommonFactory.class, factoryMethod="create")
@XmlAccessorType(XmlAccessType.FIELD)
@XmlCustomizer(FactoryCustomizer.class)
public class Foo {

    private Date creationDate;
    private Bar bar;

    // Non-default constructor
    public Foo(Date creationDate) {
        this.creationDate = creationDate;
    }

}

Bar

Again we will use the @XmlCustomizer annotation to reference the same DescriptorCustomizer that we did in the Foo class.

import java.util.Date;
import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.XmlCustomizer;

@XmlType(factoryClass=CommonFactory.class, factoryMethod="create")
@XmlAccessorType(XmlAccessType.FIELD)
@XmlCustomizer(FactoryCustomizer.class)
public class Bar {

    private Date creationDate;

    // Non-default constructor
    public Bar(Date creationDate) {
        this.creationDate = creationDate;
    }

}

FactoryCustomizer

MOXy has the concept of an InstantiationPolicy to build new objects. In this example we will swap in our own instance InstantiationPolicy that can use parameterized factory methods:

import org.eclipse.persistence.config.DescriptorCustomizer;
import org.eclipse.persistence.descriptors.ClassDescriptor;
import org.eclipse.persistence.exceptions.DescriptorException;
import org.eclipse.persistence.internal.descriptors.InstantiationPolicy;
import org.eclipse.persistence.internal.security.PrivilegedAccessHelper;
import org.eclipse.persistence.internal.sessions.AbstractSession;

public class FactoryCustomizer implements DescriptorCustomizer{

    @Override
    public void customize(ClassDescriptor descriptor) throws Exception {
        descriptor.setInstantiationPolicy(new MyInstantiationPolicy(descriptor));
    }

    private static class MyInstantiationPolicy extends InstantiationPolicy {

        public MyInstantiationPolicy(ClassDescriptor descriptor) {
            InstantiationPolicy defaultInstantiationPolicy = descriptor.getInstantiationPolicy();
            this.factoryClassName = defaultInstantiationPolicy.getFactoryClassName();
            this.factoryClass = defaultInstantiationPolicy.getFactoryClass();
            this.methodName = defaultInstantiationPolicy.getMethodName();
        }

        @Override
        public void initialize(AbstractSession session) throws DescriptorException {
            super.initialize(session);
        }

        @Override
        protected void initializeMethod() throws DescriptorException {
            Class<?>[] methodParameterTypes = new Class[] {Class.class};
            try {
                this.method = PrivilegedAccessHelper.getMethod(factoryClass, methodName, methodParameterTypes, true);
            } catch (NoSuchMethodException e) {
                throw new RuntimeException(e);
            }
        }

        @Override
        public Object buildNewInstance() throws DescriptorException {
            Object[] parameters = new Object[] {this.descriptor.getJavaClass()};
            try {
                return PrivilegedAccessHelper.invokeMethod(method, factory, parameters);
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }

    }

}

Demo

import java.io.StringReader;
import javax.xml.bind.*;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Foo.class);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        Foo foo = (Foo) unmarshaller.unmarshal(new StringReader("<foo><bar/></foo>"));

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(foo, System.out);
    }

}

Output

<?xml version="1.0" encoding="UTF-8"?>
<foo>
   <creationDate>2011-11-08T12:35:43.198</creationDate>
   <bar>
      <creationDate>2011-11-08T12:35:43.198</creationDate>
   </bar>
</foo>

For More Information

• http://blog.bdoughan.com/2011/06/jaxb-and-factory-methods.html
• http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html