Convert Java object to XML

I am trying to convert a Java object inside of a Java library to an XML file. However, I got this problem:

A a = new A();

// initializing for a

JAXBContext jc = JAXBContext.newInstance("libraryA.A");

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

Then I got this exception:

javax.xml.bind.JAXBException: "libraryA.a" doesnt contain ObjectFactory.class or jaxb.index
    at com.sun.xml.internal.bind.v2.ContextFactory.createContext(ContextFactory.java:186)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at javax.xml.bind.ContextFinder.newInstance(ContextFinder.java:128)
    at javax.xml.bind.ContextFinder.find(ContextFinder.java:290)
    at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:372)
    at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:337)
    at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:244)

If I change: JAXBContext jc = JAXBContext.newInstance("libraryA.a");

to:

JAXBContext jc = JAXBContext.newInstance(libraryA.A.class);

then I have another exception:

com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException: 2 counts of IllegalAnnotationExceptions

library.A is an interface, and JAXB can't handle interfaces.
    this problem is related to the following location:
        at library.A

library.A does not have a no-arg default constructor.
    this problem is related 

to the following location:
    at library.A

Background Info (From Related Question)

From the comment you made on my answer to your previous question the domain model is already being used with JAXB. The easiest way to have your client and server communicate via XML is to leverage the already annotated model on both ends.

I just have checked the source code of my client. In the process, we need to convert back a xml file which is generated from java objects to xml file using: javax.xml.bind.JAXBContext & javax.xml.bind.Marshaller. so my question is it possible to read back the xml file to the same java objects? then we can use the java objects for a further step. Thanks in advance!

---

UPDATE

It appears as though your issue is due to having a domain model that is defined through interfaces with backing implementation classes. Below I'll demonstrate how you can handle this using a JAXB implementation (Metro, MOXy, JaxMe, etc).

Demo Code

import java.io.File;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;

public class Demo {

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

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("input.xml");
        Customer customer = (Customer) unmarshaller.unmarshal(xml);

        Address address = customer.getAddress();
        System.out.println(address.getStreet());

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

}

Interface Model

The following interfaces represent our domain model. These interfaces when not be leveraged to bootstrap the JAXBContext.

Customer

public interface Customer {

    public Address getAddress();

    public void setAddress(Address address);

}

Address

public interface Address {

    public String getStreet();

    public void setStreet(String street);

}

Implementation Classes

The implementation classes are what will be mapped to XML using JAXB.

CustomerImpl

Note in the CustomerImpl class we use the @XmlElement annotation on the address property to specify the type is AddressImpl.

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement(name="customer")
public class CustomerImpl implements Customer {

    private Address address;

    @XmlElement(type=AddressImpl.class)
    public Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

}

AddressImpl

public class AddressImpl implements Address {

    private String street;

    public String getStreet() {
        return street;
    }

    public void setStreet(String street) {
        this.street = street;
    }

}

input.xml

<?xml version="1.0" encoding="UTF-8"?>
<customer>
    <address>
        <street>1 Any Street</street>
    </address>
</customer>