JAXB and Immutable Objects

I was recently sent a blog post:  JAXB and immutable objects, by Aniket Shaligram.  In this post the author addresses the issue by adding a default constructor and using field access, to avoid adding set methods.  This approach is perfectly valid, but leaves the class being more mutable than some people like (including the person who sent me the link).  In this post I'll discuss an alternative approach using XmlAdapter.

Java Model

The following object model will be used for this example:

package blog.immutable;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement(name="purchase-order")
public class PurchaseOrder {

    private Customer customer;

    public Customer getCustomer() {
        return customer;
    }

    public void setCustomer(Customer customer) {
        this.customer = customer;
    }

}

Customer will be our immutable object.   Note how the name and address fields are final and do not have corresponding setters, the values may only be set once via the constructor.

package blog.immutable;

public class Customer {

    private final String name;
    private final Address address;

    public Customer(String name, Address address) {
        this.name = name;
        this.address = address;
    }

    public String getName() {
        return name;
    }

    public Address getAddress() {
        return address;
    }

}

package blog.immutable;

public class Address {

    private String street;

    public String getStreet() {
        return street;
    }

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

} 

Mapping the Immutable Class

JAXB does not know how to deal with the Customer class since it does not have a no-arg constructor and its fields are final. To solve this we need to introduce a class that JAXB does know how to handle:

package blog.immutable.adpater;

import javax.xml.bind.annotation.XmlAttribute;
import blog.immutable.Address;

public class AdaptedCustomer {

    private String name;
    private Address address;

    @XmlAttribute
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Address getAddress() {
        return address;
    }

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

}

Then we will leverage an XmlAdapter to convert between the unmappable Customer and the mappable AdaptedCustomer classes:

package blog.immutable.adpater;

import javax.xml.bind.annotation.adapters.XmlAdapter;
import blog.immutable.Customer;

public class CustomerAdapter extends XmlAdapter<AdaptedCustomer, Customer> {

    @Override
    public Customer unmarshal(AdaptedCustomer adaptedCustomer) throws Exception {
        return new Customer(adaptedCustomer.getName(), adaptedCustomer.getAddress());
    }

    @Override
    public AdaptedCustomer marshal(Customer customer) throws Exception {
        AdaptedCustomer adaptedCustomer = new AdaptedCustomer();
        adaptedCustomer.setName(customer.getName());
        adaptedCustomer.setAddress(customer.getAddress());
        return adaptedCustomer;
    }

}

Then we annotate the Customer class to leverage our XmlAdapter.  Now any field/property of type Customer will automatically leverage the XmlAdapter during marshal and unmarshal operations:

package blog.immutable;

import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;
import blog.immutable.adpater.CustomerAdapter;

@XmlJavaTypeAdapter(CustomerAdapter.class)
public class Customer {

    private final String name;
    private final Address address;

    public Customer(String name, Address address) {
        this.name = name;
        this.address = address;
    }

    public String getName() {
        return name;
    }

    public Address getAddress() {
        return address;
    }

}

Summary

As can be seen from this example, with XmlAdapter there is no such thing as an unmapped class with JAXB. For more information on XmlAdapter see:

• XmlAdapter - JAXB's Secret Weapon