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Team lead for the TopLink/EclipseLink JAXB & SDO implementations, and the Oracle representative on those specifications. Blaise is a DZone MVB and is not an employee of DZone and has posted 44 posts at DZone. You can read more from them at their website. View Full User Profile

Binding to JSON & XML - Handling Collections

03.18.2013
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One of EclipseLink JAXB (MOXy)'s strengths is the ability to map an object model to both JSON and XML with a single set of metadata.  The one weakness had been that you needed to compromise on the JSON key or XML element for collection properties.  I'm happy to say that this issue has been solved in EclipseLink 2.5 (and EclipseLink 2.4.2), and I will demonstrate below with an example.
You can try this out today by downloading an EclipseLink 2.5.0 (or EclipseLink 2.4.2) nightly build starting on March 15, 2013 from:
Domain Model 
By default a JAXB (JSR-222) implementation will not output a grouping element around collection data.  This can be done through the use of the @XmlElementWrapper annotation (see:  JAXB & Collection Properties).  This grouping element often has a plural name and is a better fit for the key of a JSON array than the repeating element defined by the @XmlElement annotation is.
package blog.json.collections;
 
import java.util.*;
import javax.xml.bind.annotation.*;
 
@XmlRootElement
@XmlType(propOrder={"name", "emailAddresses"})
public class Customer {
 
    private String name;
    private List<String> emailAddresses = new ArrayList<String>();
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    @XmlElementWrapper(name="email-addresses")
    @XmlElement(name="email-address")
    public List<String> getEmailAddresses() {
        return emailAddresses;
    }
 
    public void setEmailAddresses(List<'String> emailAddresses) {
        this.emailAddresses = emailAddresses;
    }
 
}
Demo
We will specify the JSON_WRAPPER_AS_ARRAY_NAME property with a true value to tell MOXy that it should use the grouping element as the name for the JSON array value.  Then we will use the same Marshaller to output the same object to both XML and JSON.
package blog.json.collections;
 
import java.util.*;
import javax.xml.bind.*;
import org.eclipse.persistence.jaxb.MarshallerProperties;
 
public class Demo {
 
    public static void main(String[] args) throws Exception {
        Customer customer = new Customer();
        customer.setName("Jane Doe");
        customer.getEmailAddresses().add("jane.doe@example.com");
        customer.getEmailAddresses().add("jdoe@example.org");
 
        Map<String, Object> properties = new HashMap<String, Object>(1);
        properties.put(MarshallerProperties.JSON_WRAPPER_AS_ARRAY_NAME, true);
        JAXBContext jc = JAXBContext.newInstance(new Class[] {Customer.class}, properties);
 
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
 
        // Output XML
        marshaller.marshal(customer, System.out);
 
        // Output JSON
        marshaller.setProperty(MarshallerProperties.MEDIA_TYPE, "application/json");
        marshaller.marshal(customer, System.out);
    }
 
}
XML Output
Below is the XML output from running the demo code.  We see that email-addresses is marshalled as the grouping element which contains an email-address element for each item in the collection.
<?xml version="1.0" encoding="UTF-8"?>
<customer>
   <name>Jane Doe</name>
   <email-addresses>
      <email-address>jane.doe@example.com</email-address>
      <email-address>jdoe@example.org</email-address>
   </email-addresses>
</customer>
JSON Output
The following JSON output is produced from the same metadata.  The only difference is that we told MOXy to use the grouping element as the name for JSON array values.
{
   "customer" : {
      "name" : "Jane Doe",
      "email-addresses" : [ "jane.doe@example.com", "jdoe@example.org" ]
   }
}
JAX-RS 
You can easily use MOXy as your JSON-binding provider in a JAX-RS environment (see:  MOXy as your JAX-RS JSON Provider - MOXyJsonProvider).  You can specify that the grouping element should be used as the JSON array name with the wrapperAsArrayName property on MOXyJsonProvider.
package blog.json.collections;
 
import java.util.*;
import javax.ws.rs.core.Application;
import org.eclipse.persistence.jaxb.rs.MOXyJsonProvider;
  
public class CustomerApplication  extends Application {
  
    @Override
    public Set<Class<?>> getClasses() {
        HashSet<Class<?>> set = new HashSet<Class<?>>(1);
        set.add(CustomerService.class);
        return set;
    }
  
    @Override
    public Set<Object> getSingletons() {
        MOXyJsonProvider moxyJsonProvider = new MOXyJsonProvider();
        moxyJsonProvider.setWrapperAsArrayName(true);
   
        HashSet<Object> set = new HashSet<Object>(1);
        set.add(moxyJsonProvider);
        return set;
    }
  
} 
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