JAXB element name based on object property

Question

I have to create object model for following XMLs:

XML sample 1:

<InvoiceAdd>
  <TxnDate>2009-01-21</TxnDate>
  <RefNumber>1</RefNumber>
  <InvoiceLineAdd>
  </InvoiceLineAdd>
</InvoiceAdd>

XML Sample 2:

<SalesOrderAdd>
  <TxnDate>2009-01-21</TxnDate>
  <RefNumber>1</RefNumber>
  <SalesOrderLineAdd>
  </SalesOrderLineAdd>
</SalesOrderAdd>

The XML output will be based on a single string parameter or enum. String txnType = "Invoice"; (or "SalesOrder");

I would use single class TransactionAdd:

@XmlRootElement
public class TransactionAdd {  
  public String txnDate;
  public String refNumber;

  private String txnType;
  ...

  public List<LineAdd> lines;
}

instead of using subclasses or anything else. The code which creates the TransactionAdd instance is the same for both types of transaction it only differs on the type.

This XML is used by a rather known product called QuickBooks and is consumed by QuickBooks web service - so I can't change the XML, but I want to make it easy to be able to set element name based on property (txnType).

I would consider something like a method to determine target element name:

@XmlRootElement
public class TransactionAdd {  
  public String txnDate;
  public String refNumber;

  private String txnType;
  ...

  public List<LineAdd> lines;

  public String getElementName() {
     return txnType + "Add";
  }
}

Different transactions will be created using following code:

t = new TransactionAdd();
t.txnDate = "2010-12-15";
t.refNumber = "123";
t.txnType = "Invoice";

The goal is to serialize t object with the top-level element name based on txnType. E.g.:

<InvoiceAdd>
   <TxnDate>2009-01-21</TxnDate>
   <RefNumber>1</RefNumber>
</InvoiceAdd>

In case of t.txnType = "SalesOrder" the result should be

<SalesOrderAdd>
   <TxnDate>2009-01-21</TxnDate>
   <RefNumber>1</RefNumber>
</SalesOrderAdd>

At the moment I see only one workaround with subclasses InvoiceAdd and SalesOrderAdd and using @XmlElementRef annotation to have a name based on class name. But it will need to instantiate different classes based on transaction type and also will need to have two other different classes InvoiceLineAdd and SalesOrderLineAdd which looks rather ugly.

Please suggest me any solution to handle this. I would consider something simple.

Answer 1

You could use an XmlAdapter for this. Based on the String value of the txnType property you would have the XmlAdapter marshal an instance of an Object corresponding to InvoiceLineAdd or SalesOrderLineAdd.

This is how it would look:

TransactionAdd

On the txnType property we will use a combination of @XmlJavaTypeAdapter and @XmlElementRef:

import javax.xml.bind.annotation.XmlElementRef;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlRootElement
public class TransactionAdd {

    private String txnType;

    @XmlJavaTypeAdapter(MyAdapter.class)
    @XmlElementRef
    public String getTxnType() {
        return txnType;
    }

    public void setTxnType(String txnType) {
        this.txnType = txnType;
    }

}

The adapted objects will look like:

AbstractAdd

import javax.xml.bind.annotation.XmlSeeAlso;

@XmlSeeAlso({InvoiceAdd.class, SalesOrderAdd.class})
public class AbstractAdd {

}

InvoiceAdd

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class InvoiceAdd extends AbstractAdd {

}

SalesOrderAdd

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class SalesOrderAdd extends AbstractAdd {

}

The XmlAdapter to convert between the String and the adapted objects will look like:

import javax.xml.bind.annotation.adapters.XmlAdapter;

public class MyAdapter extends XmlAdapter<AbstractAdd, String> {

    @Override
    public String unmarshal(AbstractAdd v) throws Exception {
        if(v instanceof SalesOrderAdd) {
            return "salesOrderAdd";
        }
        return "invoiceAdd";
    }

    @Override
    public AbstractAdd marshal(String v) throws Exception {
        if("salesOrderAdd".equals(v)) {
            return new SalesOrderAdd();
        } 
        return new InvoiceAdd();
    }

}

The following demo code can be used:

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(TransactionAdd.class);

        File xml = new File("input.xml");
        Unmarshaller unmarshaller = jc.createUnmarshaller();
        TransactionAdd ta = (TransactionAdd) unmarshaller.unmarshal(xml);

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

}

To produce/consume the following XML:

<transactionAdd>
    <salesOrderAdd/>
</transactionAdd>

For more information see:

• http://bdoughan.blogspot.com/2010/07/xmladapter-jaxbs-secret-weapon.html