@XmlTransformation - Going Beyond XmlAdapter

XML Adapter is my favourite JAXB feature, which is why it was the topic of my first post XmlAdapter - JAXB's Secret Weapon.  With it there is no such thing as an unmappable object in JAXB.  However today I came across a use case where the MOXy's transformation mapping was a much better fit.


The Use Case

The user wanted to map the following XML to objects.  The interesting point is that they wanted to combine the values of the DATE/TIME elements into one java.util.Date object. 

<ELEM_B> 
    <B_DATE>20100825</B_DATE> 
    <B_TIME>153000</B_TIME> 
    <NUM>123</NUM> 
    <C_DATE>20100825</C_DATE> 
    <C_TIME>154500</C_TIME> 
</ELEM_B> 

XmlAdapter would normally be a good fit here except for the following factors:

• The DATE/TIME pairings are repeated throughout the document, but each time the element names change (e.g.  B_DATE/B_TIME, and C_DATE/C_TIME).
• Each pairing is missing a grouping element.  This means we would need to adapt the entire ElemB class.

The Solution

In this case the transformation mapping concept in MOXy will make this use case much easier to map.  Let's look at how this looks:

import java.util.Date;

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

import org.eclipse.persistence.oxm.annotations.XmlReadTransformer;
import org.eclipse.persistence.oxm.annotations.XmlTransformation;
import org.eclipse.persistence.oxm.annotations.XmlWriteTransformer;
import org.eclipse.persistence.oxm.annotations.XmlWriteTransformers;

@XmlAccessorType(XmlAccessType.FIELD)
@XmlRootElement(name="ELEM_B")
public class ElemB {

    @XmlReadTransformer(transformerClass=DateAttributeTransformer.class)
    @XmlWriteTransformers({
        @XmlWriteTransformer(xmlPath="B_DATE/text()", transformerClass=DateFieldTransformer.class),
        @XmlWriteTransformer(xmlPath="B_TIME/text()", transformerClass=TimeFieldTransformer.class),
    })
    private Date bDate;

    @XmlElement(name="NUM")
    private int num;

    @XmlReadTransformer(transformerClass=DateAttributeTransformer.class)
    @XmlWriteTransformers({
        @XmlWriteTransformer(xmlPath="C_DATE/text()", transformerClass=DateFieldTransformer.class),
        @XmlWriteTransformer(xmlPath="C_TIME/text()", transformerClass=TimeFieldTransformer.class),
    })
    private Date cDate;

}

XML Read Transformer

An XML read transformer is responsible for constructing the object value from XML.

import java.text.ParseException;
import java.text.SimpleDateFormat;

import org.eclipse.persistence.internal.helper.DatabaseField;
import org.eclipse.persistence.mappings.foundation.AbstractTransformationMapping;
import org.eclipse.persistence.mappings.transformers.AttributeTransformer;
import org.eclipse.persistence.sessions.Record;
import org.eclipse.persistence.sessions.Session;

public class DateAttributeTransformer implements AttributeTransformer {

    private AbstractTransformationMapping mapping;
    private SimpleDateFormat yyyyMMddHHmmss = new SimpleDateFormat("yyyyMMddHHmmss");

    public void initialize(AbstractTransformationMapping mapping) {
        this.mapping = mapping;
    }

    public Object buildAttributeValue(Record record, Object instance, Session session) {
        try {
            String dateString = null;
            String timeString = null;
            
            for(DatabaseField field : mapping.getFields()) {
                if(field.getName().contains("DATE")) {
                    dateString = (String) record.get(field);
                } else {
                    timeString = (String) record.get(field);
                }
            }
            return yyyyMMddHHmmss.parseObject(dateString + timeString);
        } catch(ParseException e) {
            throw new RuntimeException(e);
        }
    }

}

XML Write Transformer(s)

An XML write transformer is responsible for constructing an XML value from the object.  A transformation mapping may have multiple write transformers.  For this example we will have two.

The first XML write transformer is responsible for writing out the year, month, and day information in the format yyMMdd.

import java.text.SimpleDateFormat;
import java.util.Date;

import org.eclipse.persistence.mappings.foundation.AbstractTransformationMapping;
import org.eclipse.persistence.mappings.transformers.FieldTransformer;
import org.eclipse.persistence.sessions.Session;

public class DateFieldTransformer implements FieldTransformer {

    private AbstractTransformationMapping mapping;
    private SimpleDateFormat yyyyMMdd = new SimpleDateFormat("yyyyMMdd");

    public void initialize(AbstractTransformationMapping mapping) {
        this.mapping = mapping;
    }

    public Object buildFieldValue(Object instance, String xPath, Session session) {
        Date date = (Date) mapping.getAttributeValueFromObject(instance);
        return yyyyMMdd.format(date);
    }

}

The second XML write transformer is responsible for writing out the hour, minute, and second information in the format HHmmss.

import java.text.SimpleDateFormat;
import java.util.Date;

import org.eclipse.persistence.mappings.foundation.AbstractTransformationMapping;
import org.eclipse.persistence.mappings.transformers.FieldTransformer;
import org.eclipse.persistence.sessions.Session;

public class TimeFieldTransformer implements FieldTransformer {

    private AbstractTransformationMapping mapping;
    private SimpleDateFormat HHmmss = new SimpleDateFormat("HHmmss");

    public void initialize(AbstractTransformationMapping mapping) {
        this.mapping = mapping;
    }

    public Object buildFieldValue(Object instance, String xPath, Session session) {
        Date date = (Date) mapping.getAttributeValueFromObject(instance);
        return HHmmss.format(date);
    }

}

jaxb.properties

In order to use the MOXy JAXB implementation you need to add a jaxb.properties file in which your model classes with the following entry:

javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory

Demo

The following class can be used to demonstrate the mapping:

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(ElemB.class);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("input.xml");
        ElemB elemB = (ElemB) unmarshaller.unmarshal(xml);

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

}

Please Note

For this example I have specified the transformation mapping using the annotations that will be available in the upcoming EclipseLink 2.2 release.  You can try these out today by dowloading one of the nightly builds:

• http://www.eclipse.org/eclipselink/downloads/nightly.php

Equivalent functionality is available in all released versions of EclipseLink, if you are interested in how to configure it please contact me or leave a comment.