Meet Dozer

Integration between systems is one of the most typical reasons of failure in complex projects. Yet, we find ourselves doing it day after day, so after some strategic mapping you have to get your hands dirty and map your wonderful objects to that incredible garbage other vendors still insist to call code. Or was it the other way round?

Let's assume a simple interaction via web services exposed on, say, an i-Series platform. I don't know whether all RPGLE programmers work in the same way, but almost all services of that kind I've seen so far have a single port with a thousand parameters for a hundred of different purposes and it is up to you to try and understand which of them must be used in which occasion. If that was not enough (and for me it is) it seems there is a contest for the use of the most cryptic parameter names, for which vowels seem to be banished and six characters is considered more than ehough (when not a waste of space).

One of the many things you learn with DDD is that code should be as close as possibile to your mental representation of the domain: well, LDLSDC does not make me think of a list of documents, but maybe it's my fault.

That said, before I use those objects in my code I'll have to convert them. The typical mapper has a signature similar to the following one:

public MyObject convert(YourObject yours, MyObject mine)

or

public MyObject convert(YourObject yours, MyObject.class)

The implementation is normally something like this:

myObject.setPropertyOne(yourObject.getPropertyOne);
myObject.setPropertyTwo(yourObject.getPropertyTwoButHasDifferentName)

(repeat ad lib for each property to be mapped)

This is very frustrating and most of all error prone. This is where several libraries step forward, most of them based on reflection, each of them having pluses and minuses. So far, the most interesting one I've found is Dozer, a bean to bean mapper that recursively copies data from one object to another (of a different type, of course! where would the fun be otherwise?). By the way, integrating different systems is just one of many reasons you have for mapping between different objects.

Using Dozer is just as easy as downloading the jar file, adding it to the classpath (don't forget the dependencies!) and coding right away:

Mapper mapper = new DozerBeanMapper();
Destination destination = mapper.map(source, Destination.class);

(almost utterly copied from the official tutorial). That's it. All fields with the same name are automagically mapped, with Dozer taking care of the necessary conversions (at least the more common ones). Should you need to customize mappings you may add some xml files that define how Dozer should behave:

<?xml version="1.0" encoding="UTF-8"?>
<mappings xmlns="http://dozer.sourceforge.net"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://dozer.sourceforge.net
          http://dozer.sourceforge.net/schema/beanmapping.xsd">

  <configuration>
    <stop-on-errors>true</stop-on-errors>
    <date-format>MM/dd/yyyy HH:mm</date-format>
    <wildcard>true</wildcard>
  </configuration>

  <mapping>
    <class-a>yourpackage.yourSourceClassName</class-a>
    <class-b>yourpackage.yourDestinationClassName</class-b>
      <field>
        <A>yourSourceFieldName</A>
        <B>yourDestinationFieldName</B>
      </field>
  </mapping> 
            
  other custom class mappings would go here.......   
                   
</mappings>

And this is copied from the official manual.

You can also introduce a custom converter that you can reuse through your application:

<converter type="org.dozer.converters.TestCustomConverter" >
 <class-a>org.dozer.vo.CustomDoubleObject</class-a>
    <class-b>java.lang.Double</class-b>
</converter>

and its corresponding class:

public class TestCustomConverter implements CustomConverter {

    public Object convert(Object destination, Object source, Class destClass, Class sourceClass) {
        if (source == null) {
            return null;
        }
        CustomDoubleObject dest = null;
        if (source instanceof Double) {
            // check to see if the object already exists
            if (destination == null) {
                dest = new CustomDoubleObject();
            } else {
                dest = (CustomDoubleObject) destination;
            }
            dest.setTheDouble(((Double) source).doubleValue());
            return dest;
        } else if (source instanceof CustomDoubleObject) {
            double sourceObj = ((CustomDoubleObject) source).getTheDouble();
            return new Double(sourceObj);
        } else {
            throw new MappingException("Converter TestCustomConverter used incorrectly. Arguments passed in were:" + destination + " and " + source);
        }
    }
}

And now a personal reminder: if you have to map an array of objects, but you know the types that you will find in specified positions AND want to avoid class cast exceptions (and I guess you do), you can use this syntax:

<field>
 <a>myObjects[0]</a>
    <b>other.propOne</b>
    <a-hint>java.lang.Long</a-hint>    
</field>
<field>
 <a>myObjects[1]</a>
    <b>other.propTwo</b>
    <a-hint>java.lang.Short</a-hint>
</field>

Next thing I'm gonna try is the mapping for POJOs and JAXB objects...