A mapping framework is useful in a layered architecture where you are creating layers of abstraction by encapsulating changes to particular data objects vs. propagating these objects to other layers (i.e. external service data objects, domain objects, data transfer objects, internal service data objects). A mapping framework is ideal for using within Mapper type classes that are responsible for mapping data from one data object to another.
For distributed systems, a side effect is the passing of domain objects between different systems. Typically, you won't want internal domain objects exposed externally and won't allow for external domain objects to bleed into your system.
Mapping between data objects has been traditionally addressed by hand coding value object assemblers (or converters) that copy data between the objects. Most programmers will develop some sort of custom mapping framework and spend countless hours and thousands of lines of code mapping to and from their different data object.
A generic mapping framework solves these problems. Dozer is an open source mapping framework that is robust, generic, flexible, reusable, and configurable.
Data object mapping is an important part of layered service oriented architectures. Pick and choose the layers you use mapping carefully. Do not go overboard as there is maintenance and performance costs associated with mapping data objects between layers.
There could be different reasons of why application should support parallel object hierarhchies. To name a few:
In some cases it is efficient to guard your code base from frequently changing object hierarchy, which you do not control directly. In this case Dozer serves as a bridge between application and external objects. As mapping is performed in reflective manner not all changes break your API. For example if object changes from Number to String the code will keep working as this is resolved automatically.
Some frameworks impose Serialization constraints, which does not allow sending any Java objects through the wire. One of the popular examples is Google Web Toolkit (GWT) framework, which restricts developer to sending only objects compiled to JavaScript and marked as Serializable. Dozer helps to convert Rich Domain Model to Presentation Model, which satisfies GWT serialization requirements.
Dozer integrates nicely with frameworks such as Apache XmlBeans and JAXB implementations. With help of provided factory classes, conversion between domain model and Xml objects is defined in the same way as plain object to object mappings.
In complex enterprise application it is often valuable to separate design to several architectural layers. Each of them would reside on its own abstraction level. A typical simplified example would be presentation, domain and persistence layers. Each of this layers could have own set of Java Beans representing data relevant for this layer. It is not necessary that all data will travel to the upper levels of architecture. For example the same domain object could have different mappings dependant of the presentation layer requirements.