Question

I’m building a simple RESTFul Service; and for achieve that I need two tasks:

• Get an instance of my resource (i.e Book) from request parameters, so I can get that instance to be persisted
• Build an XML document from that instance to send the representation to the clients

Right now, I’m doing both things in my POJO class:

public class Book implements Serializable {

    private Long id;

    public Book(Form form) {
        //Initializing attributes
        id = Long.parseLong(form.getFirstValue(Book.CODE_ELEMENT));
    }

    public Element toXml(Document document) {
        // Getting an XML Representation of the Book
        Element bookElement = document.createElement(BOOK_ELEMENT);
    }

I’ve remembered an OO principle that said that behavior should be where the data is, but now my POJO depends from Request and XML API’s and that doesn’t feels right (also, that class has persistence anotations)

Is there any standard approach/pattern to solve that issue?

EDIT:
The libraries i’m using are Restlets and Objectify.

Answer 1

I agree with you when you say that the behavior should be where the data is. But at the same time, as you say I just don’t feel confortable polluting a POJO interface with specific methods used for serialization means (which can grow considerably depending on the way you want to do it – JSON, XML, etc.).

1) Build an XML document from that instance to send the representation to the clients

In order to decouple the object from serialization logic, I would adopt the Strategy Pattern:

interface BookSerializerStrategy {
    String serialize(Book book);
}

public class XmlBookSerializerStrategy implements BookSerializerStrategy {

    public String serialize(Book book) {
        // Do something to serialize your book.
    }

}

public class JsonBookSerializerStrategy implements BookSerializerStrategy {

    public String serialize(Book book) {
        // Do something to serialize your book.
    }

}

You POJO interface would become:

public class Book implements Serializable {

    private Long id;
    private BookSerializerStrategy serializer

    public String serialize() {
        return serializer.serialize(this);
    }

    public void setSerializer(BookSerializerStrategy serializer) {
        this.serializer = serializer;
    }
}

Using this approach you will be able to isolate the serialization logic in just one place and wouldn’t pollute your POJO with that. Additionally, returning a String I won’t need to couple you POJO with classes Document and Element.

2) Get an instance of my resource (i.e Book) from request parameters, so I can get that instance to be persisted

To find a pattern to handle the deserialization is more complex in my opinion. I really don’t see a better way than to create a Factory with static methods in order to remove this logic from your POJO.

Another approach to answer your two questions would be something like JAXB uses: two different objects, an Unmarshaller in charge of deserialization and a Marshaller for serialization. Since Java 1.6, JAXB comes by default with JDK.

Finally, those are just suggestions. I’ve become really interested in your question actually and curious about other possible solutions.