Which pattern is suitable to send multiple type of responses through REST web service?

Question

I have bunch of REST style web services which support XML and JSON type responses. Now I have to modify existing web services such that they provide Protobuf type of responses.

Developer who has developed these services has created three layers. Service layer, DAO layer and Processing layer.

Service layer calls one of the method of processing layer and processing layer calls DAO methods and returns either JSON or XML type response (POJO) to the service. Service layer then sends back this POJO to client.

For example, if service to add list of items is called then that code roughly looks as shown below:

@Path("/item")
class ShoppingItemService {
    // annotations
    public ItemAddResponse addList(/* parameters required for this method */) {
        ItemProcessor processor = new ItemProcessor();
        ItemAddResponse response = processor.process(/* pass some data to processing layer */);
        return Response.ok().format("XML").build(response);
    }
}

Here ItemAddResponse is already annotated with JAXB type annotations for XML response and other annotations for JSON response.

I cannot reuse same ItemAddResponse because Protobuf generates its own classes using schema files.

Processor class has all the business logic and If I want to add support for Protobuf then I have create new method in processor and copy-paste almost all the business logic, but just return object of protobuf generated class.

This approach is time consuming and there atleast 50+ similar methods where I have to do copy-paste work. I want to avoid this and would like to design some elegant solution.

I would like to know how can I add support for Protobuf with minimal amount of changes?

I am reading Head First Design patterns book but this book is really huge and it will take lot of time to find proper design pattern.

Is there any design pattern which can be useful in this type of situations?

Thanks,

Chandrashekhar

Answer 1

If I want to add support for Protobuf then I have create new method in processor and copy-paste almost all the business logic

NO!

An object oriented approach

• ItemProcessor "has all the business logic". Don't pollute the class, keep it strictly business logic
• Your data/response structure is universal. JSON, XML, protobuf are just different syntactical representations of the same thing.
• All your existing layers operate on the same data structure, all they need is to receive or send in their desired form (JSON, XML, protobuf)
• You need something that transforms between the various formats so that the client, ItemProcessor, whatever, all get the format they expect.

OO Design Sketch

• A ItemProcessor instance needs a Translator object to get the data to or from, or both, the desired form.

• ItemProcessor maybe needs different Translators for interacting w/ the client and DAO respectively.

• So we need to make a ItemProcessor/Translator composite at run time. creational design patterns help us do that.

• It may be that you end up designing a single, static, Translator class.

• Design patterns do not dictate your design. To start with a blank sheet of paper and say "I want to use the xyz design pattern" is wrong, wrong, wrong.

Fitting a pattern to your design

You still need to design things. Only you can define the various parts overall. From there, a creational pattern shows us how to decouple the creation of an object from its user. In this way we avoid the problem as quoted at the beginning of this answer.

Look at creation design patterns like factory, abstract factory, and builder.

Which design pattern you use is generally driven by the complexity of building the given object. That there are 3 kinds of factory has to do with complexity.

You will tell the factory, etc. to build you something with "JSON - protobuffer" transformation for example. So maybe you get back a ItemProcessor that has a JSONprotobufTranformer object, or XMLprotobufTransformer, or ... whatever.

Answer 2

I wonder each input parameter is different or not. Anyway you may be able to return result as Object and cast it. Also you may be able to get parameter as Object.

You may be looking for that how to generate methods in runtime. https://stackoverflow.com/q/6680674

I recommend you'd better make all methods. I think It may be good for readability.