Spring MVC Framework and REST

Understanding REST

Since Spring Framework is the most popular and the standard framework for developing Java web application and RESTful Web Services.

While the traditional MVC controller relies on the View technology, the RESTful web service controller simply returns the object and the object data is written directly to the HTTP response as JSON/XML.
REST (Representational State Transfer) was introduced and defined in 2000 by Roy . REST is an architectural style for designing distributed systems. It is not a standard but a set of constraints, such as being stateless, having a client/server relationship, and a uniform interface. REST is not strictly related to HTTP, but it is most commonly associated with it

Principles of REST

• Resources expose easily understood directory structure URIs.
• Representations transfer JSON or XML to represent data objects and attributes.
• Messages use HTTP methods explicitly (for example, GET, POST, PUT, and DELETE).
• Stateless interactions store no client context on the server between requests. State dependencies limit and restrict scalability. The client holds session state.

HTTP methods

Use HTTP methods to map CRUD (create, retrieve, update, delete) operations to HTTP requests.

• GET     -    @RequestingMapping (method =  RequestMethod.GET)         or     @RequestingMapping ("/topics")
• POST   -    @RequestingMapping (method =  RequestMethod.POST   ,  value = "/topics")
• PUT     -    @RequestingMapping (method =  RequestMethod.PUT)
• DELETE  -    @RequestingMapping (method =  RequestMethod.DELETE)

HTTP Verb	CRUD	Entire Collection (e.g. /customers)	Specific Item (e.g. /customers/{id})
POST	Create	201 (Created), 'Location' header with link to /customers/{id} containing new ID.	404 (Not Found), 409 (Conflict) if resource already exists..
GET	Read	200 (OK), list of customers. Use pagination, sorting and filtering to navigate big lists.	200 (OK), single customer. 404 (Not Found), if ID not found or invalid.
PUT	Update/Replace	405 (Method Not Allowed), unless you want to update/replace every resource in the entire collection.	200 (OK) or 204 (No Content). 404 (Not Found), if ID not found or invalid.
PATCH	Update/Modify	405 (Method Not Allowed), unless you want to modify the collection itself.	200 (OK) or 204 (No Content). 404 (Not Found), if ID not found or invalid.
DELETE	Delete	405 (Method Not Allowed), unless you want to delete the whole collection—not often desirable.	200 (OK). 404 (Not Found), if ID not found or invalid.

HTTP status codes

Status codes indicate the result of the HTTP request.

• 1XX - informational
• 2XX - success
• 3XX - redirection
• 4XX - client error
• 5XX - server error

Using the @ResponseBody Annotation

When you use the @ResponseBody annotation on a method, Spring converts the return value and writes it to the http response automatically. Each method in the Controller class must be annotated with @ResponseBody.

Behind the Scenes

Spring has a list of HttpMessageConverters registered in the background. The responsibility of the HTTPMessageConverter is to convert the request body to a specific class and back to the response body again, depending on a predefined mime type. Every time an issued request hits @ResponseBody, Spring loops through all registered HTTPMessageConverters seeking the first that fits the given mime type and class, and then uses it for the actual conversion.

Using the @RestController Annotation

Spring 4.0 introduced @RestController, a specialized version of the controller which is a convenience annotation that does nothing more than add the @Controller and @ResponseBody annotations. By annotating the controller class with @RestController annotation, you no longer need to add @ResponseBody to all the request mapping methods. The @ResponseBody annotation is active by default.


References : –

• spring-framework-restcontroller-vs-controller
• Using HTTP Methods for RESTful Services 
• understanding/REST