The term REST stands for REpresentational State Transfer, and it describes the architectural technique used to facilitate communication between different systems via the web. Stateless requests are sent from a client to either retrieve or make changes to a resource stored on a server. The server provides a response based on the information received in the request. Check out this article to learn more about REST architecture.

In order to provide a response to a request, the server maps the request to an endpoint. In Spring, a class is marked as a controller and each of its methods is associated with an endpoint. The method is called to formulate a response to each request. This is facilitated by two important annotations:

• The @RestController annotation is used to declare a class as a controller that can provide application-specific types of data as part of an HTTP response
• The @RequestMapping annotation is used to wire request types and endpoints to specific class methods.

The @RestController annotation should be added to each class that will handle responses to HTTP requests. The @RestController combines the functionality of two separate annotations, @Controller and @ResponseBody.

• @Controller is used to make a class identifiable to Spring as a component of the web application.
• @ResponseBody tells Spring to convert the return values of a controller’s methods to JSON and bind that JSON to the HTTP response body. Since almost every major programming language offers some type of JSON (JavaScript Object Notation) support, we can easily take an object created in Spring and translate it to JSON which can be easily parsed in the HTML and displayed on the page.

@RequestMapping accepts several arguments. The first, and perhaps most important, is the path. The path argument is used to determine where requests are mapped. For example, if a user makes a request to get a “book” resource, the server will send the request to a method with the annotation @RequestMapping(path = "/book") and that method would provide the response.

The @RequestMapping annotation also accepts several other arguments including method, params, and consumes. Let’s take a look at them.

• method: allows the developer to specify which HTTP method should be used when accessing the controller method. The most common are RequestMethod.GET, ...POST, ...PUT, and ...DELETE. Since this is an optional argument, if we do not specify an HTTP method it will be defaulted to GET.

• params: filters requests based on given parameters.

• consumes: used to specify which media type can be consumed; some common media types are "text/plain", "application/JSON", etc.

@Controller
public class VirtualLibrary{

  @RequestMapping("/books/all", RequestMethod.GET)
  public Book getAllBooks() {
    return getBook();
  }

}