In the previous exercise, we discussed the use of helper methods to specify the HTTP method being used. GET requests are used to retrieve information from the server, while POST, PUT, or DELETE requests are used to make changes to resources on the server. In this exercise, we’ll discuss HOW those changes are transmitted from the user to the endpoint.

When users need to pass data to the server, the term we use to describe the process is called binding. The prime example of binding data from a user’s request to an endpoint occurs when a user needs to submit data. Imagine an application where you enter a book’s ISBN and the response returned is either a yes, indicating the book is available for checkout, or a no, indicating the book is not available. When the user submits their request, we need a way to bind their entry to the endpoint that will provide the response. Thankfully, there’s a Spring annotation for that!

@RequestParam is an annotation that can be used at the method parameter level. It allows us to parse query parameters and capture those parameters as method arguments. This is incredibly helpful because we can take values passed in from the HTTP request, parse them, and then bind the values to a controller method for further processing. We also have the option of defining a default value for the method argument in case a value is not received from the client.

In this example, let’s say the user submits their request for a book with 2 authors and a publishing year of 1995.

libraryapp.com/book?author_count=2&published_year=1995

The author_count and published_year values would map to the method parameters as follows:

@GetMapping("/book")
public Book isBookAvailable(@RequestParam int author_count, @RequestParam int published_year) {
  return books.find(author_count, published_year);
}