Fibonacci backend service with Spring

This guide walks you through the process of creating a Fibonacci number application with Spring.

What You Will Build

You will build an backend application that will accept HTTP GET requests at: http://localhost:8080/fibonacci?n=5.

It will respond a static page that body will contain a n. fibonacci number. If the n is greater than 46 then return BAD REQUEST (500) response code.

The Fibonacci number are represented with the next expression:

f(n) = f(n - 1) + f(n - 2)


f(1) = 0
f(2) = 1
f(3) = 1
f(4) = 2
f(5) = 3
f(6) = 5

What You Need


Next steps help to build the Fibonacci application.

1. Fork the application

  1. Log in onto GitGub account to fork this application.
  2. Click on fork button top left on this page.
Click on fork button
  1. Select destination project on your own.
Click on fork button
  1. Start the development to follow the next instructions. Happy coding!

2. Develop Fibonacci application

The layers are shown on the next diagram.


2.1. Modify the Controller layer

In Spring’s approach to building web sites, HTTP requests are handled by a controller. You can easily identify the controller by the @RestController annotation.

@GetMapping(value = ["fibonacci"])
open fun fibonacci(@RequestParam n: Int): Int? {
   // TODO - If n is greater than 46 then return BAD REQUEST use HttpStatus
   return fibonacciService?.fibonacci(n)

This controller is concise and simple, but there is plenty going on. We break it down step by step.

The @GetMapping annotation ensures that HTTP GET requests to /fibonacci are mapped to the fibonacci() method.

The @RequestParam binds the value of the query string parameter n into the n parameter of the fibonacci() method. This query string parameter is required.

The point is that the controller layer call the service layer and return the business result of fibonacci calculation.

2.2. Modify Service Layer

Service layer are placed under service package. In Spring’s approach to build business tier is used with @Component or @Service annotations. In business function should be implemented the above-mentioned Fibonacci number service, that return the n. Fibonacci number.

Implement the Fibonacci algorithm under the next code:

fun fibonacci(n: Int): Int {
    return if (n == 1) 0
    else 0 // TODO instead of this logic implement fibonacci

2.3. Run the Application

After you complete the task you can run the application with the next Gradle command.

./gradlew.bat bootRun

3. Test the Application

Following the guideline test the application with two aspects

  • With unit tests let us test the service layer. Test the normal cases: f(1), f(2), ... and edge cases: f(-1).
  • With integration tests let us test controller layer with connected business tier. The the edge case too: f(47).

3.1. Unit tests

Example to test application with JUnit:

void shouldReturn0WhenCall1() {
    // given

    // when
    Integer result = underTest.fibonacci(1);
    // then
    Assertions.assertEquals(0, result);

3.2. Integration tests

Example to test controller layer with JUnit

void callFibonacciEndpoint() {
    // given

    // when
    ResponseEntity<String> entity = restTemplate.getForEntity("http://localhost:8080/fibonacci?n=5", String.class);

    // then
    Assertions.assertEquals(HttpStatus.OK, entity.getStatusCode());
    Assertions.assertEquals("3", entity.getBody());

3.3. Manual test

Now that the web site is running, visit http://localhost:8080/fibonacci?n=10, where you should see “55”


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