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Reactive Spring Boot Restful Web Service + Google Books API

alexladeira profile image Alexandre Ladeira ・4 min read

The idea of this post, the first of a series, is to demonstrate how to write a Spring Boot Restful Web Service that access a remote API in a reactive way. My intention is to add more features, like caching and database, always using the reactive paradigm. In this first one, I will describe how you can start from zero to the point that you have a functional web service.

Getting Started

I based this on the Spring tutorial Building a Reactive RESTful Web Service, and build it from the scratch, copying only the initial Gradle build file. The project structure is as this following image.

Alt Text

I created a default Spring Boot Application class, the only difference is the @ConponentScan annotation that points to the base package of the application.

package dev.alexladeira.springboot.reactive;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;

@ComponentScan(basePackages = {"dev.alexladeira.springboot.reactive"})
public class Application {
    public static void main(String[] args) {, args);

Now the game begins!

Writing the Router class

Thought the Router class I put the route that you going to expose, this is a simple class that create the link between the service that I was exposing and the handler that I create to handle the request.

package dev.alexladeira.springboot.reactive.routes;

import dev.alexladeira.springboot.reactive.handlers.GoogleBooksHandler;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.reactive.function.server.RequestPredicates;
import org.springframework.web.reactive.function.server.RouterFunction;
import org.springframework.web.reactive.function.server.RouterFunctions;
import org.springframework.web.reactive.function.server.ServerResponse;

public class SearchRouter {
    public RouterFunction<ServerResponse> search(GoogleBooksHandler googleBooksHandler) {
        return RouterFunctions.route(RequestPredicates.GET("/search"), googleBooksHandler::search);

Until now I did not need to use Spring Reactor, this changed when I wrote the handler.

Writing the Handler and The Service classes

I created the handler to handle the request, call the external resource and create a response. Here I got the searchTerm from the request object and pass it to the service that is responsible for returning some information about the books that has the word that I passed as a parameter, in this example I was using the Google Books API.

package dev.alexladeira.springboot.reactive.handlers;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.stereotype.Component;
import org.springframework.web.reactive.function.BodyInserters;
import org.springframework.web.reactive.function.server.ServerRequest;
import org.springframework.web.reactive.function.server.ServerResponse;
import reactor.core.publisher.Mono;

import java.util.ArrayList;
import java.util.List;

public class GoogleBooksHandler {

    private GoogleBooksService googleBooksService;

    public Mono<ServerResponse> search(ServerRequest request) {
        String searchTerm = request.queryParam("searchTerm").orElse(null);
        return searchTerm != null ? ServerResponse.ok().body(BodyInserters.fromPublisher(this.googleBooksService.getBooksBy(searchTerm).reduce(new ArrayList<GoogleBook>(), (list, googleBookServiceResponse) -> {
            return list;
        }), List.class)) : ServerResponse.badRequest().build();

The job of the search method is just get the books from the Google Books API, reduce then to a list of GoogleBooks type, a java class that has only the info that I want the service to return, and create the response. If the service receive and empty call (without any parameters), then an error is thrown.

The service class is as follow, a call to the API via a WebClient object


import org.springframework.stereotype.Service;
import org.springframework.web.reactive.function.client.WebClient;
import reactor.core.publisher.Flux;

public class GoogleBooksService implements GenericService<GoogleBookServiceResponse> {
    private WebClient webClient = WebClient.builder().baseUrl("").build();

    public Flux<GoogleBookServiceResponse> getBooksBy(String searchTerm) {
        return this.webClient.get().uri(uriBuilder -> uriBuilder
                .queryParam("q", searchTerm)
                .queryParam("maxResults", MAX_RESULTS)

This class extends GenericService, a class created to concentrated some information that is going to be used by the others services that I will create in the future, the constants MAX_RESULTS and TIMEOUT.

Conclusion... for now

It's time start the server and see the results, point your browser to http://localhost:8080, if everything went well, you will see this:

[{"volumeInfo":{"title":"Learning Spring Boot 2.0","authors":["Greg L. Turnquist"],"printType":"BOOK"}},{"volumeInfo":{"title":"Spring Boot 2.0 Projects","authors":["Mohamed Shazin Sadak
ath"],"printType":"BOOK"}},{"volumeInfo":{"title":"Spring: Microservices with Spring Boot","authors":["Ranga Rao Karanam"],"printType":"BOOK"}},{"volumeInfo":{"title":"Pro Spring Boot","
authors":["Felipe Gutierrez"],"printType":"BOOK"}},{"volumeInfo":{"title":"Mastering Spring Boot 2.0","authors":["Dinesh Rajput"],"printType":"BOOK"}}]

The source code is at I will follow this post with others, always trying to show what I've being learning in SpringBoot topic.


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