Creating the Backend Application

Create a Spring Boot skeleton project using Spring Initializr.

Spring Boot reactive skeleton

To start with, the easiest option is to navigate to and create a Spring Boot application skeleton project.

Make sure to select a Spring Boot version 2.0 or newer, the Reactive WebReactive MongoDB, and the classic MongoDB dependencies. We’ll use Spring Boot 2.3 and Java 14 with Maven.

Spring Boot integrates the WebFlux capabilities so let’s use its power to set up our backend. The first change we apply is already in our dependency management thanks to the Spring Boot Initializr. We don’t include the classic web starter; instead, we include the WebFlux one (spring-boot-starter-webflux on line-31 in pom.xml). This starter contains the Reactor Netty server library dependencies, so Spring Boot will know it’s the one to start at runtime. We’ll also have the Reactor API available during development.

Have a look at the dependencies in the pom.xml that is generated below.

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="" xmlns:xsi=""
<description>Reactive Web demo project</description>
<relativePath/> <!-- lookup parent from repository -->

The reactive MongoDB dependencies are included within the starter spring-boot-starter-data-mongodb-reactive (line-43 in pom.xml). This one pulls the Spring Data MongoDB dependency, containing Spring Data Commons with reactive support, and the asynchronous and reactive-streams versions of the MongoDB driver. We also need the classic MongoDB starter (line-39 in pom.xml) to compare both alternatives.

Running the skeleton project

The following widget contains the skeleton project made with Spring Initializr. The following two changes have been made to the project so that it can run at this stage of creation:

  • The MongoDB dependencies have been commented out in pom.xml (We will use MongoDB later in the course).
  • A Spring MVC REST controller has been added to, which responds with “Hello, learner!” when a request to /hello/learner is made (lines 19-26).

The following Spring skeleton project was made for the purpose of demonstration, with Java version 15, Spring Boot version 2.4.0, and the default project metadata. The rest of the course follows the specifications mentioned at the start of this lesson.

 * Copyright 2007-present the original author or authors.
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * See the License for the specific language governing permissions and
 * limitations under the License.
import java.nio.channels.*;
import java.util.Properties;

public class MavenWrapperDownloader {

    private static final String WRAPPER_VERSION = "0.5.6";
     * Default URL to download the maven-wrapper.jar from, if no 'downloadUrl' is provided.
    private static final String DEFAULT_DOWNLOAD_URL = ""
        + WRAPPER_VERSION + "/maven-wrapper-" + WRAPPER_VERSION + ".jar";

     * Path to the file, which might contain a downloadUrl property to
     * use instead of the default one.
    private static final String MAVEN_WRAPPER_PROPERTIES_PATH =

     * Path where the maven-wrapper.jar will be saved to.
    private static final String MAVEN_WRAPPER_JAR_PATH =

     * Name of the property which should be used to override the default download url for the wrapper.
    private static final String PROPERTY_NAME_WRAPPER_URL = "wrapperUrl";

    public static void main(String args[]) {
        System.out.println("- Downloader started");
        File baseDirectory = new File(args[0]);
        System.out.println("- Using base directory: " + baseDirectory.getAbsolutePath());

        // If the exists, read it and check if it contains a custom
        // wrapperUrl parameter.
        File mavenWrapperPropertyFile = new File(baseDirectory, MAVEN_WRAPPER_PROPERTIES_PATH);
        String url = DEFAULT_DOWNLOAD_URL;
        if(mavenWrapperPropertyFile.exists()) {
            FileInputStream mavenWrapperPropertyFileInputStream = null;
            try {
                mavenWrapperPropertyFileInputStream = new FileInputStream(mavenWrapperPropertyFile);
                Properties mavenWrapperProperties = new Properties();
                url = mavenWrapperProperties.getProperty(PROPERTY_NAME_WRAPPER_URL, url);
            } catch (IOException e) {
                System.out.println("- ERROR loading '" + MAVEN_WRAPPER_PROPERTIES_PATH + "'");
            } finally {
                try {
                    if(mavenWrapperPropertyFileInputStream != null) {
                } catch (IOException e) {
                    // Ignore ...
        System.out.println("- Downloading from: " + url);

        File outputFile = new File(baseDirectory.getAbsolutePath(), MAVEN_WRAPPER_JAR_PATH);
        if(!outputFile.getParentFile().exists()) {
            if(!outputFile.getParentFile().mkdirs()) {
                        "- ERROR creating output directory '" + outputFile.getParentFile().getAbsolutePath() + "'");
        System.out.println("- Downloading to: " + outputFile.getAbsolutePath());
        try {
            downloadFileFromURL(url, outputFile);
        } catch (Throwable e) {
            System.out.println("- Error downloading");

    private static void downloadFileFromURL(String urlString, File destination) throws Exception {
        if (System.getenv("MVNW_USERNAME") != null && System.getenv("MVNW_PASSWORD") != null) {
            String username = System.getenv("MVNW_USERNAME");
            char[] password = System.getenv("MVNW_PASSWORD").toCharArray();
            Authenticator.setDefault(new Authenticator() {
                protected PasswordAuthentication getPasswordAuthentication() {
                    return new PasswordAuthentication(username, password);
        URL website = new URL(urlString);
        ReadableByteChannel rbc;
        rbc = Channels.newChannel(website.openStream());
        FileOutputStream fos = new FileOutputStream(destination);
        fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);

Spring Boot application skeleton project with a REST controller