In this article, I would like to suggest a few free open-source projects developed using Spring Boot for learning purposes. So please go ahead, check out the source code, and have a hands-on experience on real projects.
Along with Spring Boot, you will also learn Spring framework modules, Microservices, Thymeleaf, Maven, JWT, React Js, Elastic Search, WebSocket, JSP, MySQL, Hibernate 5, Spring Data JPA, etc.

Learn and master in spring boot on Master in Spring Boot

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Video

This blog post is explained very well in below YouTube video:

10+ Free Open Source Projects Using Spring Boot

1. Project Sagan

This is a real-world app that powers the Spring.io website. It is in production and used by thousands of users each day.

Sagan project is an open-source and available on the GitHub repository: https://github.com/RameshMF/sagan

I highly suggest this project, you will learn everything about the Spring framework ecosystem, Elasticsearch, Gradle, and also Thymeleaf UI stuff.

Quick reference links to the Sagan project:

1. Sagan - Get source code from GitHub

2. Sagan - Run the Sagan site application locally
3. Sagan - Import Sagan project sources into your IDE of choice.
4. Sagan - Sagan Wiki
5. Live demo on - Spring.io

Learn and master in spring boot on Master in Spring Boot

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2. ReactJS Spring Boot CRUD Full Stack App

In this full-stack project, we will learn how to develop a simple end-to-end full-stack web application that is a basic Employee Management Application using React, and Spring Boot.

I have created this project for my free ReactJS + Spring Boot CRUD Full Stack App course on YouTube.

We will build two projects:

sprintboot-backend (server) – To develop REST API

react-frontend (client) – Consume REST API

In this project, you will learn how to build a full-stack web application that is basic Employee Management Application with CRUD features:

Create Employee

List Employee

Update Employee

Delete Employee

View Employee

Following is the screenshot of the final version of our application -

GitHub repository - https://github.com/RameshMF/ReactJS-Spring-Boot-CRUD-Full-Stack-App

Blog post - https://www.javaguides.net/2020/07/spring-boot-react-js-crud-example-tutorial.html

YouTube playlist - ReactJS + Spring Boot CRUD Full Stack App - free course

3. Spring Initializr

Spring Initializr is a great tool developed by the spring team to quickly bootstrap your Spring Boot projects. This is an open-source project hosted on GitHub.

There are many ways to create a Spring Boot application. The simplest way is to use Spring Initializr at http://start.spring.io/, which is an online Spring Boot application generator.

Live Demo: https://start.spring.io
GitHub Repository - https://github.com/RameshMF/initializr
Documentation - Spring Initializr Reference Guide

4. Employee Management System App (Spring Boot + Angular 8 CRUD)

A simple Employee Management System App is developed using Spring Boot and Angular 8. Here we will learn how to develop a CRUD (Create, Read, Update, Delete) Web Application using Angular 8 as a front-end and Spring boot 2 restful API as a backend.

I have developed this app and you can find step by step guide at https://www.javaguides.net/2019/06/spring-boot-angular-7-crud-example-tutorial.html.

Source Code on GitHub at https://github.com/RameshMF/Angular8-SpringBoot-CRUD-Tutorial.

Here is a sample screenshot of this application:

5. Angular 9 + Spring Boot CRUD App

In this web application, you will learn how to develop a CRUD (Create, Read, Update, Delete) Web Application (employee management system) using Angular 9 as a front-end and Spring boot 2 restful API as a backend.

Features Implementation

Create an Employee

Update an Employee

List of Employees

Delete Employee

View Employee

You will develop your first FULL STACK Application with Angular 8 and Spring Boot

You can download the source code of this web app from my GitHub repository at https://github.com/RameshMF/Angular9-SpringBoot-CRUD-Tutorial

Check out the step by step tutorial/guide to create this web app at https://www.javaguides.net/2020/01/spring-boot-angular-9-crud-example-tutorial.html

6. Spring Boot Thymeleaf Hibernate CRUD Database - Real-Time Project

You will learn how to create a simple real-time project (Employee Management System) using Spring boot and Thymeleaf.

Features:

Implement List Employee Feature
Implement Add Employee Feature
Implement Update Employee Feature
Implement Delete Employee Feature
Implement Pagination Feature
Implement Sorting Feature
Implement Search Feature

Tutorial and Source code link - https://www.javaguides.net/2020/05/spring-boot-thymeleaf-hibernate-crud-database-project.html.

The below diagram shows the application flow of our Spring MVC web application with Thymeleaf:

7. Java Blogs Aggregator

Java Blog Aggregator is a Spring boot MVC-based web application developed to aggregate top Java blogs, articles, and posts via RSS feed. This project was developed with the Spring Boot.

Source code on - https://github.com/RameshMF/java-blog-aggregator-boot

Live Demo on - https://www.topjavablogs.com/

8. Spring PetClinic Sample Application

Pet Clinic is a https://spring.io/guides/gs/spring-boot[Spring Boot] application built using https://spring.io/guides/gs/maven/[Maven]. You can build a jar file and run it from the command line.

You can clone the PetClinic application from GitHub - https://github.com/RameshMF/spring-petclinic

Check out the readme to know more about the PetClinic application.

9. PiggyMetrics

This project demonstrates the Microservice Architecture Pattern using Spring Boot, Spring Cloud, and Docker. With a pretty neat user interface.

GitHub link - https://github.com/RameshMF/PiggyMetrics.

You will learn:

Microservice Architecture
Spring Boot
Spring Cloud
Docker

PiggyMetrics was decomposed into three core microservices. All of them are independently deployable applications, organized around certain business domains.

10. Zipkin

Zipkin is a distributed tracing system. It helps gather timing data needed to troubleshoot latency problems in microservice architectures. It manages both the collection and lookup of this data.

Source code on - https://github.com/openzipkin/zipkin

Live Demo on - https://zipkin.io/

11. Polling App

Full Stack Polls App built using Spring Boot, Spring Security, JWT, React, and Ant Design.

You will learn:

Spring Boot
Spring Data JPA
Hibernate
Maven
Spring Security
JWT
React JS

Read an article on - Spring Boot + Spring Security + JWT + MySQL + React Full Stack Polling App

Source code on - https://github.com/RameshMF/spring-security-react-ant-design-polls-app

Live Demo on - https://polls.callicoder.com/

12. Spring Boot WebSocket Chat Application

This is Spring Boot WebSocket Chat Demo with SockJS fallback and STOMP protocol. You will learn how to use WebSocket API with Spring Boot and build a simple group chat application at the end.

Read an article on - Building a chat application with Spring Boot and WebSocket

Source code on - https://github.com/RameshMF/spring-boot-websocket-chat-demo

Live demo on - https://spring-ws-chat.herokuapp.com/