Java REST API with Spring Boot, Spring Data JPA, PostgreSQL with Docker

This project demonstrates a simple Java REST API built with Spring Boot, Spring Data JPA, and PostgreSQL, running the database in Docker.

Based on the course:

Spring Boot Tutorial for Beginners | Full Course 2025 by amigoscode

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🧱 Tech Stack

• Java 21
• Spring Boot
• Spring Data JPA (Hibernate)
• PostgreSQL
• Docker & Docker Compose

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🐳 PostgreSQL in Docker

docker-compose.yml

services:
  db:
    container_name: postgres-spring-boot
    image: postgres:18.1-alpine
    restart: unless-stopped
    environment:
      POSTGRES_USER: postgres   # database name will be the same as username
      POSTGRES_PASSWORD:
      PGDATA: /data/postgres
    volumes:
      - db:/data/postgres
    ports:
      - "5332:5432"
    networks:
      - db

networks:
  db:
    driver: bridge

volumes:
  db:

▶️ Run PostgreSQL

docker compose up -d

Connecting docker container to pgAdmin 4

• register - > server
• Host name: localhost
• Port: 5332
• Connect

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🧭 Navigating PostgreSQL Container

Enter the container

docker exec -it (container id) bash

Open PostgreSQL shell

psql -U (username = postgres)

Useful PostgreSQL commands

\l    \list              -- list databases
CREATE DATABASE spring;
\c spring   \connect     -- connect to database
\q 						           -- exit psql 
\c postgres				       -- exit to base database
\d                       -- list tables
\d engineers             -- show table structure

Deleting DB

DELETE DATABASE name;
docker compose down -v
-v - removes data volumes and your db (is needed to create db again with docker compose up)

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📊 Fetching Data from PostgreSQL

SELECT * FROM engineers;

Example output:

 id |  name
----+--------
  1 | Albert
  2 | Megan

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📦 Spring Data JPA Repository

JPQL Query with JOIN FETCH

@Query("SELECT DISTINCT e FROM SoftwareEngineer e LEFT JOIN FETCH e.techStack")
List<SoftwareEngineer> findAllWithTechStack();

🧠 JPQL Notes

• FETCH → forces eager loading of related collections
• LEFT JOIN → includes engineers without tech stack
• DISTINCT → removes duplicate entity instances caused by joins

What this JPQL does

• Fetches SoftwareEngineer entities
• Loads @ElementCollection techStack eagerly
• Prevents duplicate entities using DISTINCT

🔄 Equivalent SQL Generated by Hibernate

SELECT e.id, e.name, t.tech_stack
FROM software_engineer e
LEFT JOIN software_engineer_tech_stack t
ON e.id = t.software_engineer_id;

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🌐 Spring MVC Annotations

@RestController

• Used for REST APIs
• Returns JSON / XML responses
• Combines @Controller + @ResponseBody

@Controller

• Used for MVC applications
• Returns HTML views (Thymeleaf, JSP, etc.)

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🧩 JPA / Hibernate Best Practices

• JPA entities must have a no-args constructor

@NoArgsConstructor

• Avoid implementing equals() and hashCode() when using generated IDs

  • Unless explicitly designed to ignore the ID
  • Incorrect implementations can break Hibernate behavior

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✅ Summary

• Docker simplifies PostgreSQL setup
• JPQL operates on entities, not tables
• JOIN FETCH is essential for loading @ElementCollection
• Never expose entities directly in APIs — use DTOs

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