Docker Compose containers can't communicate with each other

How can I troubleshoot communication issues between my Spring Boot application, RabbitMQ, and MySQL when using Docker and Docker Compose for deployment, as my back-end application keeps restarting and fails to establish communication with RabbitMQ and MySQL?

Dockerfile:

<!-- language: lang-docker -->

FROM openjdk:17-alpine
ADD target/*.jar app.jar
ENTRYPOINT ["java","-jar","/app.jar"]

Docker compose:

<!-- language: lang-yaml -->

version: "3.9"
networks:
  net:
    driver: bridge

services:
  thermostat-app:
    image: myapp
    build: .
    restart: always
    ports:
      - "8080:8080"
    environment:
      MYSQL_USER: root
      MYSQL_PASSWORD: password
      MYSQL_PORT: 3306
    networks:
      - net
  rabbitmq:
    image: rabbitmq:3.9.11-management-alpine
    container_name: rabbitmq
    networks:
      - net
  db:
    image: mysql:latest
    command: --default-authentication-plugin=mysql_native_password
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: password
      MYSQL_DATABASE: mysqldb
    networks:
      - net

application.properties:

<!-- language: lang-properties -->

spring.datasource.url=jdbc:mysql://db:${MYSQL_PORT:3306}/thermostat
spring.datasource.username=${MYSQL_USER:root}
spring.datasource.password=${MYSQL_PASSWORD:password}
spring.jpa.hibernate.ddl-auto=create
spring.jpa.show-sql=true
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQLDialect
spring.rabbitmq.host=localhost
spring.rabbitmq.port=5672
spring.rabbitmq.username=guest
spring.rabbitmq.password=guest

In a container, localhost means the container itself. Containers on the same docker network can address each other using their service names as host names.

You've done that with the datasource URL, where you've used db as the host name.

You need to do it for RabbitMQ as well and change

spring.rabbitmq.host=localhost

to

spring.rabbitmq.host=rabbitmq