Error creating a docker-compose connecting a java and a mysql containers

I believe the issue is your application’s use of localhost for the SQL URL in the Application.yaml property file. Since your app runs on a container by itself it tries to look at localhost of the container, while your SQL server is in another container, with its own localhost. Localhost in docker container do not refer to the host, they refer to the localhost within the container itself. If you want to access the host machine, this is an excellent answer From inside of a Docker container, how do I connect to the localhost of the machine?

url: jdbc:mysql://localhost:3306/dronefeederdb

localhost should not be used, you need to use the sql continainer url.

The fastest option is to use host.docker.internal instead of localhost. But it’s not the best.

Another quick option is to run the two containers on the same docker network. Define that in your compose file the same way as the volumes. Then set each container to that network. See Networking in Compose. Then you can set your SQL url to use the SQL container name instead of localhost. So this..
url: jdbc:mysql://localhost:3306/dronefeederdb becomes url: jdbc:mysql://mysql/dronefeederdb

Neither option is robust, since you’re hardcoding the container name in the application property file. A better solution is to have an environment variable in your webApp image that can accept the URL to the SQL server. Then you can provide the SQL location when running the container, or in your compose file (Environment variables in Compose). This way the SQL server can be anywhere.

Update:

There were a couple of issues in the compose and env files that caused mySQL container to fail startup. Thus the webApp was not able to connect.

Credentials

• MYSQL_USER was set to root. mySql already creates the user root. You cannot create it again. I changed that to foo. See the Environment Variables section in the official docker image readme for more.
• MYSQL_PASSWORD was not set. This is the password for the user your app will use. I set this to pass!123
• The apps DB_PASSWORD was set to user root. That would have been ok if sql had started and it was using the root user I guess. But I changed that to the non-root user since were setting DB_USER=foo

Network was not defined

• The two containers need to be on the same "docker network" if they are to run together in docker in the same machine. There’s more to this which is beyond my experience. But in this case it needs to be on the same network for app to access mysqldb by its container name. I created dronefeederNet and added each container to it.

Files:

.env

MYSQLDB_USER=foo
MYSQLDB_PASSWORD=pass!123
MYSQLDB_ROOT_PASSWORD=12345678
MYSQLDB_DATABASE=dronefeederdb
MYSQLDB_LOCAL_PORT=3307
MYSQLDB_DOCKER_PORT=3306

APP_LOCAL_PORT=8081
APP_DOCKER_PORT=8080

docker-compose.yml

version: '3.8'

services:
  mysqldb:
    image: mysql
    platform: linux/x86_64
    env_file: ./.env
    restart: always
    environment:
      - MYSQL_USER=$MYSQLDB_USER
      - MYSQL_PASSWORD=$MYSQLDB_PASSWORD
      - MYSQL_ROOT_PASSWORD=$MYSQLDB_ROOT_PASSWORD
      - MYSQL_DATABASE=$MYSQLDB_DATABASE
    ports:
      - $MYSQLDB_LOCAL_PORT:$MYSQLDB_DOCKER_PORT
    volumes:
      - db:/var/lib/mysql
    networks:
      - dronefeederNet
  app:
    depends_on:
     - mysqldb
    build: .
    restart: always
    env_file: ./.env
    ports:
      - $APP_LOCAL_PORT:$APP_DOCKER_PORT
    environment:
      - DB_HOST=mysqldb
      - DB_USER=$MYSQLDB_USER
      - DB_PASSWORD=$MYSQLDB_PASSWORD
      - DB_NAME=$MYSQLDB_DATABASE
      - DB_PORT=$MYSQLDB_DOCKER_PORT
    stdin_open: true
    tty: true
    networks:
      - dronefeederNet
volumes: 
  db:
networks:
  dronefeederNet:

Give this a try and I hope it runs. I was able to start it up ok.