How to handle Docker-Secrets in application.properties files

How do you inject Docker secrets (files/data from /run/secrets) into the application.properties files? Is it safe to use environment variables?

First of all, usage of environment variables for secret data for the application.properties isn't safe.

You have mainly two options when talking about Secrets.

1. If you are using Docker Secrets without Docker Swarm then you can directly load the whole application.properties in a secret, mount it under /run/secrets and refer to it as configuration file with the Spring flags.

2. If you are using Docker Secrets with Docker Swarm then you can just store as secret the concrete fields that you're interested in and relate to them using the Configuration Templates of Swarm.

Example:

echo -n "myUser" | docker secret create db_user -
echo -n "myPass" | docker secret create db_password -
echo -n "jdbc://..." | docker secret create db_url -

application.properties.tmpl

spring.datasource.url={{ secret "db_url" }}
spring.datasource.user={{ secret "db_user" }}
spring.datasource.password={{ secret "db_password" }}

docker-compose.yml

version: '3.9'
services:
  api:
    image: yourapp:1.0.0
    configs:
      - source: application.properties
        target: /usr/app/config/application.properties
    secrets:
      - db_url
      - db_user
      - db_password

configs:
  application.properties:
    template_driver: golang
    file: ./application.properties.tmpl
    name: myapp.application.properties

secrets:
  db_url:
    external: true
  db_user:
    external: true
  db_password:
    external: true

When you deploy with docker stack deploy -c docker-compose.yml myapp, it will automatically populate the configuration with the contents of the secrets and it will mount it in the destination path.