Spring Boot (4.0.0) Cannot Connect to MongoDB in Docker Compose Network

I'm trying to connect my Spring Boot application (using spring-boot-starter-data-mongodb) to a MongoDB instance within a shared Docker network managed by docker-compose.

The connection is failing, and my application appears to be attempting to connect to the default MongoDB address (mongodb://localhost:27017) instead of the host/port I've configured for the Docker network. This suggests my configuration is being ignored or overridden due to Spring Boot's Externalized Configuration precedence.

My Setup

1. docker-compose.yml

YAML

services:
  # 1. MongoDB Service
  mongodb:
    image: mongo:latest
    container_name: mongodb
    ports:
      # Exposing for external access/testing, but internal communication doesn't use this port mapping
      - "27017:27017" 
    environment:
      MONGO_INITDB_ROOT_USERNAME: myuser
      MONGO_INITDB_ROOT_PASSWORD: mypassword
    volumes:
      - mongo_data:/data/db

  # 2. Spring Boot Application Service
  myapp:
    build: .
    container_name: myapp
    ports:
      - "8080:8080"
    depends_on:
      - mongodb
    # --- CRITICAL AREA ---
    environment:
      # These properties seem to be failing to correctly configure the connection:
      SPRING_DATA_MONGODB_HOST: mongodb 
      SPRING_DATA_MONGODB_PORT: 27017
      SPRING_DATA_MONGODB_USERNAME: myuser
      SPRING_DATA_MONGODB_PASSWORD: mypassword
      SPRING_DATA_MONGODB_DATABASE: mydatabase

volumes:
  mongo_data:

2. Spring Boot application.yaml (or application-prod.yaml)

This is the internal configuration I'm trying to override:

YAML

spring:
  data:
    mongodb:
      host: localhost # This is the value I expect to be overridden
      port: 27017
      database: mydatabase
      username: myuser
      password: mypassword

The Problem

1. My application logs show a connection attempt to mongodb://localhost:27017 or a connection timeout error.

2. I suspect the environment variables in my docker-compose.yml are either incorrectly formatted for Spring Boot's relaxed binding or not taking effect, allowing the default settings (or the values in application.yaml) to take precedence.

What is the correct and most robust way to configure Spring Boot's MongoDB connection string using the Docker Compose service name, ensuring it correctly overrides any configuration inside my JAR?

I am specifically looking for confirmation on:

1. The correct environment variable format for the full connection URI (e.g., SPRING_DATA_MONGODB_URI).

2. Why using SPRING_DATA_MONGODB_HOST: mongodb is failing to override the localhost setting.