Issue Summary: I’m encountering a connection error between my Spring Boot application and a MongoDB database running in separate Docker containers.

Error Description: I’m receiving the following error:

com.mongodb.MongoSocketOpenException: Exception opening socket java.net.ConnectException: Connection refused

This indicates that my Spring Boot application is unable to connect to the MongoDB server.

Root Cause: The MongoDB server might not be running or isn’t accessible at the specified host and port (mongodb_instance:27017).

application.properties

# Local MongoDB configuration
spring.data.mongodb.host=localhost
spring.data.mongodb.port=27017
spring.data.mongodb.database=mydatabase

application.yml (I think in this file I have a mistake)

spring:
  profiles:
    active: docker

  data:
    mongodb:
      host: mongodb_instance 
      port: 27017
      database: mydatabase  

docker-compose.yml

version: '3.8'

services:
  mongodb:
    image: mongo:latest
    container_name: mongodb_instance
    ports:
      - "27017:27017"
    networks:
      - backend

  spring-app:
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - "8080:8080"
    networks:
      - backend
    depends_on:
      - mongodb

networks:
  backend:
    driver: bridge

dockerfile

# Use an appropriate base image for your Java application
FROM openjdk:17-jdk-slim AS build

# Set the working directory inside the container
WORKDIR /app

# Copy the Maven wrapper script and pom.xml to the container
COPY mvnw .
COPY .mvn .mvn
COPY pom.xml .

# Copy the source code into the container
COPY src src

# Make the Maven wrapper executable
RUN chmod +x mvnw

# Build your application with Maven
RUN ./mvnw clean package -DskipTests

# Stage 2: Use a smaller base image for runtime
FROM openjdk:17-jdk-slim

# Set the working directory inside the container
WORKDIR /app

# Copy the built artifact from the build stage
COPY --from=build /app/target/simple-spring-mongodb-backup-0.0.1-SNAPSHOT.jar ./app.jar

# Expose the port your application runs on
EXPOSE 8080

# Specify the command to run your application
CMD ["java", "-Dspring.profiles.active=docker", "-jar", "./app.jar"]

I tried everything possible but no solution yet, I tried with

spring:
  profiles:
    active: docker

  data:
    mongodb:
      host: 172.17.0.2  # Replace with the IP address of your MongoDB container
      port: 27017
      database: mydatabase  # Replace with your actual database name

this config also but nothing works