Connection refused trying to connect to mongoDB docker instance

I'm trying to deploy a simple app with a MongoDB with JPA, but I can't make it work right.

When I deploy the app in a docker container, I keep getting the following error:

2020-10-05 18:47:39.770 INFO 1 --- [localhost:27017] org.mongodb.driver.cluster : Exception in monitor thread while connecting to server localhost:27017

com.mongodb.MongoSocketOpenException: Exception opening socket
    at com.mongodb.internal.connection.SocketStream.open(SocketStream.java:70) ~[mongodb-driver-core-4.0.5.jar!/:na]
    at com.mongodb.internal.connection.InternalStreamConnection.open(InternalStreamConnection.java:127) ~[mongodb-driver-core-4.0.5.jar!/:na]
    at com.mongodb.internal.connection.DefaultServerMonitor$ServerMonitorRunnable.run(DefaultServerMonitor.java:117) ~[mongodb-driver-core-4.0.5.jar!/:na]
    at java.lang.Thread.run(Thread.java:748) [na:1.8.0_212]
Caused by: java.net.ConnectException: Connection refused (Connection refused)
    at java.net.PlainSocketImpl.socketConnect(Native Method) ~[na:1.8.0_212]
    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350) ~[na:1.8.0_212]
    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206) ~[na:1.8.0_212]
    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188) ~[na:1.8.0_212]
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) ~[na:1.8.0_212]
    at java.net.Socket.connect(Socket.java:589) ~[na:1.8.0_212]
    at com.mongodb.internal.connection.SocketStreamHelper.initialize(SocketStreamHelper.java:63) ~[mongodb-driver-core-4.0.5.jar!/:na]
    at com.mongodb.internal.connection.SocketStream.initializeSocket(SocketStream.java:79) ~[mongodb-driver-core-4.0.5.jar!/:na]
    at com.mongodb.internal.connection.SocketStream.open(SocketStream.java:65) ~[mongodb-driver-core-4.0.5.jar!/:na]
    ... 3 common frames omitted

This is the dockerfile I use to build the image:

FROM openjdk:8-jdk-alpine
ARG JAR_FILE=target/*.jar
COPY ${JAR_FILE} app.jar
ENTRYPOINT ["java","-jar","/app.jar"]

And the docker-compose.yml I used to run mongoDB:

# Use root/example as user/password credentials
version: '3.1'

services:

  mongo:
    image: mongo
    restart: always
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: example

  mongo-express:
    image: mongo-express
    restart: always
    ports:
      - 8081:8081
    environment:
      ME_CONFIG_MONGODB_ADMINUSERNAME: root
      ME_CONFIG_MONGODB_ADMINPASSWORD: example

My entities all follow the following pattern (being <Name> the name of the entity):

@Document
@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class <Name>Entity {
    @Id
    private Integer <Name>Id;
    // More data
}

And my JPA Repositories (as above, being <Name> the name of the jpa class & entity):

public interface <Name>JPA extends MongoRepository<<Name>Entity, Integer>{
}

And finally, I have defined the following properties under application.properties:

spring.data.mongodb.host=localhost
spring.data.mongodb.port=27017
spring.data.mongodb.database=api-database
spring.data.mongodb.username=root
spring.data.mongodb.password=example
spring.data.mongodb.repositories.enabled=true

I am able to connect to localhost:8081 to see the mongo-express interface, but appart from that, I don't see anything.

They can't connect via loopback address because mongodb and the app are in different containers. They're like like different machines and localhost for each of them is not the host, but the container itself. Use service name (from docker-compose.yml) instead:

spring.data.mongodb.host=mongo:27017

If you are trying to connect with containerized MongoDB docker instance with localhost then you might face this issue.

Solution :

In place of

mongodb://localhost:27017

To connect the host out of the box by using the special DNS name: host.docker.internal

mongodb://host.docker.internal:27017

Replacing localhost with host.docker.internal will do the trick.