I have a Spring Boot 3.4.2 app, which has several databases and services configured, including MongoDB. There are 2 connections to MongoDB, each with their own credentials and database they access.

This stacktrace comes from a @SpringBootTest where I'm running MongoDB using TestContainers.

The app works and connects just fine when MongoDB is started via Docker Compose. I'm using the same config for the TestContainers version was for the Docker Compose version.

I can't work out from where the origin of this stacktrace is. It doesn't appear when the main app is booted for manual testing and exercising.

2025-03-27T07:58:36.305Z  INFO 110184 --- [my-app] [localhost:27017] .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.lambda$open$0(SocketStream.java:85) ~[mongodb-driver-core-5.2.1.jar:na]
    at java.base/java.util.Optional.orElseThrow(Optional.java:403) ~[na:na]
    at com.mongodb.internal.connection.SocketStream.open(SocketStream.java:85) ~[mongodb-driver-core-5.2.1.jar:na]
    at com.mongodb.internal.connection.InternalStreamConnection.open(InternalStreamConnection.java:233) ~[mongodb-driver-core-5.2.1.jar:na]
    at com.mongodb.internal.connection.DefaultServerMonitor$ServerMonitor.lookupServerDescription(DefaultServerMonitor.java:219) ~[mongodb-driver-core-5.2.1.jar:na]
    at com.mongodb.internal.connection.DefaultServerMonitor$ServerMonitor.run(DefaultServerMonitor.java:176) ~[mongodb-driver-core-5.2.1.jar:na]
Caused by: java.ConnectException: Connection refused
    at java.base/sun.nio.ch.Net.pollConnect(Native Method) ~[na:na]
    at java.base/sun.nio.ch.Net.pollConnectNow(Net.java:682) ~[na:na]
    at java.base/sun.nio.ch.NioSocketImpl.timedFinishConnect(NioSocketImpl.java:542) ~[na:na]
    at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:592) ~[na:na]
    at java.base/java.SocksSocketImpl.connect(SocksSocketImpl.java:327) ~[na:na]
    at java.base/java.Socket.connect(Socket.java:751) ~[na:na]
    at com.mongodb.internal.connection.SocketStreamHelper.initialize(SocketStreamHelper.java:76) ~[mongodb-driver-core-5.2.1.jar:na]
    at com.mongodb.internal.connection.SocketStream.initializeSocket(SocketStream.java:104) ~[mongodb-driver-core-5.2.1.jar:na]
    at com.mongodb.internal.connection.SocketStream.open(SocketStream.java:79) ~[mongodb-driver-core-5.2.1.jar:na]
    ... 3 common frames omitted