HTTP Listener replying "Server not available to handle this request, either not initialized yet or it has been disposed" in Mule 4

SYMPTOM

A Mule Application that has an HTTP Listener as source endpoint consistently replies 503 HTTP statuses with the message "Server not available to handle this request, either not initialized yet or it has been disposed". Additionally, this Mule Application includes MUnit test or dependencies in its structure.

If you enable HTTP Wire logging in your Mule Application refer to <How to Enable HTTP Wire Logging for the HTTP Connector> you'll see a similar trace to the one below.

DEBUG 2019-11-13 17:46:29,465 [http.listener.06 SelectorRunner] [event: ] org.mule.service.http.impl.service.HttpMessageLogger.HTTP_Listener_config: LISTENER
HTTP/1.1 503 Service Unavailable
Content-Type: text/plain; charset=UTF-8
Content-Length: 96
Date: Wed, 13 Nov 2019 20:46:29 GMT
Connection: close

Server not available to handle this request, either not initialized yet or it has been disposed.

CAUSE

SOLUTION

<dependency>
    <groupId>com.mulesoft.munit</groupId>
    <artifactId>munit-runner</artifactId>
    <version>2.x</version>
    <classifier>mule-plugin</classifier>
    <scope>test</scope>
</dependency>

<dependency>
    <groupId>com.mulesoft.munit</groupId>
    <artifactId>munit-tools</artifactId>
    <version>2.x</version>
    <classifier>mule-plugin</classifier>
    <scope>test</scope>
</dependency>