Failed to load ApplicationContext with OAuth2 resource server issuer URI when running @WebMvcTest

Question

My Spring Boot application defines an OAuth2 resource server like this in the application.yml:

spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          issuer-uri: ${AUTH_SERVER_METADATA_URL:http://localhost:8080/auth/realms/demo}

This works fine when actually running the application.

When, however, running tests with@WebMvcTest and MockMvc to test a @RestController class, then the test fails with the following error:

java.lang.IllegalStateException: Failed to load ApplicationContext
...
...
...
I/O error on GET request for "http://localhost:8080/auth/realms/demo/.well-known/openid-configuration": Connection refused (Connection refused); nested exception is java.net.ConnectException: Connection refused (Connection refused)

The message is pretty clear in its root cause, but I don't know how I could "mock" the connection for sliced tests without spinning up a real Keycloak or WireMock server.

Running the same test with my development Keycloak server running in the background works fine.