Here's a solution that's working from me. Spring Boot 2 & Spring-Webflux 5.3.30 using SpringBootTest

1. MyServiceClientTestConfig creates injected beans port and myServiceClient, where

   • The port Integer bean is generated using Spring's TestSocketUtils.findAvailableTcpPort()
   • MyServiceClient is a Spring service that uses WebClient and gets initialized with a base URL.

2. As documented we should not re-use a MockWebServer instance between tests so create it before each test using the port we found free at test initialization and shut it down after each test.

I suppose there is a slim chance that since I find the port once when Spring is initializing the application context and there are short periods of time where it is not in use by MockWebServer, before the first call to init and between tests, that another process could take the port but the odds of that happening during the lifetime of the test seem incredibly small.

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.NONE)
public class MyServiceClientTest {

    private static final Logger LOG = LoggerFactory.getLogger(MyServiceClientTest.class);

    private MockWebServer mockWebServer;

    @Autowired
    private MyServiceClient myServiceClient;

    @Autowired
    private Integer port;

    @BeforeEach
    void init() throws IOException {
        assertNotNull(myServiceClient);
        assertNotNull(port);

        // MockWebServer not re-usable between tests
        mockWebServer = new MockWebServer();
        mockWebServer.start(port);

        assertNotNull(mockWebServer);
        assertEquals(port, mockWebServer.getPort());
    }

    @AfterEach
    void cleanUp() throws IOException {
        // MockWebServer not re-usable between tests
        mockWebServer.shutdown();
    }

    @Test
    public void placeholderTest1() {
        LOG.info("Hello 1 {}", mockWebServer.getPort());
    }

    @Test
    public void placeholderTest2() {
        LOG.info("Hello 2 {}", mockWebServer.getPort());
    }

    @Configuration
    static class MyServiceClientTestConfig {

        @Bean
        Integer getFreePort () {
            // find a free port to use for this test
            return TestSocketUtils.findAvailableTcpPort();
        }

        @Bean
        MyServiceClient getMyServiceClient(Integer port) {
            return new MyServiceClient(String.format("http://localhost:%d", port));
        }
    }
}