Road Reactive Spring: JUnit Test, WebFlux Non Blocking API (2)

Road Reactive Spring: JUnit Test, WebFlux Non Blocking API (2)

WebFlux Non Blocking API @RestController @GetMapping return [1,2,3,4]

@RestController 

public class FluxMonoController {
    @GetMapping("/flux")
    Flux<Integer> flux() {
        return Flux.just(1,2,3,4)
                .log();     

    }
}

Running the app
  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.2.RELEASE)

2019-12-10 19:13:17.830  INFO 12520 --- [           main] com.bext.RoadReactiveSpringApplication   : Starting RoadReactiveSpringApplication on DESKTOP-NLF0058 with PID 12520 (D:\proy\roadReactiveSpring\build\classes\java\main started by bext in D:\proy\roadReactiveSpring)
2019-12-10 19:13:17.832  INFO 12520 --- [           main] com.bext.RoadReactiveSpringApplication   : No active profile set, falling back to default profiles: default
2019-12-10 19:13:18.187  INFO 12520 --- [           main] .s.d.r.c.RepositoryConfigurationDelegate : Bootstrapping Spring Data Reactive MongoDB repositories in DEFAULT mode.
2019-12-10 19:13:18.202  INFO 12520 --- [           main] .s.d.r.c.RepositoryConfigurationDelegate : Finished Spring Data repository scanning in 10ms. Found 0 Reactive MongoDB repository interfaces.
2019-12-10 19:13:18.206  INFO 12520 --- [           main] .s.d.r.c.RepositoryConfigurationDelegate : Bootstrapping Spring Data MongoDB repositories in DEFAULT mode.
2019-12-10 19:13:18.207  INFO 12520 --- [           main] .s.d.r.c.RepositoryConfigurationDelegate : Finished Spring Data repository scanning in 1ms. Found 0 MongoDB repository interfaces.
2019-12-10 19:13:18.712  INFO 12520 --- [           main] org.mongodb.driver.cluster               : Cluster created with settings {hosts=[localhost:27017], mode=SINGLE, requiredClusterType=UNKNOWN, serverSelectionTimeout='30000 ms', maxWaitQueueSize=500}
2019-12-10 19:13:18.749  INFO 12520 --- [localhost:27017] org.mongodb.driver.connection            : Opened connection [connectionId{localValue:1, serverValue:775}] to localhost:27017
2019-12-10 19:13:18.753  INFO 12520 --- [localhost:27017] org.mongodb.driver.cluster               : Monitor thread successfully connected to server with description ServerDescription{address=localhost:27017, type=STANDALONE, state=CONNECTED, ok=true, version=ServerVersion{versionList=[4, 2, 1]}, minWireVersion=0, maxWireVersion=8, maxDocumentSize=16777216, logicalSessionTimeoutMinutes=30, roundTripTimeNanos=2480000}
2019-12-10 19:13:18.926  INFO 12520 --- [           main] org.mongodb.driver.cluster               : Cluster created with settings {hosts=[localhost:27017], mode=SINGLE, requiredClusterType=UNKNOWN, serverSelectionTimeout='30000 ms', maxWaitQueueSize=500}
2019-12-10 19:13:19.324  INFO 12520 --- [           main] o.s.b.web.embedded.netty.NettyWebServer  : Netty started on port(s): 8080
2019-12-10 19:13:19.327  INFO 12520 --- [           main] com.bext.RoadReactiveSpringApplication   : Started RoadReactiveSpringApplication in 1.725 seconds (JVM running for 2.33)
2019-12-10 19:13:19.384  INFO 12520 --- [localhost:27017] org.mongodb.driver.connection            : Opened connection [connectionId{localValue:2, serverValue:776}] to localhost:27017
2019-12-10 19:13:19.385  INFO 12520 --- [localhost:27017] org.mongodb.driver.cluster               : Monitor thread successfully connected to server with description ServerDescription{address=localhost:27017, type=STANDALONE, state=CONNECTED, ok=true, version=ServerVersion{versionList=[4, 2, 1]}, minWireVersion=0, maxWireVersion=8, maxDocumentSize=16777216, logicalSessionTimeoutMinutes=30, roundTripTimeNanos=1257200}
2019-12-10 19:13:24.711  INFO 12520 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onSubscribe([Synchronous Fuseable] FluxArray.ArraySubscription)
2019-12-10 19:13:24.712  INFO 12520 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | request(unbounded)
2019-12-10 19:13:24.712  INFO 12520 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onNext(1)
2019-12-10 19:13:24.712  INFO 12520 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onNext(2)
2019-12-10 19:13:24.713  INFO 12520 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onNext(3)
2019-12-10 19:13:24.713  INFO 12520 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onNext(4)
2019-12-10 19:13:24.713  INFO 12520 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onComplete()
From the command line
C:\Users\bext>curl localhost:8080/flux
[1,2,3,4]


WebFlux Non Blocking API .delayElements( Duration.ofSeconds(1))

@RestController

    public class FluxMonoController {
    @GetMapping("/flux")
    Flux<Integer> flux() {
        return Flux.just(1,2,3,4)
                .delayElements(Duration.ofSeconds(1))
                .log();     

    }
}

Running the application
2019-12-10 19:18:04.464  INFO 16152 --- [ctor-http-nio-2] reactor.Flux.ConcatMap.1                 : onSubscribe(FluxConcatMap.ConcatMapImmediate)
2019-12-10 19:18:04.465  INFO 16152 --- [ctor-http-nio-2] reactor.Flux.ConcatMap.1                 : request(unbounded)
2019-12-10 19:18:05.470  INFO 16152 --- [     parallel-1] reactor.Flux.ConcatMap.1                 : onNext(1)
2019-12-10 19:18:06.471  INFO 16152 --- [     parallel-2] reactor.Flux.ConcatMap.1                 : onNext(2)
2019-12-10 19:18:07.473  INFO 16152 --- [     parallel-3] reactor.Flux.ConcatMap.1                 : onNext(3)
2019-12-10 19:18:08.474  INFO 16152 --- [     parallel-4] reactor.Flux.ConcatMap.1                 : onNext(4)
2019-12-10 19:18:08.474  INFO 16152 --- [     parallel-4] reactor.Flux.ConcatMap.1                 : onComplete()



The time between onNext is 1 second.

From the command line
C:\Users\bext>curl localhost:8080/flux
[1,2,3,4] 
after 4 seconds aprox. shows all the results in one time.

WebFlux Non Blocking API .delayElements( Duration.ofSeconds(1)) MediaType.APPLICATION_STREAM_JSON_VALUE

this way the results are showing one by one on the browser.

@GetMapping(value = "/fluxstream", produces = MediaType.APPLICATION_STREAM_JSON_VALUE)
public Flux<Integer> fluxStream() {
    return Flux.just(1,2,3,4)
            .delayElements(Duration.ofSeconds(1))
            .log();

 }

output on the browser

1 ( after a second)

2 ( after a second)

3 ( after a second)

4 ( after a second)

Running the application and Tested on Browser, shows successively each second a value.

JUnit Test Reactive API using WebTestClient

With the Spring class WebTestClient we can test the flux, this is instantiated by the @WebFluxTest annotation.

With StepVerifier we check the result expeted for our Junit test. (use @WebFluxTest in the case of rest controler)

@RunWith(SpringRunner.class)
@WebFluxTest 

class FluxMonoControllerTest {

    @Autowired 

    WebTestClient webTestClient;
     

    @Test     

    public void flux_webClient01() {
        Flux<Integer> fluxInteger = webTestClient.get().uri("/flux")
                .accept(MediaType.APPLICATION_JSON)
                .exchange()
                .expectStatus().isOk()
                .returnResult(Integer.class)
                .getResponseBody();
        StepVerifier.create( fluxInteger)
                .expectSubscription()
                .expectNext(1)
                .expectNext(2)
                .expectNext(3)
                .expectNext(4)
                .verifyComplete();     

    }

    @Test 

    public void flux_webClient02() {
         webTestClient.get().uri("/flux")
                .accept(MediaType.APPLICATION_JSON)
                .exchange()
                .expectStatus().isOk()
                .expectHeader().contentType(MediaType.APPLICATION_JSON)
                .expectBodyList(Integer.class)
                .hasSize(4); 

    }

    @Test 

    public void flux_webClient03() {
        List<Integer> expectedListInteger = Arrays.asList(1,2,3,4);
        EntityExchangeResult<List<Integer>> entityExchangeResult =
        webTestClient.get().uri("/flux")
                .accept(MediaType.APPLICATION_JSON)
                .exchange()
                .expectStatus().isOk()
                .expectBodyList(Integer.class)
                .returnResult();
        assertEquals( expectedListInteger, entityExchangeResult.getResponseBody());       }

    @Test 

    public void flux_webClient04() {
        List<Integer> expectedListInteger = Arrays.asList(1,2,3,4);
        webTestClient.get().uri("/flux")
                        .accept(MediaType.APPLICATION_JSON)
                        .exchange()
                        .expectStatus().isOk()
                        .expectBodyList(Integer.class)
                        .consumeWith( response ->
                         assertEquals( expectedListInteger, response.getResponseBody()));
    }

}

Running this Tests
  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.2.RELEASE)

2019-12-10 19:39:16.665  INFO 4916 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : Starting FluxMonoControllerTest on DESKTOP-NLF0058 with PID 4916 (started by bext in D:\proy\roadReactiveSpring)
2019-12-10 19:39:16.667  INFO 4916 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : No active profile set, falling back to default profiles: default
2019-12-10 19:39:17.510  INFO 4916 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : Started FluxMonoControllerTest in 1.047 seconds (JVM running for 2.097)
2019-12-10 19:39:17.924  INFO 4916 --- [     parallel-1] reactor.Flux.ConcatMap.1                 : onSubscribe(FluxConcatMap.ConcatMapImmediate)
2019-12-10 19:39:17.927  INFO 4916 --- [     parallel-1] reactor.Flux.ConcatMap.1                 : request(unbounded)
2019-12-10 19:39:18.934  INFO 4916 --- [     parallel-2] reactor.Flux.ConcatMap.1                 : onNext(1)
2019-12-10 19:39:19.934  INFO 4916 --- [     parallel-3] reactor.Flux.ConcatMap.1                 : onNext(2)
2019-12-10 19:39:20.936  INFO 4916 --- [     parallel-4] reactor.Flux.ConcatMap.1                 : onNext(3)
2019-12-10 19:39:21.937  INFO 4916 --- [     parallel-5] reactor.Flux.ConcatMap.1                 : onNext(4)
2019-12-10 19:39:21.938  INFO 4916 --- [     parallel-5] reactor.Flux.ConcatMap.1                 : onComplete()
2019-12-10 19:39:22.035  INFO 4916 --- [     parallel-6] reactor.Flux.ConcatMap.2                 : onSubscribe(FluxConcatMap.ConcatMapImmediate)
2019-12-10 19:39:22.036  INFO 4916 --- [     parallel-6] reactor.Flux.ConcatMap.2                 : request(unbounded)
2019-12-10 19:39:23.036  INFO 4916 --- [     parallel-7] reactor.Flux.ConcatMap.2                 : onNext(1)
2019-12-10 19:39:24.038  INFO 4916 --- [     parallel-8] reactor.Flux.ConcatMap.2                 : onNext(2)
2019-12-10 19:39:25.040  INFO 4916 --- [     parallel-9] reactor.Flux.ConcatMap.2                 : onNext(3)
2019-12-10 19:39:26.040  INFO 4916 --- [    parallel-10] reactor.Flux.ConcatMap.2                 : onNext(4)
2019-12-10 19:39:26.040  INFO 4916 --- [    parallel-10] reactor.Flux.ConcatMap.2                 : onComplete()
2019-12-10 19:39:26.051  INFO 4916 --- [    parallel-11] reactor.Flux.ConcatMap.3                 : onSubscribe(FluxConcatMap.ConcatMapImmediate)
2019-12-10 19:39:26.051  INFO 4916 --- [    parallel-11] reactor.Flux.ConcatMap.3                 : request(unbounded)
2019-12-10 19:39:27.054  INFO 4916 --- [    parallel-12] reactor.Flux.ConcatMap.3                 : onNext(1)
2019-12-10 19:39:28.054  INFO 4916 --- [     parallel-1] reactor.Flux.ConcatMap.3                 : onNext(2)
2019-12-10 19:39:29.057  INFO 4916 --- [     parallel-2] reactor.Flux.ConcatMap.3                 : onNext(3)
2019-12-10 19:39:30.057  INFO 4916 --- [     parallel-3] reactor.Flux.ConcatMap.3                 : onNext(4)
2019-12-10 19:39:30.057  INFO 4916 --- [     parallel-3] reactor.Flux.ConcatMap.3                 : onComplete()
2019-12-10 19:39:30.067  INFO 4916 --- [     parallel-4] reactor.Flux.ConcatMap.4                 : onSubscribe(FluxConcatMap.ConcatMapImmediate)
2019-12-10 19:39:30.068  INFO 4916 --- [     parallel-4] reactor.Flux.ConcatMap.4                 : request(unbounded)
2019-12-10 19:39:31.070  INFO 4916 --- [     parallel-5] reactor.Flux.ConcatMap.4                 : onNext(1)
2019-12-10 19:39:32.072  INFO 4916 --- [     parallel-6] reactor.Flux.ConcatMap.4                 : onNext(2)
2019-12-10 19:39:33.072  INFO 4916 --- [     parallel-7] reactor.Flux.ConcatMap.4                 : onNext(3)
2019-12-10 19:39:34.073  INFO 4916 --- [     parallel-8] reactor.Flux.ConcatMap.4                 : onNext(4)
2019-12-10 19:39:34.073  INFO 4916 --- [     parallel-8] reactor.Flux.ConcatMap.4                 : onComplete()
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 21s
5 actionable tasks: 4 executed, 1 up-to-date
07:39:34 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.controller.FluxMonoControllerTest"'.






 JUnit Test infinite Non Blocking Sequence API using WebTestClient

    @Test 

    void fluxStream() {
    Flux<Long> fluxLong = webTestClient.get().uri("/fluxstream")

            .accept(MediaType.APPLICATION_STREAM_JSON)
            .exchange()
            .expectStatus().isOk()
            .returnResult(Long.class)
            .getResponseBody();
    StepVerifier.create(fluxLong)
            .expectSubscription()
            .expectNext(0L)
            .expectNext(1L)
            .expectNext(2L)
            .thenCancel()
            .verify(); 

    }

Running this Test

/\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.2.RELEASE)

2019-12-10 20:18:40.975  INFO 20132 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : Starting FluxMonoControllerTest on DESKTOP-NLF0058 with PID 20132 (started by bext in D:\proy\roadReactiveSpring)
2019-12-10 20:18:40.977  INFO 20132 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : No active profile set, falling back to default profiles: default
2019-12-10 20:18:41.714  INFO 20132 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : Started FluxMonoControllerTest in 0.917 seconds (JVM running for 2.188)
2019-12-10 20:18:42.034  INFO 20132 --- [     parallel-1] reactor.Flux.Interval.1                  : onSubscribe(FluxInterval.IntervalRunnable)
2019-12-10 20:18:42.037  INFO 20132 --- [     parallel-1] reactor.Flux.Interval.1                  : request(1)
2019-12-10 20:18:43.039  INFO 20132 --- [     parallel-2] reactor.Flux.Interval.1                  : onNext(0)
2019-12-10 20:18:43.085  INFO 20132 --- [    Test worker] reactor.Flux.Interval.1                  : request(31)
2019-12-10 20:18:44.039  INFO 20132 --- [     parallel-2] reactor.Flux.Interval.1                  : onNext(1)
2019-12-10 20:18:45.040  INFO 20132 --- [     parallel-2] reactor.Flux.Interval.1                  : onNext(2)
2019-12-10 20:18:45.041  INFO 20132 --- [     parallel-2] reactor.Flux.Interval.1                  : cancel()
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 6s
5 actionable tasks: 2 executed, 3 up-to-date
08:18:45 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.controller.FluxMonoControllerTest.fluxStream"'.

JUnit Test simple Non Blocking API Mono using WebTestClient

@RestController

    public class FluxMonoController {
    @GetMapping("/flux")
    public Flux<Integer> flux() {
        return Flux.just(1,2,3,4)
                .delayElements(Duration.ofSeconds(1))
                .log(); 

    }

    @GetMapping(value = "/fluxstream", produces = MediaType.APPLICATION_STREAM_JSON_VALUE)
    public Flux<Long> fluxStream() {
        return Flux.interval(Duration.ofSeconds(1))  //generate infinite succesive values                 

        .log();     

    }

    @GetMapping("/mono")
    public Mono<Integer> mono(){
        return Mono.just(1)
                .log(); 

    }
}

    @Test 

    void mono_WebClient01(){
    Integer expectedValue = new Integer(1);
    EntityExchangeResult<Integer> entityExchangeResult = webTestClient.get().uri("/mono")
            .accept(MediaType.APPLICATION_JSON)
            .exchange()
            .expectStatus().isOk()
            .expectBody(Integer.class)
            .returnResult();
    assertEquals( expectedValue, entityExchangeResult.getResponseBody()); 

    }

    @Test

    void mono_WebClient02(){
    Integer expectedValue = new Integer(1);
    webTestClient.get().uri("/mono")
            .accept(MediaType.APPLICATION_JSON)
            .exchange()
            .expectStatus().isOk()
            .expectBody(Integer.class)
            .consumeWith( response -> {
                    assertEquals( expectedValue, response.getResponseBody()); 

      });

    }

 Running this test

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.2.RELEASE)

2019-12-10 20:21:20.903  INFO 20092 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : Starting FluxMonoControllerTest on DESKTOP-NLF0058 with PID 20092 (started by bext in D:\proy\roadReactiveSpring)
2019-12-10 20:21:20.905  INFO 20092 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : No active profile set, falling back to default profiles: default
2019-12-10 20:21:21.720  INFO 20092 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : Started FluxMonoControllerTest in 1.004 seconds (JVM running for 2.131)
2019-12-10 20:21:22.034  INFO 20092 --- [     parallel-1] reactor.Mono.Just.1                      : | onSubscribe([Synchronous Fuseable] Operators.ScalarSubscription)
2019-12-10 20:21:22.045  INFO 20092 --- [     parallel-1] reactor.Mono.Just.1                      : | request(unbounded)
2019-12-10 20:21:22.046  INFO 20092 --- [     parallel-1] reactor.Mono.Just.1                      : | onNext(1)
2019-12-10 20:21:22.059  INFO 20092 --- [     parallel-1] reactor.Mono.Just.1                      : | onComplete()
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 4s
5 actionable tasks: 4 executed, 1 up-to-date
08:21:22 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.controller.FluxMonoControllerTest.mono_WebClient01"'.

/\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.2.RELEASE)

2019-12-10 20:22:14.344  INFO 10988 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : Starting FluxMonoControllerTest on DESKTOP-NLF0058 with PID 10988 (started by bext in D:\proy\roadReactiveSpring)
2019-12-10 20:22:14.347  INFO 10988 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : No active profile set, falling back to default profiles: default
2019-12-10 20:22:15.059  INFO 10988 --- [    Test worker] c.b.controller.FluxMonoControllerTest    : Started FluxMonoControllerTest in 0.889 seconds (JVM running for 2.125)
2019-12-10 20:22:15.363  INFO 10988 --- [     parallel-1] reactor.Mono.Just.1                      : | onSubscribe([Synchronous Fuseable] Operators.ScalarSubscription)
2019-12-10 20:22:15.365  INFO 10988 --- [     parallel-1] reactor.Mono.Just.1                      : | request(unbounded)
2019-12-10 20:22:15.366  INFO 10988 --- [     parallel-1] reactor.Mono.Just.1                      : | onNext(1)
2019-12-10 20:22:15.376  INFO 10988 --- [     parallel-1] reactor.Mono.Just.1                      : | onComplete()
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 3s
5 actionable tasks: 2 executed, 3 up-to-date
08:22:15 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.controller.FluxMonoControllerTest.mono_WebClient02"'.

Simple Non Blocking API Flux Mono using Handler and Router

Handler

package com.bext.handler;
import org.springframework.http.MediaType; 

import org.springframework.stereotype.Component; 

import org.springframework.web.reactive.function.server.ServerRequest;

import org.springframework.web.reactive.function.server.ServerResponse;

import reactor.core.publisher.Flux; 

import reactor.core.publisher.Mono;
 

@Component

 public class SimpleHandlerFunction {

    public Mono<ServerResponse> flux(ServerRequest serverRequest) {
        return ServerResponse.ok()
                .contentType(MediaType.APPLICATION_JSON)
                .body( Flux.just(1,2,3,4).log(), Integer.class); 

    }

    public Mono<ServerResponse> fluxString(ServerRequest serverRequest) {
        return ServerResponse.ok()
                .contentType(MediaType.APPLICATION_JSON)
                .body( Flux.just("Elemento1","Elemento2","Elemento3","Elemento4").log(),

                                 String.class); 

    }

    public Mono<ServerResponse> mono(ServerRequest serverRequest) {
        return ServerResponse.ok()
                .contentType(MediaType.APPLICATION_JSON)
                .body( Mono.just(1).log(), Integer.class);     

    }

    public Mono<ServerResponse> monoString(ServerRequest serverRequest) {
        return ServerResponse.ok()
                .contentType(MediaType.TEXT_PLAIN)
                .body( Mono.just("Unico").log(), String.class);     

    }
}

Router

package com.bext.router;
import com.bext.handler.SimpleHandlerFunction; 

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Configuration;

import org.springframework.http.MediaType;

import org.springframework.web.reactive.function.server.RouterFunction; 

import org.springframework.web.reactive.function.server.RouterFunctions;

import org.springframework.web.reactive.function.server.ServerRequest;

import org.springframework.web.reactive.function.server.ServerResponse;
import static org.springframework.web.reactive.function.server.RequestPredicates.GET; 

import static org.springframework.web.reactive.function.server.RequestPredicates.accept;
 

@Configuration 

public class RouterFunctionConfig {

    @Bean 

    public RouterFunction<ServerResponse> route(SimpleHandlerFunction simpleHandlerFunction){
        return RouterFunctions
                .route(GET("/functional/flux").and(accept(MediaType.APPLICATION_JSON)), simpleHandlerFunction::flux)
                .andRoute(GET("/functional/fluxString").and(accept(MediaType.APPLICATION_JSON)), simpleHandlerFunction::fluxString)
                .andRoute(GET("/functional/mono").and(accept(MediaType.APPLICATION_JSON)), simpleHandlerFunction::mono)
                .andRoute(GET("/functional/monoString").and(accept(MediaType.APPLICATION_JSON)), simpleHandlerFunction::monoString);    }
}

Running the Application
  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.2.RELEASE)

2019-12-10 20:32:42.252  INFO 15000 --- [           main] com.bext.RoadReactiveSpringApplication   : Starting RoadReactiveSpringApplication on DESKTOP-NLF0058 with PID 15000 (D:\proy\roadReactiveSpring\build\classes\java\main started by bext in D:\proy\roadReactiveSpring)
2019-12-10 20:32:42.255  INFO 15000 --- [           main] com.bext.RoadReactiveSpringApplication   : No active profile set, falling back to default profiles: default
2019-12-10 20:32:42.614  INFO 15000 --- [           main] .s.d.r.c.RepositoryConfigurationDelegate : Bootstrapping Spring Data Reactive MongoDB repositories in DEFAULT mode.
2019-12-10 20:32:42.625  INFO 15000 --- [           main] .s.d.r.c.RepositoryConfigurationDelegate : Finished Spring Data repository scanning in 7ms. Found 0 Reactive MongoDB repository interfaces.
2019-12-10 20:32:42.627  INFO 15000 --- [           main] .s.d.r.c.RepositoryConfigurationDelegate : Bootstrapping Spring Data MongoDB repositories in DEFAULT mode.
2019-12-10 20:32:42.629  INFO 15000 --- [           main] .s.d.r.c.RepositoryConfigurationDelegate : Finished Spring Data repository scanning in 1ms. Found 0 MongoDB repository interfaces.
2019-12-10 20:32:43.031  INFO 15000 --- [           main] org.mongodb.driver.cluster               : Cluster created with settings {hosts=[localhost:27017], mode=SINGLE, requiredClusterType=UNKNOWN, serverSelectionTimeout='30000 ms', maxWaitQueueSize=500}
2019-12-10 20:32:43.066  INFO 15000 --- [localhost:27017] org.mongodb.driver.connection            : Opened connection [connectionId{localValue:1, serverValue:783}] to localhost:27017
2019-12-10 20:32:43.069  INFO 15000 --- [localhost:27017] org.mongodb.driver.cluster               : Monitor thread successfully connected to server with description ServerDescription{address=localhost:27017, type=STANDALONE, state=CONNECTED, ok=true, version=ServerVersion{versionList=[4, 2, 1]}, minWireVersion=0, maxWireVersion=8, maxDocumentSize=16777216, logicalSessionTimeoutMinutes=30, roundTripTimeNanos=1546700}
2019-12-10 20:32:43.185  INFO 15000 --- [           main] org.mongodb.driver.cluster               : Cluster created with settings {hosts=[localhost:27017], mode=SINGLE, requiredClusterType=UNKNOWN, serverSelectionTimeout='30000 ms', maxWaitQueueSize=500}
2019-12-10 20:32:43.552  INFO 15000 --- [           main] o.s.b.web.embedded.netty.NettyWebServer  : Netty started on port(s): 8080
2019-12-10 20:32:43.556  INFO 15000 --- [           main] com.bext.RoadReactiveSpringApplication   : Started RoadReactiveSpringApplication in 1.57 seconds (JVM running for 2.278)
2019-12-10 20:32:43.592  INFO 15000 --- [localhost:27017] org.mongodb.driver.connection            : Opened connection [connectionId{localValue:2, serverValue:784}] to localhost:27017
2019-12-10 20:32:43.594  INFO 15000 --- [localhost:27017] org.mongodb.driver.cluster               : Monitor thread successfully connected to server with description ServerDescription{address=localhost:27017, type=STANDALONE, state=CONNECTED, ok=true, version=ServerVersion{versionList=[4, 2, 1]}, minWireVersion=0, maxWireVersion=8, maxDocumentSize=16777216, logicalSessionTimeoutMinutes=30, roundTripTimeNanos=1314500}
2019-12-10 20:33:03.918  INFO 15000 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onSubscribe([Synchronous Fuseable] FluxArray.ArraySubscription)
2019-12-10 20:33:03.919  INFO 15000 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | request(unbounded)
2019-12-10 20:33:03.919  INFO 15000 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onNext(1)
2019-12-10 20:33:03.919  INFO 15000 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onNext(2)
2019-12-10 20:33:03.919  INFO 15000 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onNext(3)
2019-12-10 20:33:03.919  INFO 15000 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onNext(4)
2019-12-10 20:33:03.919  INFO 15000 --- [ctor-http-nio-2] reactor.Flux.Array.1                     : | onComplete()
2019-12-10 20:33:06.201  INFO 15000 --- [ctor-http-nio-3] reactor.Flux.Array.2                     : | onSubscribe([Synchronous Fuseable] FluxArray.ArraySubscription)
2019-12-10 20:33:06.201  INFO 15000 --- [ctor-http-nio-3] reactor.Flux.Array.2                     : | request(1)
2019-12-10 20:33:06.201  INFO 15000 --- [ctor-http-nio-3] reactor.Flux.Array.2                     : | onNext(Elemento1)
2019-12-10 20:33:06.204  INFO 15000 --- [ctor-http-nio-3] reactor.Flux.Array.2                     : | request(127)
2019-12-10 20:33:06.204  INFO 15000 --- [ctor-http-nio-3] reactor.Flux.Array.2                     : | onNext(Elemento2)
2019-12-10 20:33:06.204  INFO 15000 --- [ctor-http-nio-3] reactor.Flux.Array.2                     : | onNext(Elemento3)
2019-12-10 20:33:06.204  INFO 15000 --- [ctor-http-nio-3] reactor.Flux.Array.2                     : | onNext(Elemento4)
2019-12-10 20:33:06.204  INFO 15000 --- [ctor-http-nio-3] reactor.Flux.Array.2                     : | onComplete()
2019-12-10 20:33:11.608  INFO 15000 --- [ctor-http-nio-4] reactor.Mono.Just.3                      : | onSubscribe([Synchronous Fuseable] Operators.ScalarSubscription)
2019-12-10 20:33:11.608  INFO 15000 --- [ctor-http-nio-4] reactor.Mono.Just.3                      : | request(unbounded)
2019-12-10 20:33:11.608  INFO 15000 --- [ctor-http-nio-4] reactor.Mono.Just.3                      : | onNext(1)
2019-12-10 20:33:11.613  INFO 15000 --- [ctor-http-nio-4] reactor.Mono.Just.3                      : | onComplete()
2019-12-10 20:33:16.211  INFO 15000 --- [ctor-http-nio-1] reactor.Mono.Just.4                      : | onSubscribe([Synchronous Fuseable] Operators.ScalarSubscription)
2019-12-10 20:33:16.211  INFO 15000 --- [ctor-http-nio-1] reactor.Mono.Just.4                      : | request(unbounded)
2019-12-10 20:33:16.211  INFO 15000 --- [ctor-http-nio-1] reactor.Mono.Just.4                      : | onNext(Unico)
2019-12-10 20:33:16.211  INFO 15000 --- [ctor-http-nio-1] reactor.Mono.Just.4                      : | onComplete()
Test on Command line
C:\Users\bext>curl localhost:8080/functional/flux
[1,2,3,4]
C:\Users\bext>curl localhost:8080/functional/fluxString
Elemento1Elemento2Elemento3Elemento4
C:\Users\bext>curl localhost:8080/functional/mono
1
C:\Users\bext>curl localhost:8080/functional/monoString
Unico

JUnit Functional EndPoint using WebTestClient, RestController (@WebFluxTest) and
Functional (@SpringBootTest @AutoConfigureWebTestClient) Approach

In the case of functional web we use @SpringBootTest and @AutoconfigureWebTestClient

package com.bext.handler;
import org.junit.jupiter.api.Test; 

import org.junit.runner.RunWith; 

import org.springframework.beans.factory.annotation.Autowired; 

import org.springframework.boot.test.autoconfigure.web.reactive.AutoConfigureWebTestClient;

 import org.springframework.boot.test.autoconfigure.web.reactive.WebFluxTest;

 import org.springframework.boot.test.context.SpringBootTest; 

import org.springframework.http.MediaType; 

import org.springframework.test.context.junit4.SpringRunner; 

import org.springframework.test.web.reactive.server.WebTestClient; 

import reactor.core.publisher.Flux;

import reactor.test.StepVerifier;
import static org.junit.Assert.assertEquals; 

@RunWith(SpringRunner.class)
@SpringBootTest

@AutoConfigureWebTestClient 

public class SampleHandlerFunctionTest { 

    @Autowired 

    WebTestClient webTestClient;
 

    @Test 

    public void flux_webClient01() {
        Flux<Integer> fluxInteger = webTestClient.get().uri("/functional/flux")
                .accept(MediaType.APPLICATION_JSON)
                .exchange()
                .expectStatus().isOk()
                .returnResult(Integer.class)
                .getResponseBody();
        StepVerifier.create( fluxInteger)
                .expectSubscription()
                .expectNext(1)
                .expectNext(2)
                .expectNext(3)
                .expectNext(4)
                .verifyComplete();     

    }

    @Test 

    public void fluxString_webClient01() {
        Flux<String> fluxString = webTestClient.get().uri("/functional/fluxString")
                .accept(MediaType.APPLICATION_JSON)
                .exchange()
                .expectStatus().isOk()
                .returnResult(String.class)
                .getResponseBody();
        StepVerifier.create( fluxString)
                .expectSubscription()
                .expectNext("Elemento1Elemento2Elemento3Elemento4")
                .verifyComplete();     

    }

    @Test     

    void mono_WebClient02(){
        Integer expectedValue = new Integer(1);
        webTestClient.get().uri("/functional/mono")
                .accept(MediaType.APPLICATION_JSON)
                .exchange()
                .expectStatus().isOk()
                .expectBody(Integer.class)
                .consumeWith( response -> {
                    assertEquals( expectedValue, response.getResponseBody());                });    }
    }

Running this Test

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.2.RELEASE)

2019-12-10 20:52:56.273  INFO 20308 --- [    Test worker] c.b.handler.SampleHandlerFunctionTest    : Starting SampleHandlerFunctionTest on DESKTOP-NLF0058 with PID 20308 (started by bext in D:\proy\ReactiveSpringRoad)
2019-12-10 20:52:56.275  INFO 20308 --- [    Test worker] c.b.handler.SampleHandlerFunctionTest    : No active profile set, falling back to default profiles: default
2019-12-10 20:52:57.368  INFO 20308 --- [    Test worker] c.b.handler.SampleHandlerFunctionTest    : Started SampleHandlerFunctionTest in 1.319 seconds (JVM running for 2.624)
2019-12-10 20:52:57.663  INFO 20308 --- [     parallel-1] reactor.Flux.Array.1                     : | onSubscribe([Synchronous Fuseable] FluxArray.ArraySubscription)
2019-12-10 20:52:57.664  INFO 20308 --- [     parallel-1] reactor.Flux.Array.1                     : | request(1)
2019-12-10 20:52:57.665  INFO 20308 --- [     parallel-1] reactor.Flux.Array.1                     : | onNext(Elemento1)
2019-12-10 20:52:57.705  INFO 20308 --- [    Test worker] reactor.Flux.Array.1                     : | request(31)
2019-12-10 20:52:57.705  INFO 20308 --- [    Test worker] reactor.Flux.Array.1                     : | onNext(Elemento2)
2019-12-10 20:52:57.705  INFO 20308 --- [    Test worker] reactor.Flux.Array.1                     : | onNext(Elemento3)
2019-12-10 20:52:57.705  INFO 20308 --- [    Test worker] reactor.Flux.Array.1                     : | onNext(Elemento4)
2019-12-10 20:52:57.706  INFO 20308 --- [    Test worker] reactor.Flux.Array.1                     : | onComplete()
2019-12-10 20:52:57.734  INFO 20308 --- [     parallel-2] reactor.Flux.Array.2                     : | onSubscribe([Synchronous Fuseable] FluxArray.ArraySubscription)
2019-12-10 20:52:57.734  INFO 20308 --- [     parallel-2] reactor.Flux.Array.2                     : | request(unbounded)
2019-12-10 20:52:57.734  INFO 20308 --- [     parallel-2] reactor.Flux.Array.2                     : | onNext(1)
2019-12-10 20:52:57.734  INFO 20308 --- [     parallel-2] reactor.Flux.Array.2                     : | onNext(2)
2019-12-10 20:52:57.734  INFO 20308 --- [     parallel-2] reactor.Flux.Array.2                     : | onNext(3)
2019-12-10 20:52:57.734  INFO 20308 --- [     parallel-2] reactor.Flux.Array.2                     : | onNext(4)
2019-12-10 20:52:57.734  INFO 20308 --- [     parallel-2] reactor.Flux.Array.2                     : | onComplete()
2019-12-10 20:52:57.762  INFO 20308 --- [     parallel-3] reactor.Mono.Just.3                      : | onSubscribe([Synchronous Fuseable] Operators.ScalarSubscription)
2019-12-10 20:52:57.762  INFO 20308 --- [     parallel-3] reactor.Mono.Just.3                      : | request(unbounded)
2019-12-10 20:52:57.762  INFO 20308 --- [     parallel-3] reactor.Mono.Just.3                      : | onNext(1)
2019-12-10 20:52:57.763  INFO 20308 --- [     parallel-3] reactor.Mono.Just.3                      : | onComplete()
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 4s
5 actionable tasks: 2 executed, 3 up-to-date
08:52:57 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.handler.SampleHandlerFunctionTest"'.
eot

Road Reactive Spring: JUnit Test, Flux, Mono, Exception, Gradle (1)

Road Reactive Spring: JUnit Test, Flux, Mono, Exception, Gradle (1)

The Git road along this issue

Flux suscribe(Consumer <? super String>consumer)

Running this Test

17:38:54.700 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
Elemento1
Elemento2
Elemento3
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
05:38:54 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxTest"'.

Flux
suscribe(Consumer <? super String> consumer, Consumer <? super Throwable> errorConsumer)

class FluxMonoTest {

    @Test 

    void fluxTest() {
        Flux<String> fluxString = Flux.just("Elemento1", "Elemento2", "Elemento3")
           .concatWith( Flux.error( new RuntimeException("Exception generated voluntarily"))); 

        fluxString.subscribe(System.out::println, ex -> System.err.println(ex)); 

       }
}

Running this Test
 17:47:38.429 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
Elemento1
Elemento2
Elemento3
java.lang.RuntimeException: Exception generated voluntarily
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 3s
5 actionable tasks: 3 executed, 2 up-to-date
05:47:38 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxTest"'.

Flux
suscribe(Consumer <? super String> consumer, Consumer <? super Throwable> errorConsumer)

class FluxMonoTest {

    @Test 

    void fluxTest() {
        Flux<String> fluxString = Flux.just("Elemento1", "Elemento2", "Elemento3")
           .concatWith( Flux.error( new RuntimeException("Exception generated voluntarily")));         

        fluxString.subscribe(System.out::println, System.err::println);     

    }
}

Running this test

17:51:06.333 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
Elemento1
Elemento2
Elemento3
java.lang.RuntimeException: Exception generated voluntarily
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
05:51:06 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxTest"'.

Add .Log()

class FluxMonoTest {

    @Test

    void fluxTest() {
        Flux<String> fluxString = Flux.just("Elemento1", "Elemento2", "Elemento3")
          .concatWith( Flux.error( new RuntimeException("Exception generated voluntarily")))
          .log(); 

       fluxString.subscribe(System.out::println, System.err::println);     

    }
}

Running this test
17:58:56.906 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
17:58:56.918 [Test worker] INFO reactor.Flux.ConcatArray.1 - onSubscribe(FluxConcatArray.ConcatArraySubscriber)
17:58:56.920 [Test worker] INFO reactor.Flux.ConcatArray.1 - request(unbounded)
17:58:56.921 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento1)
Elemento1
17:58:56.921 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento2)
Elemento2
17:58:56.921 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento3)
Elemento3
17:58:56.922 [Test worker] ERROR reactor.Flux.ConcatArray.1 - onError(java.lang.RuntimeException: Exception generated voluntarily)
17:58:56.926 [Test worker] ERROR reactor.Flux.ConcatArray.1 -
java.lang.RuntimeException: Exception generated voluntarily
    at com.bext.fluxmonoroad.FluxMonoTest.fluxTest(FluxMonoTest.java:10)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at ....
.... java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
    at java.base/java.lang.Thread.run(Thread.java:834)
java.lang.RuntimeException: Exception generated voluntarily
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
05:58:57 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxTest"'.

concatWith( Flux.just("...")....concatWith( Flux.just() ... if Exception flux stopped

class FluxMonoTest {

    @Test 

    void fluxTest() {
        Flux<String> fluxString = Flux.just("Elemento1", "Elemento2", "Elemento3")
                .concatWith( Flux.just("Esto se Mostrara"))
                .concatWith( Flux.error( new RuntimeException("Exception generated voluntarily")))
                .concatWith( Flux.just("Esto NO se Mostara"))
                .log(); 

        fluxString.subscribe(System.out::println, ex ->System.err.println("El error es :" + ex)); 

    }
}

Run this test
18:07:26.059 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:07:26.071 [Test worker] INFO reactor.Flux.ConcatArray.1 - onSubscribe(FluxConcatArray.ConcatArraySubscriber)
18:07:26.074 [Test worker] INFO reactor.Flux.ConcatArray.1 - request(unbounded)
18:07:26.075 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento1)
Elemento1
18:07:26.075 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento2)
Elemento2
18:07:26.076 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento3)
Elemento3
18:07:26.076 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Esto se Mostrara)
Esto se Mostrara
18:07:26.077 [Test worker] ERROR reactor.Flux.ConcatArray.1 - onError(java.lang.RuntimeException: Exception generated voluntarily)
18:07:26.080 [Test worker] ERROR reactor.Flux.ConcatArray.1 -
java.lang.RuntimeException: Exception generated voluntarily
    at com.bext.fluxmonoroad.FluxMonoTest.fluxTest(FluxMonoTest.java:11)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at ... at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
    at java.base/java.lang.Thread.run(Thread.java:834)
El error es :java.lang.RuntimeException: Exception generated voluntarily
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
06:07:26 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxTest"'.

If Exception, flux stopped, also in subscribe(Consumer...,...errorConsumer, Runnable)

class FluxMonoTest {

    @Test     

    void fluxTest() {
        Flux<String> fluxString = Flux.just("Elemento1", "Elemento2", "Elemento3")
                .concatWith( Flux.just("Esto se Mostrara"))
                .concatWith( Flux.error( new RuntimeException("Exception generated voluntarily")))
                .concatWith( Flux.just("Esto NO se Mostara"))
                .log();         

        fluxString.subscribe(System.out::println, 

                         ex -> System.err.println("El error es :" + ex),

                         () -> System.out.println("Completado"));     

    }
}

Running this test 
18:13:05.861 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:13:05.872 [Test worker] INFO reactor.Flux.ConcatArray.1 - onSubscribe(FluxConcatArray.ConcatArraySubscriber)
18:13:05.874 [Test worker] INFO reactor.Flux.ConcatArray.1 - request(unbounded)
18:13:05.875 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento1)
Elemento1
18:13:05.875 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento2)
Elemento2
18:13:05.875 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento3)
Elemento3
18:13:05.876 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Esto se Mostrara)
Esto se Mostrara
18:13:05.877 [Test worker] ERROR reactor.Flux.ConcatArray.1 - onError(java.lang.RuntimeException: Exception generated voluntarily)
18:13:05.879 [Test worker] ERROR reactor.Flux.ConcatArray.1 -
java.lang.RuntimeException: Exception generated voluntarily
    at com.bext.fluxmonoroad.FluxMonoTest.fluxTest(FluxMonoTest.java:11)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at ....
org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
    at java.base/java.lang.Thread.run(Thread.java:834)
El error es :java.lang.RuntimeException: Exception generated voluntarily
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 3s
5 actionable tasks: 3 executed, 2 up-to-date
06:13:06 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxTest"'.

Exception is suprimed, then all the Flux is showed including in subscribe(.... Runnable)

class FluxMonoTest {

    @Test 

    void fluxTest() {
        Flux<String> fluxString = Flux.just("Elemento1", "Elemento2", "Elemento3")
                .concatWith( Flux.just("Esto se Mostrara"))
                //.concatWith( Flux.error( new RuntimeException("Exception generated voluntarily"))) 

                .concatWith( Flux.just("Esto NO se Mostara, Se muestra ya no hay exception"))
                .log();         

        fluxString.subscribe(System.out::println,

                              ex -> System.err.println("El error es :" + ex), 

                             () -> System.out.println("Completado")); 

    }
}

Running this Test
18:17:18.160 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:17:18.171 [Test worker] INFO reactor.Flux.ConcatArray.1 - onSubscribe(FluxConcatArray.ConcatArraySubscriber)
18:17:18.173 [Test worker] INFO reactor.Flux.ConcatArray.1 - request(unbounded)
18:17:18.174 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento1)
Elemento1
18:17:18.174 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento2)
Elemento2
18:17:18.174 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento3)
Elemento3
18:17:18.175 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Esto se Mostrara)
Esto se Mostrara
18:17:18.175 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Esto NO se Mostara, Se muestra ya no hay exception)
Esto NO se Mostara, Se muestra ya no hay exception
18:17:18.175 [Test worker] INFO reactor.Flux.ConcatArray.1 - onComplete()
Completado
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
06:17:18 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxTest"'.

JUnit Test .expectedNext("ElementX") ... verifyComplete()

    @Test

    void fluxElements_WithoutError(){
    Flux<String> fluxString = Flux.just("Elemento1","Elemento2","Elemento3")
            .log(); 

    StepVerifier.create( fluxString)
            .expectNext("Elemento1")
            .expectNext("Elemento2")
            .expectNext("Elemento3")
            .verifyComplete(); 

    }

Running this test
18:24:27.641 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:24:27.672 [Test worker] INFO reactor.Flux.Array.1 - | onSubscribe([Synchronous Fuseable] FluxArray.ArraySubscription)
18:24:27.676 [Test worker] INFO reactor.Flux.Array.1 - | request(unbounded)
18:24:27.676 [Test worker] INFO reactor.Flux.Array.1 - | onNext(Elemento1)
18:24:27.676 [Test worker] INFO reactor.Flux.Array.1 - | onNext(Elemento2)
18:24:27.676 [Test worker] INFO reactor.Flux.Array.1 - | onNext(Elemento3)
18:24:27.677 [Test worker] INFO reactor.Flux.Array.1 - | onComplete()
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
06:24:27 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxElements_WithoutError"'.

JUnit Test Not Passed elements not in order expected

    @Test

    void fluxElements_WithoutError(){
    Flux<String> fluxString = Flux.just("Elemento1","Elemento2","Elemento3")
            .log(); 

    StepVerifier.create( fluxString)
            .expectNext("Elemento3")
            .expectNext("Elemento2")
            .expectNext("Elemento1")
            .verifyComplete(); 

    }

Running this test
18:25:20.284 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:25:20.305 [Test worker] INFO reactor.Flux.Array.1 - | onSubscribe([Synchronous Fuseable] FluxArray.ArraySubscription)
18:25:20.307 [Test worker] INFO reactor.Flux.Array.1 - | request(unbounded)
18:25:20.308 [Test worker] INFO reactor.Flux.Array.1 - | onNext(Elemento1)
18:25:20.311 [Test worker] INFO reactor.Flux.Array.1 - | cancel()

expectation "expectNext(Elemento3)" failed (expected value: Elemento3; actual value: Elemento1)
java.lang.AssertionError: expectation "expectNext(Elemento3)" failed (expected value: Elemento3; actual value: Elemento1)
    at reactor.test.MessageFormatter.assertionError(MessageFormatter.java:115)
    at reactor.test.MessageFormatter.failPrefix(MessageFormatter.java:104)
    at reactor.test.MessageFormatter.fail(MessageFormatter.java:73)
    at reactor.test.MessageFormatter.failOptional(MessageFormatter.java:88)
    at.......org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
    at java.base/java.lang.Thread.run(Thread.java:834)

com.bext.fluxmonoroad.FluxMonoTest > fluxElements_WithoutError() FAILED
    java.lang.AssertionError at FluxMonoTest.java:29
1 test completed, 1 failed
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':test'.
> There were failing tests. See the report at: file:///D:/proy/roadReactiveSpring/build/reports/tests/test/index.html
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
* Get more help at https://help.gradle.org
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD FAILED in 3s
5 actionable tasks: 3 executed, 2 up-to-date

JUnit Test, StepVerifier without .verifyComplete dont is executed at all, (Lazy execution)

    @Test 

    void fluxElements_WithoutError(){
    Flux<String> fluxString = Flux.just("Elemento1","Elemento2","Elemento3")
            .log(); 

    StepVerifier.create( fluxString)
            .expectNext("Elemento1")
            .expectNext("Elemento2")
            .expectNext("Elemento3"); 

            //.verifyComplete(); 

    }

Running this Test
18:30:32.775 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
06:30:32 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxElements_WithoutError"'.

JUnit Test, RuntimeException .verifyComplete() Dont Pass

    @Test

    void fluxElements_WithError(){
    Flux<String> fluxString = Flux.just("Elemento1","Elemento2","Elemento3")
            .concatWith( Flux.error( new RuntimeException("Exception generated voluntarily")))
            .log(); 

    StepVerifier.create( fluxString)
            .expectNext("Elemento1")
            .expectNext("Elemento2")
            .expectNext("Elemento3")
            .verifyComplete();

    }

Running this test
18:34:43.775 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:34:43.795 [Test worker] INFO reactor.Flux.ConcatArray.1 - onSubscribe(FluxConcatArray.ConcatArraySubscriber)
18:34:43.798 [Test worker] INFO reactor.Flux.ConcatArray.1 - request(unbounded)
18:34:43.799 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento1)
18:34:43.799 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento2)
18:34:43.799 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento3)
18:34:43.800 [Test worker] ERROR reactor.Flux.ConcatArray.1 - onError(java.lang.RuntimeException: Exception generated voluntarily)
18:34:43.802 [Test worker] ERROR reactor.Flux.ConcatArray.1 -
java.lang.RuntimeException: Exception generated voluntarily
....
com.bext.fluxmonoroad.FluxMonoTest > fluxElements_WithError() FAILED
    java.lang.AssertionError at FluxMonoTest.java:41
1 test completed, 1 failed
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':test'.
> There were failing tests. See the report at: file:///D:/proy/roadReactiveSpring/build/reports/tests/test/index.html
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
* Get more help at https://help.gradle.org
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD FAILED in 3s
5 actionable tasks: 3 executed, 2 up-to-date

JUnit Test, RuntimeException .expectError(...).verify(); PASS

    @Test

    void fluxElements_WithError(){
    Flux<String> fluxString = Flux.just("Elemento1","Elemento2","Elemento3")
            .concatWith( Flux.error( new RuntimeException("Exception generated voluntarily")))
            .log(); 

    StepVerifier.create( fluxString)
            .expectNext("Elemento1")
            .expectNext("Elemento2")
            .expectNext("Elemento3")
            .expectError(RuntimeException.class)
            .verify(); 

    }

Running this test
18:36:55.313 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:36:55.333 [Test worker] INFO reactor.Flux.ConcatArray.1 - onSubscribe(FluxConcatArray.ConcatArraySubscriber)
18:36:55.336 [Test worker] INFO reactor.Flux.ConcatArray.1 - request(unbounded)
18:36:55.337 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento1)
18:36:55.337 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento2)
18:36:55.337 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento3)
18:36:55.338 [Test worker] ERROR reactor.Flux.ConcatArray.1 - onError(java.lang.RuntimeException: Exception generated voluntarily)
18:36:55.339 [Test worker] ERROR reactor.Flux.ConcatArray.1 -
java.lang.RuntimeException: Exception generated voluntarily
    at com.bext.fluxmonoroad.FluxMonoTest.fluxElements_WithError(FluxMonoTest.java:34)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at ...    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
    at java.base/java.lang.Thread.run(Thread.java:834)
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
06:36:55 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxElements_WithError"'.

JUnit Test, RuntimeException .expectErrorMessage("...").verify(); PASS

    @Test

    void fluxElements_WithError(){
    Flux<String> fluxString = Flux.just("Elemento1","Elemento2","Elemento3")
            .concatWith( Flux.error( new RuntimeException("Exception generated voluntarily")))
            .log();     

    StepVerifier.create( fluxString)
            .expectNext("Elemento1")
            .expectNext("Elemento2")
            .expectNext("Elemento3")
            .expectErrorMessage("Exception generated voluntarily")
            .verify();

    }

Running this test
18:42:13.106 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:42:13.128 [Test worker] INFO reactor.Flux.ConcatArray.1 - onSubscribe(FluxConcatArray.ConcatArraySubscriber)
18:42:13.131 [Test worker] INFO reactor.Flux.ConcatArray.1 - request(unbounded)
18:42:13.132 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento1)
18:42:13.132 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento2)
18:42:13.132 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento3)
18:42:13.133 [Test worker] ERROR reactor.Flux.ConcatArray.1 - onError(java.lang.RuntimeException: Exception generated voluntarily)
18:42:13.136 [Test worker] ERROR reactor.Flux.ConcatArray.1 -
java.lang.RuntimeException: Exception generated voluntarily
    at com.bext.fluxmonoroad.FluxMonoTest.fluxElements_WithError(FluxMonoTest.java:34)
...Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
06:42:13 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxElements_WithError"'.

JUnit Test, .expectNextCount(number) PASS

    @Test

    void fluxElementsCount_WithError(){
    Flux<String> fluxString = Flux.just("Elemento1","Elemento2","Elemento3")
            .concatWith( Flux.error( new RuntimeException("Exception generated voluntarily")))
            .log(); 

    StepVerifier.create( fluxString)
            .expectNextCount(3)
            .expectErrorMessage("Exception generated voluntarily")
            .verify(); 

    }

Running this test
18:44:04.457 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:44:04.477 [Test worker] INFO reactor.Flux.ConcatArray.1 - onSubscribe(FluxConcatArray.ConcatArraySubscriber)
18:44:04.480 [Test worker] INFO reactor.Flux.ConcatArray.1 - request(unbounded)
18:44:04.481 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento1)
18:44:04.482 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento2)
18:44:04.482 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento3)
18:44:04.483 [Test worker] ERROR reactor.Flux.ConcatArray.1 - onError(java.lang.RuntimeException: Exception generated voluntarily)
18:44:04.485 [Test worker] ERROR reactor.Flux.ConcatArray.1 -
java.lang.RuntimeException: Exception generated voluntarily
...
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
06:44:04 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxElementsCount_WithError"'.

JUnit Test, .expectNext(element1, element2,...,elementN) PASS

    @Test 

    void fluxElements_WithError1(){
    Flux<String> fluxString = Flux.just("Elemento1","Elemento2","Elemento3")
            .concatWith( Flux.error( new RuntimeException("Exception generated voluntarily")))
            .log(); 

    StepVerifier.create( fluxString)
            .expectNext("Elemento1","Elemento2","Elemento3")
            .expectErrorMessage("Exception generated voluntarily")
            .verify();

    }

Running this test
18:48:04.825 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:48:04.848 [Test worker] INFO reactor.Flux.ConcatArray.1 - onSubscribe(FluxConcatArray.ConcatArraySubscriber)
18:48:04.851 [Test worker] INFO reactor.Flux.ConcatArray.1 - request(unbounded)
18:48:04.852 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento1)
18:48:04.852 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento2)
18:48:04.852 [Test worker] INFO reactor.Flux.ConcatArray.1 - onNext(Elemento3)
18:48:04.853 [Test worker] ERROR reactor.Flux.ConcatArray.1 - onError(java.lang.RuntimeException: Exception generated voluntarily)
18:48:04.856 [Test worker] ERROR reactor.Flux.ConcatArray.1 -
java.lang.RuntimeException: Exception generated voluntarily
    at com.bext.fluxmonoroad.FluxMonoTest.fluxElements_WithError1(FluxMonoTest.java:58)
....
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
06:48:04 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.fluxElements_WithError1"'.

JUnit Mono Test PASS

@Test

    void monoTest(){
    Mono<String> monoString = Mono.just("unico");
    StepVerifier.create( monoString.log())
            .expectNext("unico")
            .verifyComplete(); 

}

Running this Test
18:50:05.634 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:50:05.660 [Test worker] INFO reactor.Mono.Just.1 - | onSubscribe([Synchronous Fuseable] Operators.ScalarSubscription)
18:50:05.664 [Test worker] INFO reactor.Mono.Just.1 - | request(unbounded)
18:50:05.664 [Test worker] INFO reactor.Mono.Just.1 - | onNext(unico)
18:50:05.664 [Test worker] INFO reactor.Mono.Just.1 - | onComplete()
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
06:50:05 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.monoTest"'.

JUnit Mono Test with RuntimeException PASS

@Test

    void monoTest_WithError(){
    StepVerifier.create( Mono.error(new RuntimeException("Exception generated voluntarily")).log())
            .expectError(RuntimeException.class)
            .verify(); 

}

look at the .log() that permit see all the events between the suscriber an the mono.

Running this Test
18:51:54.761 [Test worker] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
18:51:54.780 [Test worker] INFO reactor.Mono.Error.1 - onSubscribe([Fuseable] Operators.EmptySubscription)
18:51:54.782 [Test worker] INFO reactor.Mono.Error.1 - request(unbounded)
18:51:54.783 [Test worker] ERROR reactor.Mono.Error.1 - onError(java.lang.RuntimeException: Exception generated voluntarily)
18:51:54.785 [Test worker] ERROR reactor.Mono.Error.1 -
java.lang.RuntimeException: Exception generated voluntarily
...
Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.0.1/userguide/command_line_interface.html#sec:command_line_warnings
BUILD SUCCESSFUL in 2s
5 actionable tasks: 3 executed, 2 up-to-date
06:51:54 p. m.: Tasks execution finished ':cleanTest :test --tests "com.bext.fluxmonoroad.FluxMonoTest.monoTest_WithError"'.

Ref code with Dilip

eot