Make reactive user details back off with Auth Manager Resolver bean

Closes gh-31317

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/security/reactive/ReactiveUserDetailsServiceAutoConfigurationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,6 +31,7 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
 import org.springframework.security.authentication.ReactiveAuthenticationManager;
+import org.springframework.security.authentication.ReactiveAuthenticationManagerResolver;
 import org.springframework.security.config.annotation.rsocket.EnableRSocketSecurity;
 import org.springframework.security.config.annotation.web.reactive.EnableWebFluxSecurity;
 import org.springframework.security.core.userdetails.MapReactiveUserDetailsService;
@@ -91,6 +92,13 @@ class ReactiveUserDetailsServiceAutoConfigurationTests {
 				.run((context) -> assertThat(context).getBean(ReactiveUserDetailsService.class).isNull());
 	}
 
+	@Test
+	void doesNotConfigureDefaultUserIfAuthenticationManagerResolverAvailable() {
+		this.contextRunner.withUserConfiguration(AuthenticationManagerResolverConfig.class)
+				.run((context) -> assertThat(context).hasSingleBean(ReactiveAuthenticationManagerResolver.class)
+						.doesNotHaveBean(ReactiveUserDetailsService.class));
+	}
+
 	@Test
 	void doesNotConfigureDefaultUserIfResourceServerWithJWTIsUsed() {
 		this.contextRunner.withUserConfiguration(JwtDecoderConfiguration.class).run((context) -> {
@@ -179,6 +187,16 @@ class ReactiveUserDetailsServiceAutoConfigurationTests {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	static class AuthenticationManagerResolverConfig {
+
+		@Bean
+		ReactiveAuthenticationManagerResolver<?> reactiveAuthenticationManagerResolver() {
+			return mock(ReactiveAuthenticationManagerResolver.class);
+		}
+
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	@Import(TestSecurityConfiguration.class)
 	static class TestConfigWithPasswordEncoder {