Add BlockHoundIntegration for spring-web

Closes gh-26712

spring-web/spring-web.gradle

@@ -7,6 +7,7 @@ dependencies {
 	compile(project(":spring-beans"))
 	compile(project(":spring-core"))
 	compileOnly(project(":kotlin-coroutines"))
+	compileOnly("io.projectreactor.tools:blockhound")
 	optional(project(":spring-aop"))
 	optional(project(":spring-context"))
 	optional(project(":spring-oxm"))
@@ -75,6 +76,7 @@ dependencies {
 	testCompile("org.skyscreamer:jsonassert")
 	testCompile("org.xmlunit:xmlunit-assertj")
 	testCompile("org.xmlunit:xmlunit-matchers")
+	testCompile("io.projectreactor.tools:blockhound")
 	testRuntime("com.sun.mail:javax.mail")
 	testRuntime("com.sun.xml.bind:jaxb-core")
 	testRuntime("com.sun.xml.bind:jaxb-impl")

spring-web/src/main/java/org/springframework/web/server/adapter/WebHttpHandlerBuilder.java

@@ -23,6 +23,9 @@ import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.stream.Collectors;
 
+import reactor.blockhound.BlockHound;
+import reactor.blockhound.integration.BlockHoundIntegration;
+
 import org.springframework.beans.factory.NoSuchBeanDefinitionException;
 import org.springframework.context.ApplicationContext;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
@@ -418,4 +421,20 @@ public final class WebHttpHandlerBuilder {
 		return new WebHttpHandlerBuilder(this);
 	}
 
+
+	/**
+	 * {@code BlockHoundIntegration} for spring-web classes.
+	 * @since 5.3.6
+	 */
+	public static class SpringWebBlockHoundIntegration implements BlockHoundIntegration {
+
+		@Override
+		public void applyTo(BlockHound.Builder builder) {
+
+			// Avoid hard references potentially anywhere in spring-web (no need for structural dependency)
+
+			builder.allowBlockingCallsInside("org.springframework.web.util.HtmlUtils", "<clinit>");
+		}
+	}
+
 }

spring-web/src/main/resources/META-INF/services/reactor.blockhound.integration.BlockHoundIntegration

@@ -0,0 +1,15 @@
+# Copyright 2002-2021 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.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+org.springframework.web.server.adapter.WebHttpHandlerBuilder$SpringWebBlockHoundIntegration