Replace bindToHttpHandler with bindToWebHandler

This commit replaces the option to bind the WebTestClient to an
HttpHandler to bind to a WebHandler instead.

This allows testing below the WebFlux level such as WebFilter,
WebHandler, or WebSession scenarios, but still a level above
HttpHandler so that WebTestClient is in charge of creating the
ServerWebExchange and expose consistently the
WebTestClient#MockServerSpec setup across all "mock" server bindToXxx
options.

Issue: SPR-15570

spring-test/src/main/java/org/springframework/test/web/reactive/server/DefaultMockServerSpec.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2002-2017 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
+ *
+ *      http://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.
+ */
+package org.springframework.test.web.reactive.server;
+
+import org.springframework.util.Assert;
+import org.springframework.web.server.WebHandler;
+import org.springframework.web.server.adapter.WebHttpHandlerBuilder;
+
+/**
+ *
+ * @author Rossen Stoyanchev
+ * @since 5.0
+ */
+class DefaultMockServerSpec extends AbstractMockServerSpec<DefaultMockServerSpec> {
+
+	private final WebHandler webHandler;
+
+
+	DefaultMockServerSpec(WebHandler webHandler) {
+		Assert.notNull(webHandler, "'WebHandler' is required");
+		this.webHandler = webHandler;
+	}
+
+
+	@Override
+	protected WebHttpHandlerBuilder initHttpHandlerBuilder() {
+		return WebHttpHandlerBuilder.webHandler(this.webHandler);
+	}
+
+}

spring-test/src/main/java/org/springframework/test/web/reactive/server/MockServerExchangeMutator.java

@@ -33,9 +33,10 @@ import org.springframework.web.server.WebFilterChain;
  * transformations during requests from the {@code WebTestClient} to a mock
  * server -- i.e. when one of the following is in use:
  * <ul>
- * <li>{@link WebTestClient#bindToController},
+ * <li>{@link WebTestClient#bindToController}
  * <li>{@link WebTestClient#bindToRouterFunction}
- * <li>{@link WebTestClient#bindToApplicationContext}.
+ * <li>{@link WebTestClient#bindToApplicationContext}
+ * <li>{@link WebTestClient#bindToWebHandler}
  * </ul>
  *
  * <p>Example of registering a "global" transformation:

spring-test/src/main/java/org/springframework/test/web/reactive/server/WebTestClient.java

@@ -34,7 +34,6 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.http.client.reactive.ClientHttpRequest;
 import org.springframework.http.codec.ServerCodecConfigurer;
-import org.springframework.http.server.reactive.HttpHandler;
 import org.springframework.util.MultiValueMap;
 import org.springframework.validation.Validator;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolverBuilder;
@@ -51,6 +50,7 @@ import org.springframework.web.reactive.function.server.HandlerStrategies;
 import org.springframework.web.reactive.function.server.RouterFunction;
 import org.springframework.web.reactive.result.method.annotation.ArgumentResolverConfigurer;
 import org.springframework.web.server.WebFilter;
+import org.springframework.web.server.WebHandler;
 import org.springframework.web.util.UriBuilder;
 import org.springframework.web.util.UriBuilderFactory;
 
@@ -130,7 +130,7 @@ public interface WebTestClient {
 	// Static, factory methods
 
 	/**
-	 * Integration testing without a server targeting specific annotated,
+	 * Integration testing with a "mock" server targeting specific annotated,
 	 * WebFlux controllers. The default configuration is the same as for
 	 * {@link org.springframework.web.reactive.config.EnableWebFlux @EnableWebFlux}
 	 * but can also be further customized through the returned spec.
@@ -142,9 +142,9 @@ public interface WebTestClient {
 	}
 
 	/**
-	 * Integration testing without a server with WebFlux infrastructure detected
-	 * from an {@link ApplicationContext} such as {@code @EnableWebFlux}
-	 * Java config and annotated controller Spring beans.
+	 * Integration testing with a "mock" server with WebFlux infrastructure
+	 * detected from an {@link ApplicationContext} such as
+	 * {@code @EnableWebFlux} Java config and annotated controller Spring beans.
 	 * @param applicationContext the context
 	 * @return the {@link WebTestClient} builder
 	 * @see org.springframework.web.reactive.config.EnableWebFlux
@@ -163,12 +163,12 @@ public interface WebTestClient {
 	}
 
 	/**
-	 * Integration testing without a server targeting the given HttpHandler.
-	 * @param httpHandler the handler to test
+	 * Integration testing with a "mock" server targeting the given WebHandler.
+	 * @param webHandler the handler to test
 	 * @return the {@link WebTestClient} builder
 	 */
-	static Builder bindToHttpHandler(HttpHandler httpHandler) {
-		return new DefaultWebTestClientBuilder(httpHandler);
+	static MockServerSpec<?> bindToWebHandler(WebHandler webHandler) {
+		return new DefaultMockServerSpec(webHandler);
 	}
 
 	/**