Add WebTestClientConfigurer

Issue: SPR-15674

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

@@ -96,6 +96,19 @@ public class WebHttpHandlerBuilder {
 		this.webHandler = webHandler;
 	}
 
+	/**
+	 * Copy constructor.
+	 */
+	private WebHttpHandlerBuilder(WebHttpHandlerBuilder other) {
+
+		this.webHandler = other.webHandler;
+		this.filters.addAll(other.filters);
+		this.exceptionHandlers.addAll(other.exceptionHandlers);
+		this.sessionManager = other.sessionManager;
+		this.codecConfigurer = other.codecConfigurer;
+		this.localeContextResolver = other.localeContextResolver;
+	}
+
 
 	/**
 	 * Static factory method to create a new builder instance.
@@ -263,6 +276,14 @@ public class WebHttpHandlerBuilder {
 		return adapted;
 	}
 
+	/**
+	 * Clone this {@link WebHttpHandlerBuilder}.
+	 * @return the cloned builder instance
+	 */
+	public WebHttpHandlerBuilder cloneBuilder() {
+		return new WebHttpHandlerBuilder(this);
+	}
+
 
 	private static class SortedBeanContainer {
 

spring-web/src/main/java/org/springframework/web/server/handler/ExceptionHandlingWebHandler.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.server.handler;
 
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 
@@ -40,7 +41,7 @@ public class ExceptionHandlingWebHandler extends WebHandlerDecorator {
 
 	public ExceptionHandlingWebHandler(WebHandler delegate, List<WebExceptionHandler> handlers) {
 		super(delegate);
-		this.exceptionHandlers = Collections.unmodifiableList(handlers);
+		this.exceptionHandlers = Collections.unmodifiableList(new ArrayList<>(handlers));
 	}