AsyncHandlerInterceptor.afterConcurrentHandlingStarted declared as default method

Issue: SPR-15702

spring-webmvc/src/main/java/org/springframework/web/servlet/AsyncHandlerInterceptor.java

 /*
- * Copyright 2002-2016 the original author or authors.
+ * 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.
@@ -60,8 +60,8 @@ import org.springframework.web.method.HandlerMethod;
 public interface AsyncHandlerInterceptor extends HandlerInterceptor {
 
 	/**
-	 * Called instead of {@code postHandle} and {@code afterCompletion}, when
-	 * the a handler is being executed concurrently.
+	 * Called instead of {@code postHandle} and {@code afterCompletion}
+	 * when the handler is being executed concurrently.
 	 * <p>Implementations may use the provided request and response but should
 	 * avoid modifying them in ways that would conflict with the concurrent
 	 * execution of the handler. A typical use of this method would be to
@@ -72,7 +72,9 @@ public interface AsyncHandlerInterceptor extends HandlerInterceptor {
 	 * execution, for type and/or instance examination
 	 * @throws Exception in case of errors
 	 */
-	void afterConcurrentHandlingStarted(HttpServletRequest request, HttpServletResponse response, Object handler)
-			throws Exception;
+	default void afterConcurrentHandlingStarted(
+			HttpServletRequest request, HttpServletResponse response, Object handler)
+			throws Exception {
+	}
 
 }