Fail Gradle build for Javadoc warnings

In order to catch Javadoc errors in the build, we now enable the
`Xwerror` flag for the `javadoc` tool. In addition, we now use
`Xdoclint:syntax` instead of `Xdoclint:none` in order to validate
syntax within our Javadoc.

This commit fixes all resulting Javadoc errors and warnings.

This commit also upgrades to Undertow 2.2.12.Final and fixes the
artifact names for exclusions for the Servlet and annotations APIs.

The incorrect exclusion of the Servlet API resulted in the Servlet API
being on the classpath twice for the javadoc task, which resulted in the
following warnings in previous builds.

javadoc: warning - Multiple sources of package comments found for package "javax.servlet"
javadoc: warning - Multiple sources of package comments found for package "javax.servlet.http"
javadoc: warning - Multiple sources of package comments found for package "javax.servlet.descriptor"
javadoc: warning - Multiple sources of package comments found for package "javax.servlet.annotation"

Closes gh-27480

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/ExchangeFunction.java

@@ -32,7 +32,7 @@ import reactor.core.publisher.Mono;
  *
  * Mono<String> bodyMono = exchangeFunction
  *     .exchange(request)
- *     .flatMap(response -> response.bodyToMono(String.class));
+ *     .flatMap(response -> response.bodyToMono(String.class));
  * </pre>
  *
  * @author Arjen Poutsma

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/WebClient.java

@@ -522,7 +522,7 @@ public interface WebClient {
 		 * Mono<Object> entityMono = client.get()
 		 *     .uri("/persons/1")
 		 *     .accept(MediaType.APPLICATION_JSON)
-		 *     .exchangeToMono(response -> {
+		 *     .exchangeToMono(response -> {
 		 *         if (response.statusCode().equals(HttpStatus.OK)) {
 		 *             return response.bodyToMono(Person.class);
 		 *         }
@@ -554,7 +554,7 @@ public interface WebClient {
 		 * Mono<Object> entityMono = client.get()
 		 *     .uri("/persons")
 		 *     .accept(MediaType.APPLICATION_JSON)
-		 *     .exchangeToFlux(response -> {
+		 *     .exchangeToFlux(response -> {
 		 *         if (response.statusCode().equals(HttpStatus.OK)) {
 		 *             return response.bodyToFlux(Person.class);
 		 *         }
@@ -751,7 +751,7 @@ public interface WebClient {
 		 * Provide a function to map specific error status codes to an error
 		 * signal to be propagated downstream instead of the response.
 		 * <p>By default, if there are no matching status handlers, responses
-		 * with status codes >= 400 are mapped to
+		 * with status codes &gt;= 400 are mapped to
 		 * {@link WebClientResponseException} which is created with
 		 * {@link ClientResponse#createException()}.
 		 * <p>To suppress the treatment of a status code as an error and process
@@ -766,7 +766,7 @@ public interface WebClient {
 		 *     .retrieve()
 		 *     .bodyToMono(Account.class)
 		 *     .onErrorResume(WebClientResponseException.class,
-		 *          ex -> ex.getRawStatusCode() == 404 ? Mono.empty() : Mono.error(ex));
+		 *          ex -&gt; ex.getRawStatusCode() == 404 ? Mono.empty() : Mono.error(ex));
 		 * </pre>
 		 * @param statusPredicate to match responses with
 		 * @param exceptionFunction to map the response to an error signal

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RouterFunctions.java

@@ -670,7 +670,7 @@ public abstract class RouterFunctions {
 		 * <pre class="code">
 		 * RouterFunction&lt;ServerResponse&gt; nestedRoute =
 		 *   RouterFunctions.route()
-		 *     .nest(RequestPredicates.path("/user"), () ->
+		 *     .nest(RequestPredicates.path("/user"), () -&gt;
 		 *       RouterFunctions.route()
 		 *         .GET(this::listUsers)
 		 *         .POST(this::createUser)
@@ -695,7 +695,7 @@ public abstract class RouterFunctions {
 		 * <pre class="code">
 		 * RouterFunction&lt;ServerResponse&gt; nestedRoute =
 		 *   RouterFunctions.route()
-		 *     .nest(RequestPredicates.path("/user"), builder ->
+		 *     .nest(RequestPredicates.path("/user"), builder -&gt;
 		 *       builder.GET(this::listUsers)
 		 *              .POST(this::createUser))
 		 *     .build();
@@ -740,7 +740,7 @@ public abstract class RouterFunctions {
 		 * <pre class="code">
 		 * RouterFunction&lt;ServerResponse&gt; nestedRoute =
 		 *   RouterFunctions.route()
-		 *     .path("/user", builder ->
+		 *     .path("/user", builder -&gt;
 		 *       builder.GET(this::listUsers)
 		 *              .POST(this::createUser))
 		 *     .build();
@@ -762,7 +762,7 @@ public abstract class RouterFunctions {
 		 * RouterFunction&lt;ServerResponse&gt; filteredRoute =
 		 *   RouterFunctions.route()
 		 *     .GET("/user", this::listUsers)
-		 *     .filter((request, next) -> {
+		 *     .filter((request, next) -&gt; {
 		 *       // check for authentication headers
 		 *       if (isAuthenticated(request)) {
 		 *         return next.handle(request);
@@ -788,7 +788,7 @@ public abstract class RouterFunctions {
 		 * RouterFunction&lt;ServerResponse&gt; filteredRoute =
 		 *   RouterFunctions.route()
 		 *     .GET("/user", this::listUsers)
-		 *     .before(request -> {
+		 *     .before(request -&gt; {
 		 *       log(request);
 		 *       return request;
 		 *     })
@@ -809,7 +809,7 @@ public abstract class RouterFunctions {
 		 * RouterFunction&lt;ServerResponse&gt; filteredRoute =
 		 *   RouterFunctions.route()
 		 *     .GET("/user", this::listUsers)
-		 *     .after((request, response) -> {
+		 *     .after((request, response) -&gt; {
 		 *       log(response);
 		 *       return response;
 		 *     })
@@ -829,8 +829,8 @@ public abstract class RouterFunctions {
 		 * RouterFunction&lt;ServerResponse&gt; filteredRoute =
 		 *   RouterFunctions.route()
 		 *     .GET("/user", this::listUsers)
-		 *     .onError(e -> e instanceof IllegalStateException,
-		 *       (e, request) -> ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build())
+		 *     .onError(e -&gt; e instanceof IllegalStateException,
+		 *       (e, request) -&gt; ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build())
 		 *     .build();
 		 * </pre>
 		 * @param predicate the type of exception to filter
@@ -850,7 +850,7 @@ public abstract class RouterFunctions {
 		 *   RouterFunctions.route()
 		 *     .GET("/user", this::listUsers)
 		 *     .onError(IllegalStateException.class,
-		 *       (e, request) -> ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build())
+		 *       (e, request) -&gt; ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build())
 		 *     .build();
 		 * </pre>
 		 * @param exceptionType the type of exception to filter

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/ServerRequest.java

@@ -310,7 +310,7 @@ public interface ServerRequest {
 	 * public Mono<ServerResponse> myHandleMethod(ServerRequest request) {
 	 *   Instant lastModified = // application-specific calculation
 	 *	 return request.checkNotModified(lastModified)
-	 *	   .switchIfEmpty(Mono.defer(() -> {
+	 *	   .switchIfEmpty(Mono.defer(() -> {
 	 *	     // further request processing, actually building content
 	 *		 return ServerResponse.ok().body(...);
 	 *	   }));
@@ -344,7 +344,7 @@ public interface ServerRequest {
 	 * public Mono<ServerResponse> myHandleMethod(ServerRequest request) {
 	 *   String eTag = // application-specific calculation
 	 *	 return request.checkNotModified(eTag)
-	 *	   .switchIfEmpty(Mono.defer(() -> {
+	 *	   .switchIfEmpty(Mono.defer(() -> {
 	 *	     // further request processing, actually building content
 	 *		 return ServerResponse.ok().body(...);
 	 *	   }));
@@ -381,7 +381,7 @@ public interface ServerRequest {
 	 *   Instant lastModified = // application-specific calculation
 	 *   String eTag = // application-specific calculation
 	 *	 return request.checkNotModified(lastModified, eTag)
-	 *	   .switchIfEmpty(Mono.defer(() -> {
+	 *	   .switchIfEmpty(Mono.defer(() -> {
 	 *	     // further request processing, actually building content
 	 *		 return ServerResponse.ok().body(...);
 	 *	   }));

spring-webflux/src/main/java/org/springframework/web/reactive/result/view/UrlBasedViewResolver.java

@@ -45,7 +45,7 @@ import org.springframework.util.PatternMatchUtils;
  * specified prefix and/or suffix. Exporting an attribute that holds the
  * RequestContext to all views is explicitly supported.
  *
- * <p>Example: prefix="templates/", suffix=".ftl", viewname="test" ->
+ * <p>Example: prefix="templates/", suffix=".ftl", viewname="test" &rarr;
  * "templates/test.ftl"
  *
  * <p>As a special feature, redirect URLs can be specified via the "redirect:"

spring-webflux/src/main/java/org/springframework/web/reactive/socket/WebSocketHandler.java

@@ -45,13 +45,13 @@ import reactor.core.publisher.Mono;
  * 	public Mono<Void> handle(WebSocketSession session) {
  *
  * 		Flux<WebSocketMessage> output = session.receive()
- *			.doOnNext(message -> {
+ *			.doOnNext(message -> {
  * 				// ...
  * 			})
- * 			.concatMap(message -> {
+ * 			.concatMap(message -> {
  * 				// ...
  * 			})
- * 			.map(value -> session.textMessage("Echo " + value));
+ * 			.map(value -> session.textMessage("Echo " + value));
  *
  * 		return session.send(output);
  * 	}
@@ -68,10 +68,10 @@ import reactor.core.publisher.Mono;
  * 	public Mono<Void> handle(WebSocketSession session) {
  *
  * 		Mono<Void> input = session.receive()
- *			.doOnNext(message -> {
+ *			.doOnNext(message -> {
  * 				// ...
  * 			})
- * 			.concatMap(message -> {
+ * 			.concatMap(message -> {
  * 				// ...
  * 			})
  * 			.then();