Refine null-safety in the spring-webflux module

Closes gh-34163

Author: sdeleuze

Diff for: spring-webflux/src/main/java/org/springframework/web/reactive/config/ResourceChainRegistration.java

@@ -65,7 +65,7 @@ public ResourceChainRegistration(boolean cacheResources) {
 		this(cacheResources, cacheResources ? new ConcurrentMapCache(DEFAULT_CACHE_NAME) : null);
 	}
 
-	@SuppressWarnings("NullAway")
+	@SuppressWarnings("NullAway") // Dataflow analysis limitation
 	public ResourceChainRegistration(boolean cacheResources, @Nullable Cache cache) {
 		Assert.isTrue(!cacheResources || cache != null, "'cache' is required when cacheResources=true");
 		if (cacheResources) {

Diff for: spring-webflux/src/main/java/org/springframework/web/reactive/function/server/DefaultServerResponseBuilder.java

@@ -91,7 +91,7 @@ public DefaultServerResponseBuilder(HttpStatusCode status) {
 
 
 	@Override
-	@SuppressWarnings("NullAway") // TODO NullAway bug potentially due to the recursive generic type
+	@SuppressWarnings("NullAway") // https://github.com/uber/NullAway/issues/1113
 	public ServerResponse.BodyBuilder header(String headerName, @Nullable String... headerValues) {
 		Assert.notNull(headerName, "HeaderName must not be null");
 		for (String headerValue : headerValues) {

Diff for: spring-webflux/src/main/java/org/springframework/web/reactive/resource/ResourceWebHandler.java

@@ -323,7 +323,6 @@ public boolean isOptimizeLocations() {
 	 * @param mediaTypes media type mappings
 	 * @since 5.3.2
 	 */
-	@SuppressWarnings("NullAway")
 	public void setMediaTypes(Map<String, MediaType> mediaTypes) {
 		if (this.mediaTypes == null) {
 			this.mediaTypes = new HashMap<>(mediaTypes.size());
@@ -474,7 +473,7 @@ public Mono<Void> handle(ServerWebExchange exchange) {
 				});
 	}
 
-	@SuppressWarnings("NullAway")
+	@SuppressWarnings("NullAway") // Lambda
 	protected Mono<Resource> getResource(ServerWebExchange exchange) {
 		String rawPath = getResourcePath(exchange);
 		String path = processPath(rawPath);

Diff for: spring-webflux/src/main/java/org/springframework/web/reactive/result/method/AbstractHandlerMethodMapping.java

@@ -359,7 +359,7 @@ public Mono<HandlerMethod> getHandlerInternal(ServerWebExchange exchange) {
 		}
 	}
 
-	@SuppressWarnings("NullAway")
+	@SuppressWarnings("NullAway") // Dataflow analysis limitation
 	private void addMatchingMappings(Collection<T> mappings, List<Match> matches, ServerWebExchange exchange) {
 		for (T mapping : mappings) {
 			T match = getMatchingMapping(mapping, exchange);

Diff for: spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ControllerMethodResolver.java

@@ -408,7 +408,7 @@ private InvocableHandlerMethod createAttributeMethod(Object bean, Method method)
 	 * @param handlerMethod the controller method that raised the exception,
 	 *        or if {@code null}, check only {@code @ControllerAdvice} classes.
 	 */
-	@SuppressWarnings("NullAway")
+	@SuppressWarnings("NullAway") // Dataflow analysis limitation
 	public @Nullable InvocableHandlerMethod getExceptionHandlerMethod(Throwable ex, ServerWebExchange exchange, @Nullable HandlerMethod handlerMethod) {
 
 		Class<?> handlerType = (handlerMethod != null ? handlerMethod.getBeanType() : null);

Diff for: spring-webflux/src/main/java/org/springframework/web/reactive/socket/adapter/StandardWebSocketHandlerAdapter.java

@@ -64,7 +64,7 @@ public StandardWebSocketHandlerAdapter(WebSocketHandler handler,
 
 
 	@Override
-	@SuppressWarnings("NullAway")
+	@SuppressWarnings("NullAway") // Lambda
 	public void onOpen(Session session, EndpointConfig config) {
 		this.delegateSession = this.sessionFactory.apply(session);
 		Assert.state(this.delegateSession != null, "No delegate session");

Diff for: spring-webflux/src/main/java/org/springframework/web/reactive/socket/server/support/HandshakeWebSocketService.java

@@ -241,7 +241,7 @@ private Mono<Void> handleBadRequest(ServerWebExchange exchange, String reason) {
 		return null;
 	}
 
-	@SuppressWarnings("NullAway")
+	@SuppressWarnings("NullAway") // Lambda
 	private Mono<Map<String, Object>> initAttributes(ServerWebExchange exchange) {
 		if (this.sessionAttributePredicate == null) {
 			return EMPTY_ATTRIBUTES;