Add request binding to functional endpoints

This commit introduces form binding to ServerRequest in both WebMVC.fn
and WebFlux.fn's, in the form of a bind(Class) method.

Closes gh-25943

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

@@ -27,12 +27,14 @@ import java.util.Locale;
 import java.util.Map;
 import java.util.Optional;
 import java.util.OptionalLong;
+import java.util.function.Consumer;
 import java.util.function.Function;
 
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 import org.springframework.core.ParameterizedTypeReference;
+import org.springframework.core.ResolvableType;
 import org.springframework.core.codec.DecodingException;
 import org.springframework.core.codec.Hints;
 import org.springframework.core.io.buffer.DataBuffer;
@@ -49,7 +51,12 @@ import org.springframework.http.server.RequestPath;
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.http.server.reactive.ServerHttpResponse;
 import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 import org.springframework.util.MultiValueMap;
+import org.springframework.validation.BindException;
+import org.springframework.validation.BindingResult;
+import org.springframework.web.bind.WebDataBinder;
+import org.springframework.web.bind.support.WebExchangeDataBinder;
 import org.springframework.web.reactive.function.BodyExtractor;
 import org.springframework.web.reactive.function.BodyExtractors;
 import org.springframework.web.reactive.function.UnsupportedMediaTypeException;
@@ -220,6 +227,33 @@ class DefaultServerRequest implements ServerRequest {
 				.onErrorMap(DecodingException.class, DECODING_MAPPER);
 	}
 
+	@Override
+	@SuppressWarnings("unchecked")
+	public <T> Mono<T> bind(Class<T> bindType, Consumer<WebDataBinder> dataBinderCustomizer) {
+		Assert.notNull(bindType, "BindType must not be null");
+		Assert.notNull(dataBinderCustomizer, "DataBinderCustomizer must not be null");
+
+		return Mono.defer(() -> {
+			WebExchangeDataBinder dataBinder = new WebExchangeDataBinder(null);
+			dataBinder.setTargetType(ResolvableType.forClass(bindType));
+			dataBinderCustomizer.accept(dataBinder);
+
+			ServerWebExchange exchange = exchange();
+			return dataBinder.construct(exchange)
+					.then(dataBinder.bind(exchange))
+					.then(Mono.defer(() -> {
+						BindingResult bindingResult = dataBinder.getBindingResult();
+						if (bindingResult.hasErrors()) {
+							return Mono.error(new BindException(bindingResult));
+						}
+						else {
+							T result = (T) bindingResult.getTarget();
+							return Mono.justOrEmpty(result);
+						}
+					}));
+		});
+	}
+
 	@Override
 	public Map<String, Object> attributes() {
 		return this.exchange.getAttributes();

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -29,6 +29,7 @@ import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Predicate;
 
@@ -52,6 +53,7 @@ import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.MimeTypeUtils;
 import org.springframework.util.MultiValueMap;
+import org.springframework.web.bind.WebDataBinder;
 import org.springframework.web.cors.reactive.CorsUtils;
 import org.springframework.web.reactive.function.BodyExtractor;
 import org.springframework.web.server.ServerWebExchange;
@@ -1044,6 +1046,16 @@ public abstract class RequestPredicates {
 			return this.request.bodyToFlux(typeReference);
 		}
 
+		@Override
+		public <T> Mono<T> bind(Class<T> bindType) {
+			return this.request.bind(bindType);
+		}
+
+		@Override
+		public <T> Mono<T> bind(Class<T> bindType, Consumer<WebDataBinder> dataBinderCustomizer) {
+			return this.request.bind(bindType, dataBinderCustomizer);
+		}
+
 		@Override
 		public Map<String, Object> attributes() {
 			return this.attributes;

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -48,6 +48,8 @@ import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.MultiValueMap;
+import org.springframework.validation.BindException;
+import org.springframework.web.bind.WebDataBinder;
 import org.springframework.web.reactive.function.BodyExtractor;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.WebSession;
@@ -197,6 +199,31 @@ public interface ServerRequest {
 	 */
 	<T> Flux<T> bodyToFlux(ParameterizedTypeReference<T> typeReference);
 
+	/**
+	 * Bind to this request and return an instance of the given type.
+	 * @param bindType the type of class to bind this request to
+	 * @param <T> the type to bind to
+	 * @return a mono containing either a constructed and bound instance of
+	 * {@code bindType}, or a {@link BindException} in case of binding errors
+	 * @since 6.1
+	 */
+	default <T> Mono<T> bind(Class<T> bindType) {
+		return bind(bindType, dataBinder -> {});
+	}
+
+	/**
+	 * Bind to this request and return an instance of the given type.
+	 * @param bindType the type of class to bind this request to
+	 * @param dataBinderCustomizer used to customize the data binder, e.g. set
+	 * (dis)allowed fields
+	 * @param <T> the type to bind to
+	 * @return a mono containing either a constructed and bound instance of
+	 * {@code bindType}, or a {@link BindException} in case of binding errors
+	 * @since 6.1
+	 */
+	<T> Mono<T> bind(Class<T> bindType, Consumer<WebDataBinder> dataBinderCustomizer);
+
+
 	/**
 	 * Get the request attribute value if present.
 	 * @param name the attribute name

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -25,6 +25,7 @@ import java.util.Locale;
 import java.util.Map;
 import java.util.Optional;
 import java.util.OptionalLong;
+import java.util.function.Consumer;
 
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -42,6 +43,7 @@ import org.springframework.http.server.RequestPath;
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.util.Assert;
 import org.springframework.util.MultiValueMap;
+import org.springframework.web.bind.WebDataBinder;
 import org.springframework.web.reactive.function.BodyExtractor;
 import org.springframework.web.reactive.function.server.ServerRequest;
 import org.springframework.web.server.ServerWebExchange;
@@ -171,6 +173,16 @@ public class ServerRequestWrapper implements ServerRequest {
 		return this.delegate.bodyToFlux(typeReference);
 	}
 
+	@Override
+	public <T> Mono<T> bind(Class<T> bindType) {
+		return this.delegate.bind(bindType);
+	}
+
+	@Override
+	public <T> Mono<T> bind(Class<T> bindType, Consumer<WebDataBinder> dataBinderCustomizer) {
+		return this.delegate.bind(bindType, dataBinderCustomizer);
+	}
+
 	@Override
 	public Optional<Object> attribute(String name) {
 		return this.delegate.attribute(name);