Sync semantics for WebExchangeDataBinder factory

The createBinder method in BindingContext now returns the binder
instance rather than a Mono with the expectation that binder
initialization (e.g. @InitBinder) does not require blocking.

Issue: SPR-14543

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/BindingContext.java

@@ -17,8 +17,6 @@ package org.springframework.web.reactive.result.method;
 
 import java.lang.reflect.Field;
 
-import reactor.core.publisher.Mono;
-
 import org.springframework.beans.TypeConverter;
 import org.springframework.beans.TypeMismatchException;
 import org.springframework.core.MethodParameter;
@@ -42,7 +40,7 @@ public class BindingContext implements TypeConverter {
 
 	private final WebBindingInitializer initializer;
 
-	private final WebDataBinder simpleValueDataBinder;
+	private final TypeConverter simpleValueTypeConverter;
 
 
 	public BindingContext() {
@@ -51,10 +49,15 @@ public class BindingContext implements TypeConverter {
 
 	public BindingContext(WebBindingInitializer initializer) {
 		this.initializer = initializer;
-		this.simpleValueDataBinder = new WebExchangeDataBinder(null);
+		this.simpleValueTypeConverter = initTypeConverter(initializer);
+	}
+
+	private static WebExchangeDataBinder initTypeConverter(WebBindingInitializer initializer) {
+		WebExchangeDataBinder binder = new WebExchangeDataBinder(null);
 		if (initializer != null) {
-			initializer.initBinder(this.simpleValueDataBinder);
+			initializer.initBinder(binder);
 		}
+		return binder;
 	}
 
 
@@ -73,7 +76,7 @@ public class BindingContext implements TypeConverter {
 	 * @param objectName the name of the target object
 	 * @return a Mono for the created {@link WebDataBinder} instance
 	 */
-	public Mono<WebExchangeDataBinder> createBinder(ServerWebExchange exchange, Object target,
+	public WebExchangeDataBinder createBinder(ServerWebExchange exchange, Object target,
 			String objectName) {
 
 		WebExchangeDataBinder dataBinder = createBinderInstance(target, objectName);
@@ -87,24 +90,24 @@ public class BindingContext implements TypeConverter {
 		return new WebExchangeDataBinder(target, objectName);
 	}
 
-	protected Mono<WebExchangeDataBinder> initBinder(WebExchangeDataBinder binder, ServerWebExchange exchange) {
-		return Mono.just(binder);
+	protected WebExchangeDataBinder initBinder(WebExchangeDataBinder binder, ServerWebExchange exchange) {
+		return binder;
 	}
 
 	public <T> T convertIfNecessary(Object value, Class<T> requiredType) throws TypeMismatchException {
-		return this.simpleValueDataBinder.convertIfNecessary(value, requiredType);
+		return this.simpleValueTypeConverter.convertIfNecessary(value, requiredType);
 	}
 
 	public <T> T convertIfNecessary(Object value, Class<T> requiredType, MethodParameter methodParam)
 			throws TypeMismatchException {
 
-		return this.simpleValueDataBinder.convertIfNecessary(value, requiredType, methodParam);
+		return this.simpleValueTypeConverter.convertIfNecessary(value, requiredType, methodParam);
 	}
 
 	public <T> T convertIfNecessary(Object value, Class<T> requiredType, Field field)
 			throws TypeMismatchException {
 
-		return this.simpleValueDataBinder.convertIfNecessary(value, requiredType, field);
+		return this.simpleValueTypeConverter.convertIfNecessary(value, requiredType, field);
 	}
 
 }

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/annotation/AbstractMessageReaderArgumentResolver.java

@@ -19,7 +19,6 @@ import java.lang.annotation.Annotation;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
-import java.util.function.Function;
 import java.util.stream.Collectors;
 
 import reactor.core.publisher.Flux;
@@ -39,6 +38,7 @@ import org.springframework.http.server.reactive.ServerHttpResponse;
 import org.springframework.util.Assert;
 import org.springframework.validation.Validator;
 import org.springframework.validation.annotation.Validated;
+import org.springframework.web.bind.WebExchangeDataBinder;
 import org.springframework.web.reactive.result.method.BindingContext;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.ServerWebInputException;
@@ -143,7 +143,8 @@ public abstract class AbstractMessageReaderArgumentResolver {
 					}
 					Object[] hints = extractValidationHints(bodyParameter);
 					if (hints != null) {
-						flux = flux.concatMap(getValidator(hints, bodyParameter, bindingContext, exchange));
+						flux = flux.doOnNext(target ->
+								validate(target, hints, bodyParameter, bindingContext, exchange));
 					}
 					return Mono.just(adapter.fromPublisher(flux));
 				}
@@ -162,7 +163,8 @@ public abstract class AbstractMessageReaderArgumentResolver {
 					}
 					Object[] hints = extractValidationHints(bodyParameter);
 					if (hints != null) {
-						mono = mono.then(getValidator(hints, bodyParameter, bindingContext, exchange));
+						mono = mono.doOnNext(target ->
+								validate(target, hints, bodyParameter, bindingContext, exchange));
 					}
 					if (adapter != null) {
 						return Mono.just(adapter.fromPublisher(mono));
@@ -207,19 +209,15 @@ public abstract class AbstractMessageReaderArgumentResolver {
 		return null;
 	}
 
-	protected <T> Function<T, Mono<T>> getValidator(Object[] validationHints,
+	protected void validate(Object target, Object[] validationHints,
 			MethodParameter param, BindingContext binding, ServerWebExchange exchange) {
 
 		String name = Conventions.getVariableNameForParameter(param);
-
-		return target -> binding.createBinder(exchange, target, name)
-				.map(binder -> {
-					binder.validate(validationHints);
-					if (binder.getBindingResult().hasErrors()) {
-						throw new ServerWebInputException("Validation failed", param);
-					}
-					return target;
-				});
+		WebExchangeDataBinder binder = binding.createBinder(exchange, target, name);
+		binder.validate(validationHints);
+		if (binder.getBindingResult().hasErrors()) {
+			throw new ServerWebInputException("Validation failed", param);
+		}
 	}
 
 }