Introduce RouterFunction attributes

This commit introduces support for router function attributes, a
way to associate meta-data with a route.

Closes: gh-25938

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

@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.function.server;
 
+import java.util.Map;
 import java.util.function.Function;
 
 import reactor.core.publisher.Mono;
@@ -60,6 +61,10 @@ class ChangePathPatternParserVisitor implements RouterFunctions.Visitor {
 	public void resources(Function<ServerRequest, Mono<Resource>> lookupFunction) {
 	}
 
+	@Override
+	public void attributes(Map<String, Object> attributes) {
+	}
+
 	@Override
 	public void unknown(RouterFunction<?> routerFunction) {
 	}

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

@@ -16,8 +16,14 @@
 
 package org.springframework.web.reactive.function.server;
 
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.function.Consumer;
+
 import reactor.core.publisher.Mono;
 
+import org.springframework.util.Assert;
+
 /**
  * Represents a function that routes to a {@linkplain HandlerFunction handler function}.
  *
@@ -115,4 +121,39 @@ public interface RouterFunction<T extends ServerResponse> {
 		visitor.unknown(this);
 	}
 
+	/**
+	 * Return a new routing function with the given attribute.
+	 * @param name the attribute name
+	 * @param value the attribute value
+     * @return a function that has the specified attributes
+     * @since 5.3
+	 */
+	default RouterFunction<T> withAttribute(String name, Object value) {
+		Assert.hasLength(name, "Name must not be empty");
+		Assert.notNull(value, "Value must not be null");
+
+		Map<String, Object> attributes = new LinkedHashMap<>();
+		attributes.put(name, value);
+		return new RouterFunctions.AttributesRouterFunction<>(this, attributes);
+	}
+
+	/**
+	 * Return a new routing function with attributes manipulated with the given consumer.
+	 * <p>The map provided to the consumer is "live", so that the consumer can be used
+	 * to {@linkplain Map#put(Object, Object) overwrite} existing attributes,
+	 * {@linkplain Map#remove(Object) remove} attributes, or use any of the other
+	 * {@link Map} methods.
+	 * @param attributesConsumer a function that consumes the attributes map
+	 * @return this builder
+	 * @since 5.3
+	 */
+	default RouterFunction<T> withAttributes(Consumer<Map<String, Object>> attributesConsumer) {
+		Assert.notNull(attributesConsumer, "AttributesConsumer must not be null");
+
+		Map<String, Object> attributes = new LinkedHashMap<>();
+		attributesConsumer.accept(attributes);
+		return new RouterFunctions.AttributesRouterFunction<>(this, attributes);
+	}
+
+
 }

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

@@ -18,6 +18,7 @@ package org.springframework.web.reactive.function.server;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Map;
 import java.util.function.BiFunction;
 import java.util.function.Consumer;
 import java.util.function.Function;
@@ -330,6 +331,35 @@ class RouterFunctionBuilder implements RouterFunctions.Builder {
 		return this;
 	}
 
+	@Override
+	public RouterFunctions.Builder withAttribute(String name, Object value) {
+		Assert.hasLength(name, "Name must not be empty");
+		Assert.notNull(value, "Value must not be null");
+
+		if (this.routerFunctions.isEmpty()) {
+			throw new IllegalStateException("attributes can only be called after any other method (GET, path, etc.)");
+		}
+		int lastIdx = this.routerFunctions.size() - 1;
+		RouterFunction<ServerResponse> attributed = this.routerFunctions.get(lastIdx)
+				.withAttribute(name, value);
+		this.routerFunctions.set(lastIdx, attributed);
+		return this;
+	}
+
+	@Override
+	public RouterFunctions.Builder withAttributes(Consumer<Map<String, Object>> attributesConsumer) {
+		Assert.notNull(attributesConsumer, "AttributesConsumer must not be null");
+
+		if (this.routerFunctions.isEmpty()) {
+			throw new IllegalStateException("attributes can only be called after any other method (GET, path, etc.)");
+		}
+		int lastIdx = this.routerFunctions.size() - 1;
+		RouterFunction<ServerResponse> attributed = this.routerFunctions.get(lastIdx)
+				.withAttributes(attributesConsumer);
+		this.routerFunctions.set(lastIdx, attributed);
+		return this;
+	}
+
 	@Override
 	public RouterFunction<ServerResponse> build() {
 		if (this.routerFunctions.isEmpty()) {

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

@@ -16,6 +16,8 @@
 
 package org.springframework.web.reactive.function.server;
 
+import java.util.Collections;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.function.BiFunction;
@@ -858,6 +860,27 @@ public abstract class RouterFunctions {
 		<T extends Throwable> Builder onError(Class<T> exceptionType,
 				BiFunction<? super T, ServerRequest, Mono<ServerResponse>> responseProvider);
 
+		/**
+		 * Add an attribute with the given name and value to the last route built with this builder.
+		 * @param name the attribute name
+		 * @param value the attribute value
+		 * @return this builder
+		 * @since 5.3
+		 */
+		Builder withAttribute(String name, Object value);
+
+		/**
+		 * Manipulate the attributes of the last route built with the given consumer.
+		 * <p>The map provided to the consumer is "live", so that the consumer can be used
+		 * to {@linkplain Map#put(Object, Object) overwrite} existing attributes,
+		 * {@linkplain Map#remove(Object) remove} attributes, or use any of the other
+		 * {@link Map} methods.
+		 * @param attributesConsumer a function that consumes the attributes map
+		 * @return this builder
+		 * @since 5.3
+		 */
+		Builder withAttributes(Consumer<Map<String, Object>> attributesConsumer);
+
 		/**
 		 * Builds the {@code RouterFunction}. All created routes are
 		 * {@linkplain RouterFunction#and(RouterFunction) composed} with one another, and filters
@@ -902,6 +925,14 @@ public abstract class RouterFunctions {
 		 */
 		void resources(Function<ServerRequest, Mono<Resource>> lookupFunction);
 
+		/**
+		 * Receive notification of a router function with attributes. The
+		 * given attributes apply to the router notification that follows this one.
+		 * @param attributes the attributes that apply to the following router
+		 * @since 5.3
+		 */
+		void attributes(Map<String, Object> attributes);
+
 		/**
 		 * Receive notification of an unknown router function. This method is called for router
 		 * functions that were not created via the various {@link RouterFunctions} methods.
@@ -1126,6 +1157,58 @@ public abstract class RouterFunctions {
 	}
 
 
+	static final class AttributesRouterFunction<T extends ServerResponse> extends AbstractRouterFunction<T> {
+
+		private final RouterFunction<T> delegate;
+
+		private final Map<String,Object> attributes;
+
+		public AttributesRouterFunction(RouterFunction<T> delegate, Map<String, Object> attributes) {
+			this.delegate = delegate;
+			this.attributes = initAttributes(attributes);
+		}
+
+		private static Map<String, Object> initAttributes(Map<String, Object> attributes) {
+			if (attributes.isEmpty()) {
+				return Collections.emptyMap();
+			}
+			else {
+				return Collections.unmodifiableMap(new LinkedHashMap<>(attributes));
+			}
+		}
+
+		@Override
+		public Mono<HandlerFunction<T>> route(ServerRequest request) {
+			return this.delegate.route(request);
+		}
+
+		@Override
+		public void accept(Visitor visitor) {
+			visitor.attributes(this.attributes);
+			this.delegate.accept(visitor);
+		}
+
+		@Override
+		public RouterFunction<T> withAttribute(String name, Object value) {
+			Assert.hasLength(name, "Name must not be empty");
+			Assert.notNull(value, "Value must not be null");
+
+			Map<String, Object> attributes = new LinkedHashMap<>(this.attributes);
+			attributes.put(name, value);
+			return new AttributesRouterFunction<>(this.delegate, attributes);
+		}
+
+		@Override
+		public RouterFunction<T> withAttributes(Consumer<Map<String, Object>> attributesConsumer) {
+			Assert.notNull(attributesConsumer, "AttributesConsumer must not be null");
+
+			Map<String, Object> attributes = new LinkedHashMap<>(this.attributes);
+			attributesConsumer.accept(attributes);
+			return new AttributesRouterFunction<>(this.delegate, attributes);
+		}
+	}
+
+
 	private static class HandlerStrategiesResponseContext implements ServerResponse.Context {
 
 		private final HandlerStrategies strategies;

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

@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.function.server;
 
+import java.util.Map;
 import java.util.Set;
 import java.util.function.Function;
 
@@ -69,6 +70,10 @@ class ToStringVisitor implements RouterFunctions.Visitor, RequestPredicates.Visi
 		this.builder.append(lookupFunction).append('\n');
 	}
 
+	@Override
+	public void attributes(Map<String, Object> attributes) {
+	}
+
 	@Override
 	public void unknown(RouterFunction<?> routerFunction) {
 		indent();