Add "mutate" builder to ServerWebExchange

This commit adds a default mutate method to ServerWebExchange which
prepares an immutable wrapper and returns the provided mutated
properties.

spring-web/src/main/java/org/springframework/web/server/DefaultServerWebExchangeMutativeBuilder.java

@@ -0,0 +1,134 @@
+/*
+ * Copyright 2002-2016 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.web.server;
+
+import java.security.Principal;
+import java.util.Optional;
+
+import reactor.core.publisher.Mono;
+
+import org.springframework.http.server.reactive.ServerHttpRequest;
+import org.springframework.http.server.reactive.ServerHttpResponse;
+import org.springframework.util.Assert;
+
+/**
+ * Default implementation of
+ * {@link org.springframework.web.server.ServerWebExchange.MutativeBuilder}.
+ *
+ * @author Rossen Stoyanchev
+ * @since 5.0
+ */
+class DefaultServerWebExchangeMutativeBuilder implements ServerWebExchange.MutativeBuilder {
+
+	private final ServerWebExchange delegate;
+
+	private ServerHttpRequest request;
+
+	private ServerHttpResponse response;
+
+	private Principal user;
+
+	private Mono<WebSession> session;
+
+
+	public DefaultServerWebExchangeMutativeBuilder(ServerWebExchange delegate) {
+		Assert.notNull(delegate, "'delegate' is required.");
+		this.delegate = delegate;
+	}
+
+
+	@Override
+	public ServerWebExchange.MutativeBuilder setRequest(ServerHttpRequest request) {
+		this.request = request;
+		return this;
+	}
+
+	@Override
+	public ServerWebExchange.MutativeBuilder setResponse(ServerHttpResponse response) {
+		this.response = response;
+		return this;
+	}
+
+	@Override
+	public ServerWebExchange.MutativeBuilder setPrincipal(Principal user) {
+		this.user = user;
+		return this;
+	}
+
+	@Override
+	public ServerWebExchange.MutativeBuilder setSession(Mono<WebSession> session) {
+		this.session = session;
+		return this;
+	}
+
+	@Override
+	public ServerWebExchange build() {
+		return new MutativeDecorator(this.delegate,
+				this.request, this.response, this.user, this.session);
+	}
+
+
+	/**
+	 * An immutable wrapper of an exchange returning property overrides -- given
+	 * to the constructor -- or original values otherwise.
+	 */
+	private static class MutativeDecorator extends ServerWebExchangeDecorator {
+
+		private final ServerHttpRequest request;
+
+		private final ServerHttpResponse response;
+
+		private final Principal user;
+
+		private final Mono<WebSession> session;
+
+
+		public MutativeDecorator(ServerWebExchange delegate,
+				ServerHttpRequest request, ServerHttpResponse response, Principal user,
+				Mono<WebSession> session) {
+
+			super(delegate);
+			this.request = request;
+			this.response = response;
+			this.user = user;
+			this.session = session;
+		}
+
+
+		@Override
+		public ServerHttpRequest getRequest() {
+			return (this.request != null ? this.request : getDelegate().getRequest());
+		}
+
+		@Override
+		public ServerHttpResponse getResponse() {
+			return (this.response != null ? this.response : getDelegate().getResponse());
+		}
+
+		@Override
+		public Mono<WebSession> getSession() {
+			return (this.session != null ? this.session : getDelegate().getSession());
+		}
+
+		@SuppressWarnings("unchecked")
+		@Override
+		public <T extends Principal> Optional<T> getPrincipal() {
+			return (this.user != null ? Optional.of((T) this.user) : getDelegate().getPrincipal());
+		}
+	}
+
+}
+

spring-web/src/main/java/org/springframework/web/server/ServerWebExchange.java

@@ -118,4 +118,48 @@ public interface ServerWebExchange {
 	 */
 	boolean checkNotModified(String etag, Instant lastModified);
 
+
+	/**
+	 * Return a builder to mutate properties of this exchange. The resulting
+	 * new exchange is an immutable {@link ServerWebExchangeDecorator decorator}
+	 * around the current exchange instance that returns mutated values, where
+	 * provided, or delegating to the decorated instance otherwise.
+	 */
+	default MutativeBuilder mutate() {
+		return new DefaultServerWebExchangeMutativeBuilder(this);
+	}
+
+
+	/**
+	 * Builder for mutating properties of a {@link ServerWebExchange}.
+	 */
+	interface MutativeBuilder {
+
+		/**
+		 * Set the request to use.
+		 */
+		MutativeBuilder setRequest(ServerHttpRequest request);
+
+		/**
+		 * Set the response to use.
+		 */
+		MutativeBuilder setResponse(ServerHttpResponse response);
+
+		/**
+		 * Set the principal to use.
+		 */
+		MutativeBuilder setPrincipal(Principal user);
+
+		/**
+		 * Set the session to use.
+		 */
+		MutativeBuilder setSession(Mono<WebSession> session);
+
+		/**
+		 * Build an immutable wrapper that returning the mutated properties.
+		 */
+		ServerWebExchange build();
+
+	}
+
 }

spring-web/src/main/java/org/springframework/web/server/ServerWebExchangeDecorator.java

@@ -27,20 +27,26 @@ import org.springframework.http.server.reactive.ServerHttpResponse;
 import org.springframework.util.Assert;
 
 /**
- * Wraps another {@link ServerWebExchange} and delegates all methods to it.
- * Sub-classes can override specific methods, e.g. {@link #getPrincipal()} to
- * return the authenticated user for the request.
+ * A convenient base class for classes that need to wrap another
+ * {@link ServerWebExchange}. Pre-implements all methods by delegating to the
+ * wrapped instance.
+ *
+ * <p>Note that if the purpose for wrapping is simply to override specific
+ * properties, e.g. {@link #getPrincipal()}, consider using
+ * {@link ServerWebExchange#mutate()} instead.
  *
  * @author Rossen Stoyanchev
  * @since 5.0
+ *
+ * @see ServerWebExchange#mutate()
  */
 public class ServerWebExchangeDecorator implements ServerWebExchange {
 
 	private final ServerWebExchange delegate;
 
 
-	public ServerWebExchangeDecorator(ServerWebExchange delegate) {
-		Assert.notNull(delegate, "'delegate' is required.");
+	protected ServerWebExchangeDecorator(ServerWebExchange delegate) {
+		Assert.notNull(delegate, "ServerWebExchange 'delegate' is required.");
 		this.delegate = delegate;
 	}