Add RSocketRequest.Builder in Spring Messaging

Prior to this commit, `RSocketRequester` would have a single
`RSocketRequester.create` static method taking a fully built
`RSocket` as an argument. Developers need to build an `RSocket`
instance using the `RSocketFactory` and then use it to create
a requester.

To help developers set up a requester, this commit adds a new
`RSocketRequester.Builder` interface and implementation. The
`RSocket` building phase and codecs configuration are part of a
single call chain. Subscribing to the returned
`Mono<RSocketRequester>` will configure and connect to the remote
RSocket server.

This design should be improved in gh-22798, since we will need to
support metadata in a broader fashion.

Closes gh-22806

spring-messaging/src/main/java/org/springframework/messaging/rsocket/DefaultRSocketRequesterBuilder.java

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2002-2019 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.messaging.rsocket;
+
+import java.net.URI;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Consumer;
+
+import io.rsocket.RSocketFactory;
+import io.rsocket.transport.ClientTransport;
+import io.rsocket.transport.netty.client.TcpClientTransport;
+import io.rsocket.transport.netty.client.WebsocketClientTransport;
+import reactor.core.publisher.Mono;
+
+import org.springframework.lang.Nullable;
+import org.springframework.util.MimeType;
+
+/**
+ * Default implementation of {@link RSocketRequester.Builder}.
+ *
+ * @author Brian Clozel
+ * @since 5.2
+ */
+final class DefaultRSocketRequesterBuilder implements RSocketRequester.Builder {
+
+	@Nullable
+	private List<Consumer<RSocketFactory.ClientRSocketFactory>> factoryConfigurers = new ArrayList<>();
+
+	@Nullable
+	private List<Consumer<RSocketStrategies.Builder>> strategiesConfigurers = new ArrayList<>();
+
+	@Override
+	public RSocketRequester.Builder rsocketFactory(Consumer<RSocketFactory.ClientRSocketFactory> configurer) {
+		this.factoryConfigurers.add(configurer);
+		return this;
+	}
+
+	@Override
+	public RSocketRequester.Builder rsocketStrategies(Consumer<RSocketStrategies.Builder> configurer) {
+		this.strategiesConfigurers.add(configurer);
+		return this;
+	}
+
+	@Override
+	public Mono<RSocketRequester> connect(ClientTransport transport, MimeType dataMimeType) {
+		return Mono.defer(() -> {
+			RSocketStrategies.Builder strategiesBuilder = RSocketStrategies.builder();
+			this.strategiesConfigurers.forEach(configurer -> configurer.accept(strategiesBuilder));
+			RSocketFactory.ClientRSocketFactory clientFactory = RSocketFactory.connect()
+					.dataMimeType(dataMimeType.toString());
+			this.factoryConfigurers.forEach(configurer -> configurer.accept(clientFactory));
+			return clientFactory.transport(transport).start()
+					.map(rsocket -> RSocketRequester.create(rsocket, dataMimeType, strategiesBuilder.build()));
+		});
+	}
+
+	@Override
+	public Mono<RSocketRequester> connectTcp(String host, int port, MimeType dataMimeType) {
+		return connect(TcpClientTransport.create(host, port), dataMimeType);
+	}
+
+	@Override
+	public Mono<RSocketRequester> connectWebSocket(URI uri, MimeType dataMimeType) {
+		return connect(WebsocketClientTransport.create(uri), dataMimeType);
+	}
+
+}

spring-messaging/src/main/java/org/springframework/messaging/rsocket/RSocketRequester.java

@@ -16,7 +16,12 @@
 
 package org.springframework.messaging.rsocket;
 
+import java.net.URI;
+import java.util.function.Consumer;
+
 import io.rsocket.RSocket;
+import io.rsocket.RSocketFactory;
+import io.rsocket.transport.ClientTransport;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -32,6 +37,7 @@ import org.springframework.util.MimeType;
  * methods specify routing and other metadata.
  *
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  * @since 5.2
  */
 public interface RSocketRequester {
@@ -55,6 +61,61 @@ public interface RSocketRequester {
 		return new DefaultRSocketRequester(rsocket, dataMimeType, strategies);
 	}
 
+	/**
+	 * Obtain a {@code RSocketRequester} builder.
+	 */
+	static RSocketRequester.Builder builder() {
+		return new DefaultRSocketRequesterBuilder();
+	}
+
+	/**
+	 * A mutable builder for creating a client {@link RSocketRequester}.
+	 */
+	interface Builder {
+
+		/**
+		 * Configure the client {@code RSocketFactory}. This is useful for
+		 * customizing protocol options and add RSocket plugins.
+		 * @param configurer the configurer to apply
+		 */
+		RSocketRequester.Builder rsocketFactory(
+				Consumer<RSocketFactory.ClientRSocketFactory> configurer);
+
+		/**
+		 * Configure the builder for {@link RSocketStrategies}.
+		 * <p>The builder starts with an empty {@code RSocketStrategies}.
+		 * @param configurer the configurer to apply
+		 */
+		RSocketRequester.Builder rsocketStrategies(Consumer<RSocketStrategies.Builder> configurer);
+
+		/**
+		 * Configure the {@code ClientTransport} for the RSocket connection
+		 * and connect to the RSocket server
+		 * @param transport the chosen client transport
+		 * @return a mono containing the connected {@code RSocketRequester}
+		 */
+		Mono<RSocketRequester> connect(ClientTransport transport, MimeType dataMimeType);
+
+		/**
+		 * Connect to the RSocket server over TCP transport using the
+		 * provided connection parameters
+		 * @param host the RSocket server host
+		 * @param port the RSocket server port
+		 * @param dataMimeType the data MimeType
+		 * @return a mono containing the connected {@code RSocketRequester}
+		 */
+		Mono<RSocketRequester> connectTcp(String host, int port, MimeType dataMimeType);
+
+		/**
+		 * Connect to the RSocket server over WebSocket transport using the
+		 * provided connection parameters
+		 * @param uri the RSocket server endpoint URI
+		 * @param dataMimeType the data MimeType
+		 * @return a mono containing the connected {@code RSocketRequester}
+		 */
+		Mono<RSocketRequester> connectWebSocket(URI uri, MimeType dataMimeType);
+
+	}
 
 	// For now we treat metadata as a simple string that is the route.
 	// This will change after the resolution of: