Minor refactoring of suspend/resumeReceiving

suspend/resumeReceiving in the AbstractListenerWebSocketSession are
now abstract methods. In Tomcat/Jetty these methods are no-op
implementations that are then coupled with a buffering strategy via
Flux#onBackpressureBuffer. In Undertow they rely on flow control for
receiving WebSocket messages.

Issue: SPR-14527

spring-web-reactive/src/main/java/org/springframework/web/reactive/socket/adapter/AbstractListenerWebSocketSession.java

@@ -36,11 +36,17 @@ import org.springframework.web.reactive.socket.WebSocketSession;
  * Base class for Listener-based {@link WebSocketSession} adapters.
  *
  * @author Violeta Georgieva
+ * @author Rossen Stoyanchev
  * @since 5.0
  */
 public abstract class AbstractListenerWebSocketSession<T> extends WebSocketSessionSupport<T> {
 
-	private static final int BUFFER_SIZE = 8192;
+	/**
+	 * The "back-pressure" buffer size to use if the underlying WebSocket API
+	 * does not have flow control for receiving messages.
+	 */
+	private static final int RECEIVE_BUFFER_SIZE = 8192;
+
 
 	private final AtomicBoolean sendCalled = new AtomicBoolean();
 
@@ -52,8 +58,6 @@ public abstract class AbstractListenerWebSocketSession<T> extends WebSocketSessi
 
 	private volatile WebSocketSendProcessor sendProcessor;
 
-	private volatile boolean suspended = false;
-
 
 	public AbstractListenerWebSocketSession(T delegate, String id, URI uri) {
 		super(delegate);
@@ -80,7 +84,9 @@ public abstract class AbstractListenerWebSocketSession<T> extends WebSocketSessi
 
 	@Override
 	public Flux<WebSocketMessage> receive() {
-		return Flux.from(this.receivePublisher).onBackpressureBuffer(BUFFER_SIZE);
+		return canSuspendReceiving() ?
+				Flux.from(this.receivePublisher) :
+				Flux.from(this.receivePublisher).onBackpressureBuffer(RECEIVE_BUFFER_SIZE);
 	}
 
 	@Override
@@ -97,18 +103,33 @@ public abstract class AbstractListenerWebSocketSession<T> extends WebSocketSessi
 		}
 	}
 
-	protected void resumeReceives() {
-		this.suspended = false;
-	}
-
-	protected void suspendReceives() {
-		this.suspended = true;
-	}
-
-	protected boolean isSuspended() {
-		return this.suspended;
-	}
-
+	/**
+	 * Resume receiving new message(s) after demand is generated by the
+	 * downstream Subscriber.
+	 * <p><strong>Note:</strong> if the underlying WebSocket API does not provide
+	 * flow control for receiving messages, and this method should be a no-op
+	 * and {@link #canSuspendReceiving()} should return {@code false}.
+	 */
+	protected abstract void resumeReceiving();
+
+	/**
+	 * Suspend receiving until received message(s) are processed and more demand
+	 * is generated by the downstream Subscriber.
+	 * <p><strong>Note:</strong> if the underlying WebSocket API does not provide
+	 * flow control for receiving messages, and this method should be a no-op
+	 * and {@link #canSuspendReceiving()} should return {@code false}.
+	 */
+	protected abstract void suspendReceiving();
+
+	/**
+	 * Whether the underlying WebSocket API has flow control and can suspend and
+	 * resume the receiving of messages.
+	 */
+	protected abstract boolean canSuspendReceiving();
+
+	/**
+	 * Send the given WebSocket message.
+	 */
 	protected abstract boolean sendMessage(WebSocketMessage message) throws IOException;
 
 	/** Handle a message callback from the WebSocketHandler adapter */
@@ -151,7 +172,7 @@ public abstract class AbstractListenerWebSocketSession<T> extends WebSocketSessi
 			if (this.webSocketMessage != null) {
 				WebSocketMessage result = this.webSocketMessage;
 				this.webSocketMessage = null;
-				resumeReceives();
+				resumeReceiving();
 				return result;
 			}
 
@@ -160,7 +181,7 @@ public abstract class AbstractListenerWebSocketSession<T> extends WebSocketSessi
 
 		void handleMessage(WebSocketMessage webSocketMessage) {
 			this.webSocketMessage = webSocketMessage;
-			suspendReceives();
+			suspendReceiving();
 			onDataAvailable();
 		}
 	}

spring-web-reactive/src/main/java/org/springframework/web/reactive/socket/adapter/JettyWebSocketHandlerAdapter.java

@@ -71,7 +71,7 @@ public class JettyWebSocketHandlerAdapter {
 
 	@OnWebSocketMessage
 	public void onWebSocketText(String message) {
-		if (this.wsSession != null && !this.wsSession.isSuspended()) {
+		if (this.wsSession != null) {
 			WebSocketMessage wsMessage = toMessage(Type.TEXT, message);
 			this.wsSession.handleMessage(wsMessage.getType(), wsMessage);
 		}
@@ -79,7 +79,7 @@ public class JettyWebSocketHandlerAdapter {
 
 	@OnWebSocketMessage
 	public void onWebSocketBinary(byte[] message, int offset, int length) {
-		if (this.wsSession != null && !this.wsSession.isSuspended()) {
+		if (this.wsSession != null) {
 			WebSocketMessage wsMessage = toMessage(Type.BINARY, ByteBuffer.wrap(message, offset, length));
 			wsSession.handleMessage(wsMessage.getType(), wsMessage);
 		}
@@ -87,7 +87,7 @@ public class JettyWebSocketHandlerAdapter {
 
 	@OnWebSocketFrame
 	public void onWebSocketFrame(Frame frame) {
-		if (this.wsSession != null && !this.wsSession.isSuspended()) {
+		if (this.wsSession != null) {
 			if (OpCode.PONG == frame.getOpCode()) {
 				ByteBuffer message = frame.getPayload() != null ? frame.getPayload() : EMPTY_PAYLOAD;
 				WebSocketMessage wsMessage = toMessage(Type.PONG, message);

spring-web-reactive/src/main/java/org/springframework/web/reactive/socket/adapter/JettyWebSocketSession.java

@@ -21,13 +21,13 @@ import java.nio.charset.StandardCharsets;
 
 import org.eclipse.jetty.websocket.api.Session;
 import org.eclipse.jetty.websocket.api.WriteCallback;
+import reactor.core.publisher.Mono;
+
 import org.springframework.util.ObjectUtils;
 import org.springframework.web.reactive.socket.CloseStatus;
 import org.springframework.web.reactive.socket.WebSocketMessage;
 import org.springframework.web.reactive.socket.WebSocketSession;
 
-import reactor.core.publisher.Mono;
-
 /**
  * Spring {@link WebSocketSession} adapter for Jetty's
  * {@link org.eclipse.jetty.websocket.api.Session}.
@@ -37,11 +37,13 @@ import reactor.core.publisher.Mono;
  */
 public class JettyWebSocketSession extends AbstractListenerWebSocketSession<Session> {
 
+
 	public JettyWebSocketSession(Session session) {
 		super(session, ObjectUtils.getIdentityHexString(session),
 				session.getUpgradeRequest().getRequestURI());
 	}
 
+
 	@Override
 	protected Mono<Void> closeInternal(CloseStatus status) {
 		getDelegate().close(status.getCode(), status.getReason());
@@ -73,6 +75,22 @@ public class JettyWebSocketSession extends AbstractListenerWebSocketSession<Sess
 		return true;
 	}
 
+	@Override
+	protected void resumeReceiving() {
+		// No-op
+	}
+
+	@Override
+	protected void suspendReceiving() {
+		// No-op
+	}
+
+	@Override
+	protected boolean canSuspendReceiving() {
+		return false;
+	}
+
+
 	private final class WebSocketMessageWriteCallback implements WriteCallback {
 
 		@Override

spring-web-reactive/src/main/java/org/springframework/web/reactive/socket/adapter/TomcatWebSocketHandlerAdapter.java

@@ -64,10 +64,8 @@ public class TomcatWebSocketHandlerAdapter extends Endpoint {
 
 			@Override
 			public void onMessage(String message) {
-				if (!wsSession.isSuspended()) {
-					WebSocketMessage wsMessage = toMessage(message);
-					wsSession.handleMessage(wsMessage.getType(), wsMessage);
-				}
+				WebSocketMessage wsMessage = toMessage(message);
+				wsSession.handleMessage(wsMessage.getType(), wsMessage);
 			}
 
 		});
@@ -75,10 +73,8 @@ public class TomcatWebSocketHandlerAdapter extends Endpoint {
 
 			@Override
 			public void onMessage(ByteBuffer message) {
-				if (!wsSession.isSuspended()) {
-					WebSocketMessage wsMessage = toMessage(message);
-					wsSession.handleMessage(wsMessage.getType(), wsMessage);
-				}
+				WebSocketMessage wsMessage = toMessage(message);
+				wsSession.handleMessage(wsMessage.getType(), wsMessage);
 			}
 
 		});
@@ -86,10 +82,8 @@ public class TomcatWebSocketHandlerAdapter extends Endpoint {
 
 			@Override
 			public void onMessage(PongMessage message) {
-				if (!wsSession.isSuspended()) {
-					WebSocketMessage wsMessage = toMessage(message);
-					wsSession.handleMessage(wsMessage.getType(), wsMessage);
-				}
+				WebSocketMessage wsMessage = toMessage(message);
+				wsSession.handleMessage(wsMessage.getType(), wsMessage);
 			}
 
 		});

spring-web-reactive/src/main/java/org/springframework/web/reactive/socket/adapter/TomcatWebSocketSession.java

@@ -18,19 +18,18 @@ package org.springframework.web.reactive.socket.adapter;
 
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
-
 import javax.websocket.CloseReason;
+import javax.websocket.CloseReason.CloseCodes;
 import javax.websocket.SendHandler;
 import javax.websocket.SendResult;
 import javax.websocket.Session;
-import javax.websocket.CloseReason.CloseCodes;
+
+import reactor.core.publisher.Mono;
 
 import org.springframework.web.reactive.socket.CloseStatus;
 import org.springframework.web.reactive.socket.WebSocketMessage;
 import org.springframework.web.reactive.socket.WebSocketSession;
 
-import reactor.core.publisher.Mono;
-
 /**
  * Spring {@link WebSocketSession} adapter for Tomcat's
  * {@link javax.websocket.Session}.
@@ -40,10 +39,12 @@ import reactor.core.publisher.Mono;
  */
 public class TomcatWebSocketSession extends AbstractListenerWebSocketSession<Session> {
 
+
 	public TomcatWebSocketSession(Session session) {
 		super(session, session.getId(), session.getRequestURI());
 	}
 
+
 	@Override
 	protected Mono<Void> closeInternal(CloseStatus status) {
 		try {
@@ -81,6 +82,22 @@ public class TomcatWebSocketSession extends AbstractListenerWebSocketSession<Ses
 		return true;
 	}
 
+	@Override
+	protected void resumeReceiving() {
+		// No-op
+	}
+
+	@Override
+	protected void suspendReceiving() {
+		// No-op
+	}
+
+	@Override
+	protected boolean canSuspendReceiving() {
+		return false;
+	}
+
+
 	private final class WebSocketMessageSendHandler implements SendHandler {
 
 		@Override

spring-web-reactive/src/main/java/org/springframework/web/reactive/socket/adapter/UndertowWebSocketSession.java

@@ -21,17 +21,17 @@ import java.net.URI;
 import java.net.URISyntaxException;
 import java.nio.charset.StandardCharsets;
 
-import org.springframework.util.ObjectUtils;
-import org.springframework.web.reactive.socket.CloseStatus;
-import org.springframework.web.reactive.socket.WebSocketMessage;
-import org.springframework.web.reactive.socket.WebSocketSession;
-
 import io.undertow.websockets.core.CloseMessage;
 import io.undertow.websockets.core.WebSocketCallback;
 import io.undertow.websockets.core.WebSocketChannel;
 import io.undertow.websockets.core.WebSockets;
 import reactor.core.publisher.Mono;
 
+import org.springframework.util.ObjectUtils;
+import org.springframework.web.reactive.socket.CloseStatus;
+import org.springframework.web.reactive.socket.WebSocketMessage;
+import org.springframework.web.reactive.socket.WebSocketSession;
+
 /**
  * Spring {@link WebSocketSession} adapter for Undertow's
  * {@link io.undertow.websockets.core.WebSocketChannel}.
@@ -54,16 +54,19 @@ public class UndertowWebSocketSession extends AbstractListenerWebSocketSession<W
 		return Mono.empty();
 	}
 
-	protected void resumeReceives() {
-		super.resumeReceives();
+	protected void resumeReceiving() {
 		getDelegate().resumeReceives();
 	}
 
-	protected void suspendReceives() {
-		super.suspendReceives();
+	protected void suspendReceiving() {
 		getDelegate().suspendReceives();
 	}
 
+	@Override
+	protected boolean canSuspendReceiving() {
+		return true;
+	}
+
 	@Override
 	protected boolean sendMessage(WebSocketMessage message) throws IOException {
 		if (WebSocketMessage.Type.TEXT.equals(message.getType())) {