Do not raise exception for undelivered empty messages

Closes gh-23828

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/session/AbstractSockJsSession.java

@@ -380,10 +380,16 @@ public abstract class AbstractSockJsSession implements SockJsSession {
 	public void delegateMessages(String... messages) throws SockJsMessageDeliveryException {
 		for (int i = 0; i < messages.length; i++) {
 			try {
-				if (isClosed()) {
+				if (!isClosed()) {
-					throw new SockJsMessageDeliveryException(this.id, getUndelivered(messages, i), "Session closed");
+					this.handler.handleMessage(this, new TextMessage(messages[i]));
+				}
+				else {
+					List<String> undelivered = getUndelivered(messages, i);
+					if (undelivered.isEmpty()) {
+						return;
+					}
+					throw new SockJsMessageDeliveryException(this.id, undelivered, "Session closed");
 				}
-				this.handler.handleMessage(this, new TextMessage(messages[i]));
 			}
 			catch (Exception ex) {
 				throw new SockJsMessageDeliveryException(this.id, getUndelivered(messages, i), ex);

spring-websocket/src/test/java/org/springframework/web/socket/sockjs/transport/session/SockJsSessionTests.java

@@ -106,7 +106,7 @@ public class SockJsSessionTests extends AbstractSockJsSessionTests<TestSockJsSes
 	}
 
 	@Test
-	public void delegateMessagesWithErrorAndConnectionClosing() throws Exception {
+	public void delegateMessagesWithError() throws Exception {
 
 		TestSockJsSession session = new TestSockJsSession("1", this.sockJsConfig,
 				new ExceptionWebSocketHandlerDecorator(this.webSocketHandler), Collections.emptyMap());
@@ -130,6 +130,23 @@ public class SockJsSessionTests extends AbstractSockJsSessionTests<TestSockJsSes
 		verifyNoMoreInteractions(this.webSocketHandler);
 	}
 
+	@Test // gh-23828
+	public void delegateMessagesEmptyAfterConnectionClosed() throws Exception {
+
+		TestSockJsSession session = new TestSockJsSession("1", this.sockJsConfig,
+				new ExceptionWebSocketHandlerDecorator(this.webSocketHandler), Collections.emptyMap());
+
+		session.delegateConnectionEstablished();
+		session.close(CloseStatus.NORMAL);
+		session.delegateMessages("", " ", "\n");
+
+		// No exception for empty messages
+
+		verify(this.webSocketHandler).afterConnectionEstablished(session);
+		verify(this.webSocketHandler).afterConnectionClosed(session, CloseStatus.NORMAL);
+		verifyNoMoreInteractions(this.webSocketHandler);
+	}
+
 	@Test
 	public void delegateConnectionClosed() throws Exception {
 		this.session.delegateConnectionEstablished();