GH-8600: Fix WebSocket removeRegistration (#8601)

* GH-8600: Fix WebSocket `removeRegistration`

Fixes https://github.com/spring-projects/spring-integration/issues/8600

When we register a dynamic WebSocket endpoint and use a `WebSocketHandlerDecoratorFactory`
such an endpoint is not removed on an `IntegrationFlow` destruction.
The actual `WebSocketHandler` is decorated, however we still use an initial one
for condition.

* Refactor `IntegrationWebSocketContainer` to expose a `protected` setter for the
`WebSocketHandler` which is called from the `ServerWebSocketContainer` after decoration

**Cherry-pick to `6.0.x` & `5.5.x`**

* Fix language in Javadocs

Co-authored-by: Gary Russell <grussell@vmware.com>

---------

Co-authored-by: Gary Russell <grussell@vmware.com>

spring-integration-websocket/src/test/java/org/springframework/integration/websocket/dsl/WebSocketDslTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2021-2022 the original author or authors.
+ * Copyright 2021-2023 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.
@@ -16,6 +16,8 @@
 
 package org.springframework.integration.websocket.dsl;
 
+import java.util.concurrent.atomic.AtomicReference;
+
 import jakarta.websocket.DeploymentException;
 import org.junit.jupiter.api.Test;
 
@@ -37,6 +39,7 @@ import org.springframework.messaging.support.GenericMessage;
 import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
 import org.springframework.web.context.support.AnnotationConfigWebApplicationContext;
+import org.springframework.web.socket.WebSocketHandler;
 import org.springframework.web.socket.client.WebSocketClient;
 import org.springframework.web.socket.client.standard.StandardWebSocketClient;
 import org.springframework.web.socket.server.HandshakeHandler;
@@ -46,6 +49,9 @@ import org.springframework.web.socket.server.support.DefaultHandshakeHandler;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.awaitility.Awaitility.await;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.verify;
 
 @SpringJUnitConfig(classes = WebSocketDslTests.ClientConfig.class)
 @DirtiesContext
@@ -61,13 +67,19 @@ public class WebSocketDslTests {
 	IntegrationFlowContext integrationFlowContext;
 
 	@Test
-	void testDynamicServerEndpointRegistration() {
+	void testDynamicServerEndpointRegistration() throws Exception {
 		// Dynamic server flow
 		AnnotationConfigWebApplicationContext serverContext = this.server.getServerContext();
 		IntegrationFlowContext serverIntegrationFlowContext = serverContext.getBean(IntegrationFlowContext.class);
+		AtomicReference<WebSocketHandler> decoratedHandler = new AtomicReference<>();
 		ServerWebSocketContainer serverWebSocketContainer =
 				new ServerWebSocketContainer("/dynamic")
 						.setHandshakeHandler(serverContext.getBean(HandshakeHandler.class))
+						.setDecoratorFactories(handler -> {
+							WebSocketHandler spy = spy(handler);
+							decoratedHandler.set(spy);
+							return spy;
+						})
 						.withSockJs();
 
 		WebSocketInboundChannelAdapter webSocketInboundChannelAdapter =
@@ -106,6 +118,8 @@ public class WebSocketDslTests {
 				.extracting(Message::getPayload)
 				.isEqualTo("dynamic test");
 
+		verify(decoratedHandler.get()).handleMessage(any(), any());
+
 		dynamicServerFlow.destroy();
 
 		await() // Looks like endpoint is removed on the server side somewhat async