Support WebSocket ServletServerContainerFB in the TCF

Prior to this commit, any attempt to include a bean of type
ServletServerContainerFactoryBean in the WebApplicationContext for an
integration test class annotated with @WebAppConfiguration in
conjunction the Spring TestContext Framework (TCF) would have resulted
in an IllegalStateException stating that "A ServletContext is required
to access the javax.websocket.server.ServerContainer instance."

In such scenarios, the MockServletContext was in fact present in the
WebApplicationContext; however there was no WebSocket ServerContainer
stored in the ServletContext.

This commit addresses this issue by introducing the following.

- MockServerContainer: a private mock implementation of the
  javax.websocket.server.ServerContainer interface.

- MockServerContainerContextCustomizer: a ContextCustomizer that
  instantiates a new MockServerContainer and stores it in the
  ServletContext under the attribute named
  "javax.websocket.server.ServerContainer".

- MockServerContainerContextCustomizerFactory: a
  ContextCustomizerFactory which creates a
  MockServerContainerContextCustomizer if WebSocket support is present
  in the classpath and the test class is annotated with
  @WebAppConfiguration. This factory is registered by default via the
  spring.factories mechanism.

Issue: SPR-14367

spring-test/src/test/java/org/springframework/test/context/web/socket/WebSocketServletServerContainerFactoryBeanTests.java

@@ -0,0 +1,67 @@
+/*
+ * 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.test.context.web.socket;
+
+import javax.websocket.server.ServerContainer;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.test.context.junit4.SpringRunner;
+import org.springframework.test.context.web.WebAppConfiguration;
+import org.springframework.web.socket.config.annotation.EnableWebSocket;
+import org.springframework.web.socket.server.standard.ServletServerContainerFactoryBean;
+
+import static org.junit.Assert.*;
+
+/**
+ * Integration tests that validate support for {@link ServletServerContainerFactoryBean}
+ * in conjunction with {@link WebAppConfiguration @WebAppConfiguration} and the
+ * Spring TestContext Framework.
+ *
+ * @author Sam Brannen
+ * @since 4.3.1
+ */
+@RunWith(SpringRunner.class)
+@WebAppConfiguration
+public class WebSocketServletServerContainerFactoryBeanTests {
+
+	@Autowired
+	ServerContainer serverContainer;
+
+
+	@Test
+	public void servletServerContainerFactoryBeanSupport() {
+		assertEquals(42, serverContainer.getDefaultMaxTextMessageBufferSize());
+	}
+
+
+	@Configuration
+	@EnableWebSocket
+	static class WebSocketConfig {
+
+		@Bean
+		ServletServerContainerFactoryBean createWebSocketContainer() {
+			ServletServerContainerFactoryBean container = new ServletServerContainerFactoryBean();
+			container.setMaxTextMessageBufferSize(42);
+			return container;
+		}
+	}
+
+}