Fix issue with obtaining WebSocketContainer

spring-websocket/src/main/java/org/springframework/web/socket/server/endpoint/ServerEndpointExporter.java

@@ -35,7 +35,6 @@ import org.springframework.beans.factory.config.BeanPostProcessor;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;
 import org.springframework.util.Assert;
-import org.springframework.util.ClassUtils;
 import org.springframework.util.ReflectionUtils;
 
 /**
@@ -58,9 +57,6 @@ import org.springframework.util.ReflectionUtils;
  */
 public class ServerEndpointExporter implements InitializingBean, BeanPostProcessor, ApplicationContextAware {
 
-	private static final boolean isServletApiPresent =
-			ClassUtils.isPresent("javax.servlet.ServletContext", ServerEndpointExporter.class.getClassLoader());
-
 	private static Log logger = LogFactory.getLog(ServerEndpointExporter.class);
 
 
@@ -103,12 +99,19 @@ public class ServerEndpointExporter implements InitializingBean, BeanPostProcess
 	}
 
 	protected ServerContainer getServerContainer() {
-		if (isServletApiPresent) {
+
+		Class<?> servletContextClass;
+		try {
+			servletContextClass = Class.forName("javax.servlet.ServletContext");
+		}
+		catch (Throwable e) {
+			return null;
+		}
+
 		try {
 			Method getter = ReflectionUtils.findMethod(this.applicationContext.getClass(), "getServletContext");
 			Object servletContext = getter.invoke(this.applicationContext);
-
-				Method attrMethod = ReflectionUtils.findMethod(servletContext.getClass(), "getAttribute", String.class);
+			Method attrMethod = ReflectionUtils.findMethod(servletContextClass, "getAttribute", String.class);
 			return (ServerContainer) attrMethod.invoke(servletContext, "javax.websocket.server.ServerContainer");
 		}
 		catch (Exception ex) {
@@ -116,8 +119,6 @@ public class ServerEndpointExporter implements InitializingBean, BeanPostProcess
 					"Failed to get javax.websocket.server.ServerContainer via ServletContext attribute", ex);
 		}
 	}
-		return null;
-	}
 
 	@Override
 	public void afterPropertiesSet() throws Exception {