Fix serverless dispatcher init ordering and harden proxy filter chain; add regression coverage and override caution docs

Signed-off-by: Oblio <oblio.leitch@vermont.gov>

Author: Oblio

spring-cloud-function-adapters/spring-cloud-function-serverless-web/README.md

@@ -8,6 +8,12 @@ A sample is provided in [sample](https://github.com/spring-cloud/spring-cloud-fu
 
 _NOTE: Although this module is AWS specific, this dependency is protocol only (not binary), therefore there is no AWS dependnecies._
 
+_NOTE: The serverless `ServletWebServerFactory` is declared with `@ConditionalOnMissingBean`. If your
+application defines its own `ServletWebServerFactory` bean (for example Tomcat/Jetty/Undertow customization),
+that custom bean will take precedence and can disable the serverless adapter path. For serverless-web usage,
+do not provide a competing `ServletWebServerFactory` bean unless it delegates to
+`ServerlessAutoConfiguration.ServerlessServletWebServerFactory`._
+
 The aformentioned proxy is identified as AWS Lambda [handler](https://github.com/spring-cloud/spring-cloud-function/blob/serverless-web/spring-cloud-function-adapters/spring-cloud-function-adapter-aws-web/sample/pet-store/template.yml#L14)
 
 The main Spring Boot configuration file is identified as [MAIN_CLASS](https://github.com/spring-cloud/spring-cloud-function/blob/serverless-web/spring-cloud-function-adapters/spring-cloud-function-adapter-aws-web/sample/pet-store/template.yml#L22)

spring-cloud-function-adapters/spring-cloud-function-serverless-web/src/main/java/org/springframework/cloud/function/serverless/web/ServerlessAutoConfiguration.java

@@ -21,6 +21,7 @@
 
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.InitializingBean;
+import org.springframework.boot.autoconfigure.AutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.web.server.WebServer;
 import org.springframework.boot.web.server.WebServerException;
@@ -39,13 +40,15 @@
  * @author Oleg Zhurakousky
  * @since 4.x
  */
+@AutoConfiguration(beforeName = "org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration")
 @Configuration(proxyBeanMethods = false)
 public class ServerlessAutoConfiguration {
 	private static Log logger = LogFactory.getLog(ServerlessAutoConfiguration.class);
 
 	@Bean
 	@ConditionalOnMissingBean
 	public ServletWebServerFactory servletWebServerFactory() {
+		// A user-defined ServletWebServerFactory bean will override this and may bypass serverless initialization.
 		return new ServerlessServletWebServerFactory();
 	}
 

spring-cloud-function-adapters/spring-cloud-function-serverless-web/src/main/java/org/springframework/cloud/function/serverless/web/ServerlessMVC.java

@@ -23,6 +23,7 @@
 import java.util.Enumeration;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Map;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.stream.Stream;
@@ -33,6 +34,7 @@
 import jakarta.servlet.Filter;
 import jakarta.servlet.FilterChain;
 import jakarta.servlet.FilterConfig;
+import jakarta.servlet.FilterRegistration;
 import jakarta.servlet.RequestDispatcher;
 import jakarta.servlet.Servlet;
 import jakarta.servlet.ServletConfig;
@@ -126,6 +128,8 @@ private void initContext(Class<?>... componentClasses) {
 		if (this.applicationContext.containsBean(DispatcherServletAutoConfiguration.DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)) {
 			this.dispatcher = this.applicationContext.getBean(DispatcherServlet.class);
 		}
+		Assert.state(this.dispatcher != null, "DispatcherServlet bean was not initialized. "
+				+ "Ensure ServerlessAutoConfiguration is active and selected as the ServletWebServerFactory.");
 	}
 
 	public ConfigurableWebApplicationContext getApplicationContext() {
@@ -161,6 +165,8 @@ public void service(HttpServletRequest request, HttpServletResponse response) th
 	}
 
 	public void service(HttpServletRequest request, HttpServletResponse response, CountDownLatch latch) throws Exception {
+		Assert.state(this.dispatcher != null, "DispatcherServlet is not initialized. "
+				+ "Ensure ServerlessAutoConfiguration is active and selected as the ServletWebServerFactory.");
 		ProxyFilterChain filterChain = new ProxyFilterChain(this.dispatcher);
 		filterChain.doFilter(request, response);
 
@@ -217,7 +223,17 @@ private static class ProxyFilterChain implements FilterChain {
 		 */
 		ProxyFilterChain(DispatcherServlet servlet) {
 			List<Filter> filters = new ArrayList<>();
-			servlet.getServletContext().getFilterRegistrations().values().forEach(fr -> filters.add(((ServerlessFilterRegistration) fr).getFilter()));
+			for (Map.Entry<String, ? extends FilterRegistration> entry : servlet.getServletContext().getFilterRegistrations()
+					.entrySet()) {
+				FilterRegistration registration = entry.getValue();
+				if (registration instanceof ServerlessFilterRegistration serverlessFilterRegistration) {
+					filters.add(serverlessFilterRegistration.getFilter());
+				}
+				else {
+					LOG.debug("Skipping unsupported filter registration type '" + registration.getClass().getName()
+							+ "' for filter '" + entry.getKey() + "'");
+				}
+			}
 			Assert.notNull(filters, "filters cannot be null");
 			Assert.noNullElements(filters, "filters cannot contain null values");
 			this.filters = initFilterList(servlet, filters.toArray(new Filter[] {}));

spring-cloud-function-adapters/spring-cloud-function-serverless-web/src/test/java/org/springframework/cloud/function/serverless/web/RequestResponseTests.java

@@ -16,8 +16,12 @@
 
 package org.springframework.cloud.function.serverless.web;
 
+import java.lang.reflect.Field;
+import java.lang.reflect.Proxy;
 import java.util.List;
+import java.util.Map;
 
+import jakarta.servlet.FilterRegistration;
 import jakarta.servlet.http.HttpServletRequest;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -195,4 +199,38 @@ public void validatePostAsyncWithBody() throws Exception {
 		assertThat(pet.getName()).isNotEmpty();
 	}
 
+	@Test
+	public void validateNonServerlessFilterRegistrationIsSkipped() throws Exception {
+		ServerlessServletContext servletContext = (ServerlessServletContext) this.mvc.getServletContext();
+		Field registrationsField = ServerlessServletContext.class.getDeclaredField("filterRegistrations");
+		registrationsField.setAccessible(true);
+		@SuppressWarnings("unchecked")
+		Map<String, FilterRegistration> registrations =
+			(Map<String, FilterRegistration>) registrationsField.get(servletContext);
+
+		FilterRegistration nonServerlessRegistration = (FilterRegistration) Proxy.newProxyInstance(
+			FilterRegistration.class.getClassLoader(),
+			new Class[]{FilterRegistration.class},
+			(proxy, method, args) -> {
+				if ("getName".equals(method.getName())) {
+					return "nonServerless";
+				}
+				if (method.getReturnType().isPrimitive()) {
+					if (method.getReturnType() == boolean.class) {
+						return false;
+					}
+					return 0;
+				}
+				return null;
+			});
+
+		registrations.put("nonServerless", nonServerlessRegistration);
+
+		HttpServletRequest request = new ServerlessHttpServletRequest(null, "GET", "/pets");
+		ServerlessHttpServletResponse response = new ServerlessHttpServletResponse();
+		this.mvc.service(request, response);
+
+		assertThat(response.getStatus()).isEqualTo(HttpStatus.OK.value());
+	}
+
 }

spring-cloud-function-adapters/spring-cloud-function-serverless-web/src/test/java/org/springframework/cloud/function/serverless/web/ServerlessWebServerFactoryTests.java

@@ -20,6 +20,8 @@
 
 import org.springframework.boot.autoconfigure.SpringBootApplication;
 
+import static org.assertj.core.api.Assertions.assertThat;
+
 /**
  * @author Oleg Zhurakousky
  */
@@ -29,6 +31,7 @@ public class ServerlessWebServerFactoryTests {
 	public void testServerFactoryExists() {
 		ServerlessMVC mvc = ServerlessMVC.INSTANCE(TestApplication.class);
 		mvc.getApplicationContext();
+		assertThat(mvc.getServletContext()).isNotNull();
 	}
 
 	@SpringBootApplication