Use consistent current thread context classloader for initialization

Ensure `Thread.currentThread().getContextClassLoader()` returns the same
classloader for all types of initialization.

Prior to this commit, `JettyEmbeddedWebAppContext` would return a
different classloader when initializing Servlet and Filter classes. This
was due to the fact that our `deferredInitialize()` method has called
outside of a `getContext().call(...)`.

Fixes gh-37649

Co-authored-by: Phillip Webb <phil.webb@broadcom.com>

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/jetty/JettyEmbeddedWebAppContext.java

@@ -38,7 +38,8 @@ class JettyEmbeddedWebAppContext extends WebAppContext {
 	}
 
 	void deferredInitialize() throws Exception {
-		((JettyEmbeddedServletHandler) getServletHandler()).deferredInitialize();
+		JettyEmbeddedServletHandler handler = (JettyEmbeddedServletHandler) getServletHandler();
+		getContext().call(handler::deferredInitialize, null);
 	}
 
 	private static final class JettyEmbeddedServletHandler extends ServletHandler {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/servlet/server/AbstractServletWebServerFactoryTests.java

@@ -40,6 +40,7 @@ import java.util.Collection;
 import java.util.Date;
 import java.util.EnumSet;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Locale;
@@ -65,6 +66,7 @@ import jakarta.servlet.Filter;
 import jakarta.servlet.FilterChain;
 import jakarta.servlet.FilterConfig;
 import jakarta.servlet.GenericServlet;
+import jakarta.servlet.ServletConfig;
 import jakarta.servlet.ServletContext;
 import jakarta.servlet.ServletContextEvent;
 import jakarta.servlet.ServletContextListener;
@@ -1382,6 +1384,26 @@ public abstract class AbstractServletWebServerFactoryTests {
 				+ " \\(http(/1.1)?\\), [0-9]+ \\(http(/1.1)?\\)( with context path '(/)?')?");
 	}
 
+	@Test
+	void servletComponentsAreInitializedWithTheSameThreadContextClassLoader() {
+		AbstractServletWebServerFactory factory = getFactory();
+		ThreadContextClassLoaderCapturingServlet servlet = new ThreadContextClassLoaderCapturingServlet();
+		ThreadContextClassLoaderCapturingFilter filter = new ThreadContextClassLoaderCapturingFilter();
+		ThreadContextClassLoaderCapturingListener listener = new ThreadContextClassLoaderCapturingListener();
+		this.webServer = factory.getWebServer((context) -> {
+			context.addServlet("tcclCapturingServlet", servlet).setLoadOnStartup(0);
+			context.addFilter("tcclCapturingFilter", filter);
+			context.addListener(listener);
+		});
+		this.webServer.start();
+		assertThat(servlet.contextClassLoader).isNotNull();
+		assertThat(filter.contextClassLoader).isNotNull();
+		assertThat(listener.contextClassLoader).isNotNull();
+		assertThat(new HashSet<>(
+				Arrays.asList(servlet.contextClassLoader, filter.contextClassLoader, listener.contextClassLoader)))
+			.hasSize(1);
+	}
+
 	protected Future<Object> initiateGetRequest(int port, String path) {
 		return initiateGetRequest(HttpClients.createMinimal(), port, path);
 	}
@@ -1455,7 +1477,7 @@ public abstract class AbstractServletWebServerFactoryTests {
 			compression.setExcludedUserAgents(excludedUserAgents);
 		}
 		factory.setCompression(compression);
-		factory.addInitializers(new ServletRegistrationBean<HttpServlet>(new HttpServlet() {
+		factory.addInitializers(new ServletRegistrationBean<>(new HttpServlet() {
 
 			@Override
 			protected void service(HttpServletRequest req, HttpServletResponse resp) throws IOException {
@@ -1833,4 +1855,43 @@ public abstract class AbstractServletWebServerFactoryTests {
 
 	}
 
+	static class ThreadContextClassLoaderCapturingServlet extends HttpServlet {
+
+		private ClassLoader contextClassLoader;
+
+		@Override
+		public void init(ServletConfig config) throws ServletException {
+			this.contextClassLoader = Thread.currentThread().getContextClassLoader();
+		}
+
+	}
+
+	static class ThreadContextClassLoaderCapturingListener implements ServletContextListener {
+
+		private ClassLoader contextClassLoader;
+
+		@Override
+		public void contextInitialized(ServletContextEvent sce) {
+			this.contextClassLoader = Thread.currentThread().getContextClassLoader();
+		}
+
+	}
+
+	static class ThreadContextClassLoaderCapturingFilter implements Filter {
+
+		private ClassLoader contextClassLoader;
+
+		@Override
+		public void init(FilterConfig filterConfig) throws ServletException {
+			this.contextClassLoader = Thread.currentThread().getContextClassLoader();
+		}
+
+		@Override
+		public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
+				throws IOException, ServletException {
+			chain.doFilter(request, response);
+		}
+
+	}
+
 }