Handle servlet startup failures consistently

Ensure that all servlet containers handle servlet startup failures
consistently and throw a `WebServerException` that wraps the original
cause.

Both Undertow and Jetty already dealt with startup failures in this
way, but Tomcat did not. The `TomcatEmbeddedContext` has now been
changed to no longer call `super.loadOnStartup` but instead re-implement
a version of that method that wraps and rethrows the original exception
(as long as `failCtxIfServletStartFails` is `true`, which it now is by
default).

Closes gh-14790

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatEmbeddedContext.java

@@ -16,13 +16,23 @@
 
 package org.springframework.boot.web.embedded.tomcat;
 
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.TreeMap;
+import java.util.stream.Stream;
+
+import javax.servlet.ServletException;
+
 import org.apache.catalina.Container;
 import org.apache.catalina.LifecycleException;
 import org.apache.catalina.Manager;
+import org.apache.catalina.Wrapper;
 import org.apache.catalina.core.StandardContext;
+import org.apache.catalina.core.StandardWrapper;
 import org.apache.catalina.session.ManagerBase;
 
-import org.springframework.util.Assert;
+import org.springframework.boot.web.server.WebServerException;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -52,11 +62,37 @@ class TomcatEmbeddedContext extends StandardContext {
 
 	public void deferredLoadOnStartup() throws LifecycleException {
 		doWithThreadContextClassLoader(getLoader().getClassLoader(), () -> {
-			boolean started = super.loadOnStartup(findChildren());
-			Assert.state(started, "Unable to start embedded tomcat context " + getName());
+			getLoadOnStartupWrappers(findChildren()).forEach(this::load);
 		});
 	}
 
+	private Stream<Wrapper> getLoadOnStartupWrappers(Container[] children) {
+		Map<Integer, List<Wrapper>> grouped = new TreeMap<>();
+		for (Container child : children) {
+			Wrapper wrapper = (Wrapper) child;
+			int order = wrapper.getLoadOnStartup();
+			if (order >= 0) {
+				grouped.computeIfAbsent(order, ArrayList::new);
+				grouped.get(order).add(wrapper);
+			}
+		}
+		return grouped.values().stream().flatMap(List::stream);
+	}
+
+	private void load(Wrapper wrapper) {
+		try {
+			wrapper.load();
+		}
+		catch (ServletException ex) {
+			String message = sm.getString("standardContext.loadOnStartup.loadException",
+					getName(), wrapper.getName());
+			if (getComputedFailCtxIfServletStartFails()) {
+				throw new WebServerException(message, ex);
+			}
+			getLogger().error(message, StandardWrapper.getRootCause(ex));
+		}
+	}
+
 	/**
 	 * Some older Servlet frameworks (e.g. Struts, BIRT) use the Thread context class
 	 * loader to create servlet instances in this phase. If they do that and then try to

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatServletWebServerFactory.java

@@ -329,8 +329,9 @@ public class TomcatServletWebServerFactory extends AbstractServletWebServerFacto
 			ServletContextInitializer[] initializers) {
 		TomcatStarter starter = new TomcatStarter(initializers);
 		if (context instanceof TomcatEmbeddedContext) {
-			// Should be true
-			((TomcatEmbeddedContext) context).setStarter(starter);
+			TomcatEmbeddedContext embeddedContext = (TomcatEmbeddedContext) context;
+			embeddedContext.setStarter(starter);
+			embeddedContext.setFailCtxIfServletStartFails(true);
 		}
 		context.addServletContainerInitializer(starter, NO_CLASSES);
 		for (LifecycleListener lifecycleListener : this.contextLifecycleListeners) {

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatWebServer.java

@@ -284,7 +284,9 @@ public class TomcatWebServer implements WebServer {
 			}
 		}
 		catch (Exception ex) {
-			logger.error("Cannot start connector: ", ex);
+			if (ex instanceof WebServerException) {
+				throw (WebServerException) ex;
+			}
 			throw new WebServerException("Unable to start embedded Tomcat connectors",
 					ex);
 		}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/embedded/tomcat/TomcatServletWebServerFactoryTests.java

@@ -30,7 +30,6 @@ import java.util.Set;
 import javax.naming.InitialContext;
 import javax.naming.NamingException;
 import javax.servlet.ServletException;
-import javax.servlet.http.HttpServlet;
 
 import org.apache.catalina.Container;
 import org.apache.catalina.Context;
@@ -440,6 +439,20 @@ public class TomcatServletWebServerFactoryTests
 				.isThrownBy(this.webServer::start);
 	}
 
+	@Test
+	public void exceptionThrownOnLoadFailureWhenFailCtxIfServletStartFailsIsFalse() {
+		TomcatServletWebServerFactory factory = getFactory();
+		factory.addContextCustomizers((context) -> {
+			if (context instanceof StandardContext) {
+				((StandardContext) context).setFailCtxIfServletStartFails(false);
+			}
+		});
+		this.webServer = factory.getWebServer((context) -> {
+			context.addServlet("failing", FailingServlet.class).setLoadOnStartup(0);
+		});
+		this.webServer.start();
+	}
+
 	@Override
 	protected JspServlet getJspServlet() throws ServletException {
 		Tomcat tomcat = ((TomcatWebServer) this.webServer).getTomcat();
@@ -488,13 +501,4 @@ public class TomcatServletWebServerFactoryTests
 		assertThat(((ConnectorStartFailedException) ex).getPort()).isEqualTo(blockedPort);
 	}
 
-	static class FailingServlet extends HttpServlet {
-
-		@Override
-		public void init() throws ServletException {
-			throw new RuntimeException("Init Failure");
-		}
-
-	}
-
 }

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

@@ -1046,6 +1046,28 @@ public abstract class AbstractServletWebServerFactoryTests {
 		verify(listener).contextDestroyed(any(ServletContextEvent.class));
 	}
 
+	@Test
+	public void exceptionThrownOnLoadFailureIsRethrown() {
+		AbstractServletWebServerFactory factory = getFactory();
+		this.webServer = factory.getWebServer((context) -> {
+			context.addServlet("failing", FailingServlet.class).setLoadOnStartup(0);
+		});
+		assertThatExceptionOfType(WebServerException.class)
+				.isThrownBy(this.webServer::start)
+				.satisfies(this::wrapsFailingServletException);
+	}
+
+	private void wrapsFailingServletException(WebServerException ex) {
+		Throwable cause = ex.getCause();
+		while (cause != null) {
+			if (cause instanceof FailingServletException) {
+				return;
+			}
+			cause = cause.getCause();
+		}
+		fail("Exception did not wrap FailingServletException");
+	}
+
 	protected abstract void addConnector(int port,
 			AbstractServletWebServerFactory factory);
 
@@ -1344,4 +1366,21 @@ public abstract class AbstractServletWebServerFactoryTests {
 
 	}
 
+	public static class FailingServlet extends HttpServlet {
+
+		@Override
+		public void init() throws ServletException {
+			throw new FailingServletException();
+		}
+
+	}
+
+	private static class FailingServletException extends RuntimeException {
+
+		FailingServletException() {
+			super("Init Failure");
+		}
+
+	}
+
 }