Replace TomcatStarter to break servlet dependency

Replace `TomcatStarter` with a `DeferredStartupExceptions` interface
to break the direct dependency on `Servlet`.

Closes gh-44325

module/spring-boot-tomcat/src/main/java/org/springframework/boot/tomcat/TomcatEmbeddedContext.java

@@ -34,6 +34,7 @@ import org.jspecify.annotations.Nullable;
 
 import org.springframework.boot.web.server.MimeMappings;
 import org.springframework.boot.web.server.WebServerException;
+import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -46,7 +47,7 @@ import org.springframework.util.ClassUtils;
  */
 public class TomcatEmbeddedContext extends StandardContext {
 
-	private @Nullable TomcatStarter starter;
+	private DeferredStartupExceptions deferredStartupExceptions = DeferredStartupExceptions.NONE;
 
 	private @Nullable MimeMappings mimeMappings;
 
@@ -116,12 +117,17 @@ public class TomcatEmbeddedContext extends StandardContext {
 		}
 	}
 
-	public void setStarter(@Nullable TomcatStarter starter) {
-		this.starter = starter;
+	/**
+	 * Set the a strategy used to capture and rethrow deferred startup exceptions.
+	 * @param deferredStartupExceptions the strategy to use
+	 */
+	public void setDeferredStartupExceptions(DeferredStartupExceptions deferredStartupExceptions) {
+		Assert.notNull(deferredStartupExceptions, "'deferredStartupExceptions' must not be null");
+		this.deferredStartupExceptions = deferredStartupExceptions;
 	}
 
-	@Nullable TomcatStarter getStarter() {
-		return this.starter;
+	DeferredStartupExceptions getDeferredStartupExceptions() {
+		return this.deferredStartupExceptions;
 	}
 
 	public void setMimeMappings(MimeMappings mimeMappings) {
@@ -146,4 +152,24 @@ public class TomcatEmbeddedContext extends StandardContext {
 		return (this.mimeMappings != null) ? this.mimeMappings.get(extension) : null;
 	}
 
+	/**
+	 * Strategy interface that can be used to rethrow deferred startup exceptions.
+	 */
+	@FunctionalInterface
+	public interface DeferredStartupExceptions {
+
+		/**
+		 * {@link DeferredStartupExceptions} that does nothing.
+		 */
+		DeferredStartupExceptions NONE = () -> {
+		};
+
+		/**
+		 * Rethrow deferred startup exceptions if there are any.
+		 * @throws Exception the deferred startup exception
+		 */
+		void rethrow() throws Exception;
+
+	}
+
 }

module/spring-boot-tomcat/src/main/java/org/springframework/boot/tomcat/TomcatWebServer.java

@@ -200,13 +200,7 @@ public class TomcatWebServer implements WebServer {
 		Container[] children = this.tomcat.getHost().findChildren();
 		for (Container container : children) {
 			if (container instanceof TomcatEmbeddedContext embeddedContext) {
-				TomcatStarter tomcatStarter = embeddedContext.getStarter();
-				if (tomcatStarter != null) {
-					Exception exception = tomcatStarter.getStartUpException();
-					if (exception != null) {
-						throw exception;
-					}
-				}
+				embeddedContext.getDeferredStartupExceptions().rethrow();
 			}
 			if (!LifecycleState.STARTED.equals(container.getState())) {
 				throw new IllegalStateException(container + " failed to start");

module/spring-boot-tomcat/src/main/java/org/springframework/boot/tomcat/servlet/DeferredServletContainerInitializers.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.springframework.boot.tomcat;
+package org.springframework.boot.tomcat.servlet;
 
 import java.util.Set;
 
@@ -25,6 +25,7 @@ import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 import org.jspecify.annotations.Nullable;
 
+import org.springframework.boot.tomcat.TomcatEmbeddedContext;
 import org.springframework.boot.web.servlet.ServletContextInitializer;
 
 /**
@@ -33,17 +34,17 @@ import org.springframework.boot.web.servlet.ServletContextInitializer;
  *
  * @author Phillip Webb
  * @author Andy Wilkinson
- * @since 4.0.0
  */
-public class TomcatStarter implements ServletContainerInitializer {
+class DeferredServletContainerInitializers
+		implements ServletContainerInitializer, TomcatEmbeddedContext.DeferredStartupExceptions {
 
-	private static final Log logger = LogFactory.getLog(TomcatStarter.class);
+	private static final Log logger = LogFactory.getLog(DeferredServletContainerInitializers.class);
 
 	private final Iterable<ServletContextInitializer> initializers;
 
 	private volatile @Nullable Exception startUpException;
 
-	public TomcatStarter(Iterable<ServletContextInitializer> initializers) {
+	DeferredServletContainerInitializers(Iterable<ServletContextInitializer> initializers) {
 		this.initializers = initializers;
 	}
 
@@ -65,8 +66,11 @@ public class TomcatStarter implements ServletContainerInitializer {
 		}
 	}
 
-	@Nullable Exception getStartUpException() {
-		return this.startUpException;
+	@Override
+	public void rethrow() throws Exception {
+		if (this.startUpException != null) {
+			throw this.startUpException;
+		}
 	}
 
 }

module/spring-boot-tomcat/src/main/java/org/springframework/boot/tomcat/servlet/TomcatServletWebServerFactory.java

@@ -70,7 +70,6 @@ import org.springframework.boot.tomcat.DisableReferenceClearingContextCustomizer
 import org.springframework.boot.tomcat.TomcatContextCustomizer;
 import org.springframework.boot.tomcat.TomcatEmbeddedContext;
 import org.springframework.boot.tomcat.TomcatEmbeddedWebappClassLoader;
-import org.springframework.boot.tomcat.TomcatStarter;
 import org.springframework.boot.tomcat.TomcatWebServer;
 import org.springframework.boot.tomcat.TomcatWebServerFactory;
 import org.springframework.boot.web.error.ErrorPage;
@@ -289,12 +288,13 @@ public class TomcatServletWebServerFactory extends TomcatWebServerFactory
 	 * @param initializers initializers to apply
 	 */
 	protected void configureContext(Context context, Iterable<ServletContextInitializer> initializers) {
-		TomcatStarter starter = new TomcatStarter(initializers);
+		DeferredServletContainerInitializers deferredInitializers = new DeferredServletContainerInitializers(
+				initializers);
 		if (context instanceof TomcatEmbeddedContext embeddedContext) {
-			embeddedContext.setStarter(starter);
+			embeddedContext.setDeferredStartupExceptions(deferredInitializers);
 			embeddedContext.setFailCtxIfServletStartFails(true);
 		}
-		context.addServletContainerInitializer(starter, NO_CLASSES);
+		context.addServletContainerInitializer(deferredInitializers, NO_CLASSES);
 		for (LifecycleListener lifecycleListener : this.getContextLifecycleListeners()) {
 			context.addLifecycleListener(lifecycleListener);
 		}