Fix JSP EL support with Tomcat

Fix TomcatEmbeddedServletContainerFactory to set a MERGED_WEB_XML
attribute when JSPs are used. This is required for EL support with
JSPs since Jasper checks the version number in the web.xml. Without
any web.xml Jasper default to disabling EL.

Issue: #55752948

spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatEmbeddedServletContainerFactory.java

@@ -18,14 +18,20 @@ package org.springframework.boot.context.embedded.tomcat;
 
 import java.io.File;
 import java.io.IOException;
+import java.io.InputStream;
 import java.lang.reflect.Method;
+import java.nio.charset.Charset;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.List;
 
+import javax.servlet.ServletContext;
+
 import org.apache.catalina.Context;
 import org.apache.catalina.Host;
+import org.apache.catalina.Lifecycle;
+import org.apache.catalina.LifecycleEvent;
 import org.apache.catalina.LifecycleListener;
 import org.apache.catalina.Valve;
 import org.apache.catalina.Wrapper;
@@ -48,6 +54,7 @@ import org.springframework.core.io.ResourceLoader;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ReflectionUtils;
+import org.springframework.util.StreamUtils;
 
 /**
  * {@link EmbeddedServletContainerFactory} that can be used to create
@@ -161,6 +168,7 @@ public class TomcatEmbeddedServletContainerFactory extends
 				&& ClassUtils.isPresent(getJspServletClassName(), getClass()
 						.getClassLoader())) {
 			addJspServlet(context);
+			context.addLifecycleListener(new StoreMergedWebXmlListener());
 		}
 		ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);
 		configureContext(context, initializersToUse);
@@ -443,4 +451,45 @@ public class TomcatEmbeddedServletContainerFactory extends
 
 	}
 
+	/**
+	 * {@link LifecycleListener} that stores an empty merged web.xml. This is critical for
+	 * Jasper to prevent warnings about missing web.xml files and to enable EL.
+	 */
+	private static class StoreMergedWebXmlListener implements LifecycleListener {
+
+		private String MERGED_WEB_XML = org.apache.tomcat.util.scan.Constants.MERGED_WEB_XML;
+
+		@Override
+		public void lifecycleEvent(LifecycleEvent event) {
+			if (event.getType().equals(Lifecycle.CONFIGURE_START_EVENT)) {
+				onStart((Context) event.getLifecycle());
+			}
+		}
+
+		private void onStart(Context context) {
+			ServletContext servletContext = context.getServletContext();
+			if (servletContext.getAttribute(this.MERGED_WEB_XML) == null) {
+				servletContext.setAttribute(this.MERGED_WEB_XML, getEmptyWebXml());
+			}
+		}
+
+		private String getEmptyWebXml() {
+			InputStream stream = TomcatEmbeddedServletContainerFactory.class
+					.getResourceAsStream("empty-web.xml");
+			Assert.state(stream != null, "Unable to read empty web.xml");
+			try {
+				try {
+					return StreamUtils.copyToString(stream, Charset.forName("UTF-8"));
+				}
+				finally {
+					stream.close();
+				}
+			}
+			catch (IOException ex) {
+				throw new IllegalStateException(ex);
+			}
+		}
+
+	}
+
 }

spring-boot/src/main/resources/org/springframework/boot/context/embedded/tomcat/empty-web.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="ISO-8859-1"?>
+<web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
+	http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
+</web-app>