Use servlet context that can access classloader

Update `SpringBootServletInitializer` to use the `ServletContext` that
was provided to the initial `onStartup` call rather than the
one from the `ServletContextEvent`. This allows the `getClassLoader()`
call to complete without throwing an `UnsupportedOperationException`.

Fixes gh-21684

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/support/SpringBootServletInitializer.java

@@ -92,27 +92,9 @@ public void onStartup(ServletContext servletContext) throws ServletException {
 		// Logger initialization is deferred in case an ordered
 		// LogServletContextInitializer is being used
 		this.logger = LogFactory.getLog(getClass());
-		WebApplicationContext rootAppContext = createRootApplicationContext(servletContext);
-		if (rootAppContext != null) {
-			servletContext.addListener(new ContextLoaderListener(rootAppContext) {
-
-				@Override
-				public void contextInitialized(ServletContextEvent event) {
-					// no-op because the application context is already initialized
-				}
-
-				@Override
-				public void contextDestroyed(ServletContextEvent event) {
-					try {
-						super.contextDestroyed(event);
-					}
-					finally {
-						deregisterJdbcDrivers(event.getServletContext());
-					}
-				}
-
-			});
-
+		WebApplicationContext rootApplicationContext = createRootApplicationContext(servletContext);
+		if (rootApplicationContext != null) {
+			servletContext.addListener(new SpringBootContextLoaderListener(rootApplicationContext, servletContext));
 		}
 		else {
 			this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not "
@@ -212,6 +194,10 @@ protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
 		return builder;
 	}
 
+	/**
+	 * {@link ApplicationListener} to trigger
+	 * {@link ConfigurableWebEnvironment#initPropertySources(ServletContext, javax.servlet.ServletConfig)}.
+	 */
 	private static final class WebEnvironmentPropertySourceInitializer
 			implements ApplicationListener<ApplicationEnvironmentPreparedEvent>, Ordered {
 
@@ -236,4 +222,34 @@ public int getOrder() {
 
 	}
 
+	/**
+	 * {@link ContextLoaderListener} for the initialized context.
+	 */
+	private class SpringBootContextLoaderListener extends ContextLoaderListener {
+
+		private final ServletContext servletContext;
+
+		SpringBootContextLoaderListener(WebApplicationContext applicationContext, ServletContext servletContext) {
+			super(applicationContext);
+			this.servletContext = servletContext;
+		}
+
+		@Override
+		public void contextInitialized(ServletContextEvent event) {
+			// no-op because the application context is already initialized
+		}
+
+		@Override
+		public void contextDestroyed(ServletContextEvent event) {
+			try {
+				super.contextDestroyed(event);
+			}
+			finally {
+				// Use original context so that the classloader can be accessed
+				deregisterJdbcDrivers(this.servletContext);
+			}
+		}
+
+	}
+
 }