Document how to obtain ServletContext with an embedded container setup

Closes gh-24561

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/developing-web-applications.adoc

@@ -818,6 +818,17 @@ Usually a `TomcatServletWebServerFactory`, `JettyServletWebServerFactory`, or `U
 NOTE: You usually do not need to be aware of these implementation classes.
 Most applications are auto-configured, and the appropriate `ApplicationContext` and `ServletWebServerFactory` are created on your behalf.
 
+In an embedded container setup, the `ServletContext` is set as part of server startup which happens during application context initialization.
+Because of this beans in the `ApplicationContext` cannot be reliably initialized with a `ServletContext`.
+One way to get around this is to inject `ApplicationContext` as a dependency of the bean and access the `ServletContext` only when it is needed.
+Another way is to use a callback once the server has started.
+This can be done using an `ApplicationListener` which listens for the `ApplicationStartedEvent` as follows:
+
+[source,java,indent=0,subs="verbatim"]
+----
+include::{docs-java}/features/developingwebapplications/embeddedcontainer/applicationcontext/MyDemoBean.java[]
+----
+
 
 
 [[features.developing-web-applications.embedded-container.customizing]]

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/features/developingwebapplications/embeddedcontainer/applicationcontext/MyDemoBean.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.features.developingwebapplications.embeddedcontainer.applicationcontext;
+
+import javax.servlet.ServletContext;
+
+import org.springframework.boot.context.event.ApplicationStartedEvent;
+import org.springframework.context.ApplicationContext;
+import org.springframework.context.ApplicationListener;
+import org.springframework.web.context.WebApplicationContext;
+
+public class MyDemoBean implements ApplicationListener<ApplicationStartedEvent> {
+
+	private ServletContext servletContext;
+
+	@Override
+	public void onApplicationEvent(ApplicationStartedEvent event) {
+		ApplicationContext applicationContext = event.getApplicationContext();
+		this.servletContext = ((WebApplicationContext) applicationContext).getServletContext();
+	}
+
+}