Future proof SpringApplication.from(...) by returning a wrapper type

Update `SpringApplication.run` to that it returns a wrapper which
provides access to the `ApplicationContext` rather than the
`ApplicationContext` directly.

This should provide a natural place to add more accessors in the future
should we need to.

Closes gh-35451

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -1455,37 +1455,61 @@ public class SpringApplication {
 		 * @param args the main method args
 		 * @return the running {@link ApplicationContext}
 		 */
-		public ConfigurableApplicationContext run(String... args) {
-			ContextLoaderHook hook = new ContextLoaderHook(this.sources);
+		public SpringApplication.Running run(String... args) {
+			RunListener runListener = new RunListener();
+			SpringApplicationHook hook = (springApplication) -> {
+				springApplication.addPrimarySources(this.sources);
+				return runListener;
+			};
 			withHook(hook, () -> this.main.accept(args));
-			return hook.applicationContext;
+			return runListener;
 		}
 
-		private static class ContextLoaderHook implements SpringApplicationHook {
+		/**
+		 * {@link SpringApplicationRunListener} to capture {@link Running} application
+		 * details.
+		 */
+		private static class RunListener implements SpringApplicationRunListener, Running {
 
-			private final Set<Class<?>> sources;
+			private final List<ConfigurableApplicationContext> contexts = Collections
+				.synchronizedList(new ArrayList<>());
 
-			private ConfigurableApplicationContext applicationContext;
-
-			ContextLoaderHook(Set<Class<?>> sources) {
-				this.sources = sources;
+			@Override
+			public void contextLoaded(ConfigurableApplicationContext context) {
+				this.contexts.add(context);
 			}
 
 			@Override
-			public SpringApplicationRunListener getRunListener(SpringApplication springApplication) {
-				springApplication.addPrimarySources(this.sources);
-				return new SpringApplicationRunListener() {
-					@Override
-					public void contextPrepared(ConfigurableApplicationContext context) {
-						ContextLoaderHook.this.applicationContext = context;
-					}
-				};
+			public ConfigurableApplicationContext getApplicationContext() {
+				List<ConfigurableApplicationContext> rootContexts = this.contexts.stream()
+					.filter((context) -> context.getParent() == null)
+					.toList();
+				Assert.state(!rootContexts.isEmpty(), "No root application context located");
+				Assert.state(rootContexts.size() == 1, "No unique root application context located");
+				return rootContexts.get(0);
 			}
 
 		}
 
 	}
 
+	/**
+	 * Provides access to details of a {@link SpringApplication} run using
+	 * {@link Augmented#run(String...)}.
+	 *
+	 * @since 3.1.0
+	 */
+	public interface Running {
+
+		/**
+		 * Return the root {@link ConfigurableApplicationContext} of the running
+		 * application.
+		 * @return the root application context
+		 */
+		ConfigurableApplicationContext getApplicationContext();
+
+	}
+
 	/**
 	 * {@link BeanFactoryPostProcessor} to re-order our property sources below any
 	 * {@code @PropertySource} items added by the {@link ConfigurationClassPostProcessor}.

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/SpringApplicationTests.java

@@ -1372,7 +1372,8 @@ class SpringApplicationTests {
 	void fromReturnsApplicationContext() {
 		ConfigurableApplicationContext context = SpringApplication.from(ExampleFromMainMethod::main)
 			.with(ExampleAdditionalConfig.class)
-			.run();
+			.run()
+			.getApplicationContext();
 		assertThat(context).isNotNull();
 	}