Report friendly error when failing to find AOT initializer

See gh-38188

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

@@ -168,6 +168,7 @@ import org.springframework.util.function.ThrowingSupplier;
  * @author Chris Bono
  * @author Moritz Halbritter
  * @author Tadaya Tsuyukubo
+ * @author Yanming Zhou
  * @since 1.0.0
  * @see #run(Class, String[])
  * @see #run(Class[], String[])
@@ -436,6 +437,12 @@ public class SpringApplication {
 					initializers.stream().filter(AotApplicationContextInitializer.class::isInstance).toList());
 			if (aotInitializers.isEmpty()) {
 				String initializerClassName = this.mainApplicationClass.getName() + "__ApplicationContextInitializer";
+				if (!ClassUtils.isPresent(initializerClassName, getClassLoader())) {
+					throw new IllegalArgumentException(
+							"You are starting application with AOT mode but not AOT-processed,"
+									+ " please build your application with AOT first."
+									+ " Or remove system property 'spring.aot.enabled=true' to run as regular mode.");
+				}
 				aotInitializers.add(AotApplicationContextInitializer.forInitializerClasses(initializerClassName));
 			}
 			initializers.removeAll(aotInitializers);

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

@@ -163,6 +163,7 @@ import static org.mockito.Mockito.spy;
  * @author Sebastien Deleuze
  * @author Moritz Halbritter
  * @author Tadaya Tsuyukubo
+ * @author Yanming Zhou
  */
 @ExtendWith(OutputCaptureExtension.class)
 class SpringApplicationTests {
@@ -1375,6 +1376,21 @@ class SpringApplicationTests {
 		}
 	}
 
+	@Test
+	void shouldReportFriendlyErrorIfAotInitializerNotFound() {
+		SpringApplication application = new SpringApplication(TestSpringApplication.class);
+		application.setWebApplicationType(WebApplicationType.NONE);
+		application.setMainApplicationClass(TestSpringApplication.class);
+		System.setProperty(AotDetector.AOT_ENABLED, "true");
+		try {
+			assertThatIllegalArgumentException().isThrownBy(application::run)
+				.withMessageContaining(AotDetector.AOT_ENABLED);
+		}
+		finally {
+			System.clearProperty(AotDetector.AOT_ENABLED);
+		}
+	}
+
 	@Test
 	void fromRunsWithAdditionalSources() {
 		assertThat(ExampleAdditionalConfig.local.get()).isNull();