Register runtime hints for ApplicationContextInitializers in the TCF

This commit introduces automatic registration of runtime hints for
ApplicationContextInitializers configured via the `initializers`
attribute in @ContextConfiguration.

See gh-29021

spring-test/src/main/java/org/springframework/test/context/aot/TestContextAotGenerator.java

@@ -112,6 +112,7 @@ public class TestContextAotGenerator {
 		mergedConfigMappings.forEach((mergedConfig, testClasses) -> {
 			logger.debug(LogMessage.format("Generating AOT artifacts for test classes %s",
 					testClasses.stream().map(Class::getName).toList()));
+			registerHintsForMergedConfig(mergedConfig);
 			try {
 				// Use first test class discovered for a given unique MergedContextConfiguration.
 				Class<?> testClass = testClasses.get(0);
@@ -174,8 +175,6 @@ public class TestContextAotGenerator {
 				Consider annotating test class [%s] with @ContextConfiguration or \
 				@ContextHierarchy.""".formatted(testClass.getName()));
 
-		registerDeclaredConstructors(contextLoader.getClass());
-
 		if (contextLoader instanceof AotContextLoader aotContextLoader) {
 			try {
 				ApplicationContext context = aotContextLoader.loadContextForAotProcessing(mergedConfig);
@@ -227,6 +226,14 @@ public class TestContextAotGenerator {
 				.registerType(TypeReference.of(className), MemberCategory.INVOKE_PUBLIC_METHODS);
 	}
 
+	private void registerHintsForMergedConfig(MergedContextConfiguration mergedConfig) {
+		ContextLoader contextLoader = mergedConfig.getContextLoader();
+		if (contextLoader != null) {
+			registerDeclaredConstructors(contextLoader.getClass());
+		}
+		mergedConfig.getContextInitializerClasses().forEach(this::registerDeclaredConstructors);
+	}
+
 	private void registerDeclaredConstructors(Class<?> type) {
 		ReflectionHints reflectionHints = this.runtimeHints.reflection();
 		reflectionHints.registerType(type, MemberCategory.INVOKE_DECLARED_CONSTRUCTORS);

spring-test/src/test/java/org/springframework/test/context/aot/TestContextAotGeneratorTests.java

@@ -123,6 +123,7 @@ class TestContextAotGeneratorTests extends AbstractAotTests {
 		).forEach(type -> assertReflectionRegistered(runtimeHints, type, INVOKE_PUBLIC_CONSTRUCTORS));
 
 		Set.of(
+			org.springframework.test.context.aot.samples.basic.BasicSpringTestNGTests.CustomInitializer.class,
 			org.springframework.test.context.support.AnnotationConfigContextLoader.class,
 			org.springframework.test.context.support.DefaultTestContextBootstrapper.class,
 			org.springframework.test.context.support.DelegatingSmartContextLoader.class,

spring-test/src/test/java/org/springframework/test/context/aot/samples/basic/BasicSpringTestNGTests.java

@@ -19,8 +19,11 @@ package org.springframework.test.context.aot.samples.basic;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.context.ApplicationContext;
+import org.springframework.context.ApplicationContextInitializer;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.mock.env.MockPropertySource;
 import org.springframework.test.context.ContextConfiguration;
-import org.springframework.test.context.TestPropertySource;
+import org.springframework.test.context.aot.samples.basic.BasicSpringTestNGTests.CustomInitializer;
 import org.springframework.test.context.aot.samples.common.MessageService;
 import org.springframework.test.context.testng.AbstractTestNGSpringContextTests;
 
@@ -30,8 +33,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * @author Sam Brannen
  * @since 6.0
  */
-@ContextConfiguration(classes = BasicTestConfiguration.class)
-@TestPropertySource(properties = "test.engine = testng")
+@ContextConfiguration(classes = BasicTestConfiguration.class, initializers = CustomInitializer.class)
 public class BasicSpringTestNGTests extends AbstractTestNGSpringContextTests {
 
 	@Autowired
@@ -51,4 +53,14 @@ public class BasicSpringTestNGTests extends AbstractTestNGSpringContextTests {
 			.as("@TestPropertySource").isEqualTo("testng");
 	}
 
+	public static class CustomInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext> {
+
+		@Override
+		public void initialize(ConfigurableApplicationContext applicationContext) {
+			applicationContext.getEnvironment().getPropertySources()
+				.addFirst(new MockPropertySource().withProperty("test.engine", "testng"));
+		}
+
+	}
+
 }