Provide a way to create custom ApplicationContextFactory

Update `SpringBootContextLoader` so that `getApplicationContextFactory`
is now a protected that may be overridden to provide a custom
`ApplicationContextFactory` instance.

Closes gh-38205

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootContextLoader.java

@@ -196,8 +196,7 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 		else {
 			application.setWebApplicationType(WebApplicationType.NONE);
 		}
-		application.setApplicationContextFactory(
-				(webApplicationType) -> getApplicationContextFactory(mergedConfig, webApplicationType));
+		application.setApplicationContextFactory(getApplicationContextFactory(mergedConfig));
 		if (mergedConfig.getParent() != null) {
 			application.setBannerMode(Banner.Mode.OFF);
 		}
@@ -212,17 +211,26 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 		}
 	}
 
-	private ConfigurableApplicationContext getApplicationContextFactory(MergedContextConfiguration mergedConfig,
-			WebApplicationType webApplicationType) {
-		if (webApplicationType != WebApplicationType.NONE && !isEmbeddedWebEnvironment(mergedConfig)) {
-			if (webApplicationType == WebApplicationType.REACTIVE) {
-				return new GenericReactiveWebApplicationContext();
+	/**
+	 * Return the {@link ApplicationContextFactory} that should be used for the test. By
+	 * default this method will return a factory that will create an appropriate
+	 * {@link ApplicationContext} for the {@link WebApplicationType}.
+	 * @param mergedConfig the merged context configuration
+	 * @return the application context factory to use
+	 * @since 3.2.0
+	 */
+	protected ApplicationContextFactory getApplicationContextFactory(MergedContextConfiguration mergedConfig) {
+		return (webApplicationType) -> {
+			if (webApplicationType != WebApplicationType.NONE && !isEmbeddedWebEnvironment(mergedConfig)) {
+				if (webApplicationType == WebApplicationType.REACTIVE) {
+					return new GenericReactiveWebApplicationContext();
+				}
+				if (webApplicationType == WebApplicationType.SERVLET) {
+					return new GenericWebApplicationContext();
+				}
 			}
-			if (webApplicationType == WebApplicationType.SERVLET) {
-				return new GenericWebApplicationContext();
-			}
-		}
-		return ApplicationContextFactory.DEFAULT.create(webApplicationType);
+			return ApplicationContextFactory.DEFAULT.create(webApplicationType);
+		};
 	}
 
 	private void prepareEnvironment(MergedContextConfiguration mergedConfig, SpringApplication application,

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/context/SpringBootContextLoaderTests.java

@@ -26,12 +26,14 @@ import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.factory.BeanCreationException;
+import org.springframework.boot.ApplicationContextFactory;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.SpringBootConfiguration;
 import org.springframework.boot.test.context.SpringBootTest.UseMainMethod;
 import org.springframework.boot.test.util.TestPropertyValues;
 import org.springframework.boot.web.reactive.context.GenericReactiveWebApplicationContext;
 import org.springframework.context.ApplicationContext;
+import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.env.ConfigurableEnvironment;
@@ -246,6 +248,13 @@ class SpringBootContextLoaderTests {
 			.withMessage("UseMainMethod.ALWAYS cannot be used with @ContextHierarchy tests");
 	}
 
+	@Test
+	void whenSubclassProvidesCustomApplicationContextFactory() {
+		TestContext testContext = new ExposedTestContextManager(CustomApplicationContextTest.class)
+			.getExposedTestContext();
+		assertThat(testContext.getApplicationContext()).isInstanceOf(CustomAnnotationConfigApplicationContext.class);
+	}
+
 	private String[] getActiveProfiles(Class<?> testClass) {
 		TestContext testContext = new ExposedTestContextManager(testClass).getExposedTestContext();
 		ApplicationContext applicationContext = testContext.getApplicationContext();
@@ -370,6 +379,25 @@ class SpringBootContextLoaderTests {
 
 	}
 
+	@SpringBootTest
+	@ContextConfiguration(classes = Config.class, loader = CustomApplicationContextSpringBootContextLoader.class)
+	static class CustomApplicationContextTest {
+
+	}
+
+	static class CustomApplicationContextSpringBootContextLoader extends SpringBootContextLoader {
+
+		@Override
+		protected ApplicationContextFactory getApplicationContextFactory(MergedContextConfiguration mergedConfig) {
+			return (webApplicationType) -> new CustomAnnotationConfigApplicationContext();
+		}
+
+	}
+
+	static class CustomAnnotationConfigApplicationContext extends AnnotationConfigApplicationContext {
+
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class Config {