Honor final web application type when creating context in tests

Previously, SpringBootContextLoader would configure its
SpringApplication with an ApplicationContextFactory that ignores
the WebApplicationType with which its called and instead returns a
hard-coded type of context based on the type of the
MergedContextConfiguration. This hard-coding would result in the
wrong type of context being used if a subsequent change was made to
the application's WebApplicationType, for example due to binding of
the spring.main.web-application-type configuration property.

This commit updates SpringBootContextLoader to configure
SpringApplication with an ApplicationContextFactory that takes the
WebApplicationType with which it is called into consideration.

Fixes gh-29170

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

@@ -62,6 +62,7 @@ import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StringUtils;
+import org.springframework.web.context.ConfigurableWebApplicationContext;
 import org.springframework.web.context.support.GenericWebApplicationContext;
 
 /**
@@ -113,14 +114,21 @@ public class SpringBootContextLoader extends AbstractContextLoader {
 		}
 		else if (config instanceof ReactiveWebMergedContextConfiguration) {
 			application.setWebApplicationType(WebApplicationType.REACTIVE);
-			if (!isEmbeddedWebEnvironment(config)) {
-				application.setApplicationContextFactory(
-						ApplicationContextFactory.of(GenericReactiveWebApplicationContext::new));
-			}
 		}
 		else {
 			application.setWebApplicationType(WebApplicationType.NONE);
 		}
+		application.setApplicationContextFactory((type) -> {
+			if (type != WebApplicationType.NONE && !isEmbeddedWebEnvironment(config)) {
+				if (type == WebApplicationType.REACTIVE) {
+					return new GenericReactiveWebApplicationContext();
+				}
+				else if (type == WebApplicationType.SERVLET) {
+					return new GenericWebApplicationContext();
+				}
+			}
+			return ApplicationContextFactory.DEFAULT.create(type);
+		});
 		application.setInitializers(initializers);
 		boolean customEnvironent = ReflectionUtils.findMethod(getClass(), "getEnvironment")
 				.getDeclaringClass() != SpringBootContextLoader.class;
@@ -285,14 +293,38 @@ public class SpringBootContextLoader extends AbstractContextLoader {
 				List<ApplicationContextInitializer<?>> initializers) {
 			WebMergedContextConfiguration webConfiguration = (WebMergedContextConfiguration) configuration;
 			addMockServletContext(initializers, webConfiguration);
-			application.setApplicationContextFactory((webApplicationType) -> new GenericWebApplicationContext());
 		}
 
 		private void addMockServletContext(List<ApplicationContextInitializer<?>> initializers,
 				WebMergedContextConfiguration webConfiguration) {
 			SpringBootMockServletContext servletContext = new SpringBootMockServletContext(
 					webConfiguration.getResourceBasePath());
-			initializers.add(0, new ServletContextApplicationContextInitializer(servletContext, true));
+			initializers.add(0, new DefensiveWebApplicationContextInitializer(
+					new ServletContextApplicationContextInitializer(servletContext, true)));
+		}
+
+		/**
+		 * Decorator for {@link ServletContextApplicationContextInitializer} that prevents
+		 * a failure when the context type is not as was predicted when the initializer
+		 * was registered. This can occur when spring.main.web-application-type is set to
+		 * something other than servlet.
+		 */
+		private static final class DefensiveWebApplicationContextInitializer
+				implements ApplicationContextInitializer<ConfigurableApplicationContext> {
+
+			private final ServletContextApplicationContextInitializer delegate;
+
+			private DefensiveWebApplicationContextInitializer(ServletContextApplicationContextInitializer delegate) {
+				this.delegate = delegate;
+			}
+
+			@Override
+			public void initialize(ConfigurableApplicationContext applicationContext) {
+				if (applicationContext instanceof ConfigurableWebApplicationContext) {
+					this.delegate.initialize((ConfigurableWebApplicationContext) applicationContext);
+				}
+			}
+
 		}
 
 	}

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

@@ -24,6 +24,7 @@ import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
 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.Configuration;
 import org.springframework.core.env.ConfigurableEnvironment;
@@ -36,6 +37,7 @@ import org.springframework.test.context.TestContextManager;
 import org.springframework.test.context.TestPropertySource;
 import org.springframework.test.context.support.TestPropertySourceUtils;
 import org.springframework.test.util.ReflectionTestUtils;
+import org.springframework.web.context.WebApplicationContext;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -146,6 +148,20 @@ class SpringBootContextLoaderTests {
 		assertThat(last).startsWith("Config resource");
 	}
 
+	@Test
+	void whenEnvironmentChangesWebApplicationTypeToNoneThenContextTypeChangesAccordingly() {
+		TestContext context = new ExposedTestContextManager(ChangingWebApplicationTypeToNone.class)
+				.getExposedTestContext();
+		assertThat(context.getApplicationContext()).isNotInstanceOf(WebApplicationContext.class);
+	}
+
+	@Test
+	void whenEnvironmentChangesWebApplicationTypeToReactiveThenContextTypeChangesAccordingly() {
+		TestContext context = new ExposedTestContextManager(ChangingWebApplicationTypeToReactive.class)
+				.getExposedTestContext();
+		assertThat(context.getApplicationContext()).isInstanceOf(GenericReactiveWebApplicationContext.class);
+	}
+
 	private String[] getActiveProfiles(Class<?> testClass) {
 		TestContext testContext = new ExposedTestContextManager(testClass).getExposedTestContext();
 		ApplicationContext applicationContext = testContext.getApplicationContext();
@@ -228,6 +244,16 @@ class SpringBootContextLoaderTests {
 
 	}
 
+	@SpringBootTest(classes = Config.class, args = "--spring.main.web-application-type=none")
+	static class ChangingWebApplicationTypeToNone {
+
+	}
+
+	@SpringBootTest(classes = Config.class, args = "--spring.main.web-application-type=reactive")
+	static class ChangingWebApplicationTypeToReactive {
+
+	}
+
 	/**
 	 * {@link TestContextManager} which exposes the {@link TestContext}.
 	 */