Fix Kotlin main method detection for UseMainMethod

Update `SpringBootContextLoader` to detect main methods on `*Kt`
classes.

Fixes gh-33114

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

@@ -44,6 +44,7 @@ import org.springframework.context.ApplicationContextInitializer;
 import org.springframework.context.ApplicationListener;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.aot.AotApplicationContextInitializer;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.Ordered;
 import org.springframework.core.PriorityOrdered;
 import org.springframework.core.SpringVersion;
@@ -65,6 +66,7 @@ import org.springframework.test.context.support.AnnotationConfigContextLoaderUti
 import org.springframework.test.context.support.TestPropertySourceUtils;
 import org.springframework.test.context.web.WebMergedContextConfiguration;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StringUtils;
@@ -153,6 +155,15 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 				"Cannot use main method as no @SpringBootConfiguration-annotated class is available");
 		Method mainMethod = (springBootConfiguration != null)
 				? ReflectionUtils.findMethod(springBootConfiguration, "main", String[].class) : null;
+		if (mainMethod == null && KotlinDetector.isKotlinPresent()) {
+			try {
+				Class<?> kotlinClass = ClassUtils.forName(springBootConfiguration.getName() + "Kt",
+						springBootConfiguration.getClassLoader());
+				mainMethod = ReflectionUtils.findMethod(kotlinClass, "main", String[].class);
+			}
+			catch (ClassNotFoundException ex) {
+			}
+		}
 		Assert.state(mainMethod != null || useMainMethod == UseMainMethod.WHEN_AVAILABLE,
 				() -> "Main method not found on '%s'".formatted(springBootConfiguration.getName()));
 		return mainMethod;

spring-boot-project/spring-boot-test/src/test/kotlin/org/springframework/boot/test/context/KotlinApplicationWithMainThrowingException.kt

@@ -0,0 +1,15 @@
+package org.springframework.boot.test.context
+
+import org.springframework.boot.SpringBootConfiguration
+import org.springframework.boot.runApplication
+import org.springframework.context.annotation.Configuration
+
+@Configuration(proxyBeanMethods = false)
+@SpringBootConfiguration
+open class KotlinApplicationWithMainThrowingException {
+}
+
+fun main(args: Array<String>) {
+	runApplication<KotlinApplicationWithMainThrowingException>(*args)
+	throw IllegalStateException("ThrownFromMain")
+}

spring-boot-project/spring-boot-test/src/test/kotlin/org/springframework/boot/test/context/SpringBootContextLoaderKotlinTests.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.test.context
+
+import org.assertj.core.api.Assertions.assertThatIllegalStateException
+import org.junit.jupiter.api.Test
+import org.springframework.boot.SpringBootConfiguration
+import org.springframework.boot.test.context.SpringBootTest.UseMainMethod
+import org.springframework.context.annotation.Configuration
+import org.springframework.test.context.TestContext
+import org.springframework.test.context.TestContextManager
+
+/**
+ * Kotlin tests for [SpringBootContextLoader].
+ */
+class SpringBootContextLoaderKotlinTests {
+
+	@Test
+	fun `when UseMainMethod ALWAYS and main method throws exception`() {
+		val testContext = ExposedTestContextManager(
+			UseMainMethodAlwaysAndKotlinMainMethodThrowsException::class.java
+		).exposedTestContext
+		assertThatIllegalStateException().isThrownBy { testContext.applicationContext }
+			.havingCause()
+			.withMessageContaining("ThrownFromMain")
+	}
+
+	/**
+	 * [TestContextManager] which exposes the [TestContext].
+	 */
+	internal class ExposedTestContextManager(testClass: Class<*>) : TestContextManager(testClass) {
+		val exposedTestContext: TestContext
+			get() = super.getTestContext()
+	}
+
+	@SpringBootTest(classes = [KotlinApplicationWithMainThrowingException::class], useMainMethod = UseMainMethod.ALWAYS)
+	internal class UseMainMethodAlwaysAndKotlinMainMethodThrowsException
+
+}