Add 'fromApplication' and 'with' Kotlin extension functions

Update `SpringApplicationExtensions.kt` with `fromApplication` and
`with` functions that make `SpringApplication.from(...)` easier to use
with Kotlin.

Fixes gh-35756

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/features/testing/testcontainers/atdevelopmenttime/launch/TestMyApplication.kt

@@ -16,12 +16,8 @@
 
 package org.springframework.boot.docs.features.testing.testcontainers.atdevelopmenttime.launch
 
-import org.springframework.boot.SpringApplication
-import org.springframework.boot.docs.features.testing.testcontainers.atdevelopmenttime.launch.main as myApplicationMain
+import org.springframework.boot.fromApplication
 
-object Main {
-	@JvmStatic
-	fun main(args: Array<String>) {
-		SpringApplication.from(::myApplicationMain).run(*args)
-	}
+fun main(args: Array<String>) {
+	fromApplication<MyApplication>().run(*args)
 }

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/features/testing/testcontainers/atdevelopmenttime/test/TestMyApplication.kt

@@ -16,12 +16,9 @@
 
 package org.springframework.boot.docs.features.testing.testcontainers.atdevelopmenttime.test
 
-import org.springframework.boot.SpringApplication
-import org.springframework.boot.docs.features.testing.testcontainers.atdevelopmenttime.test.main as myApplicationMain
+import org.springframework.boot.fromApplication
+import org.springframework.boot.with
 
-object Main {
-	@JvmStatic
-	fun main(args: Array<String>) {
-		SpringApplication.from(::myApplicationMain).with(MyContainersConfiguration::class.java).run(*args)
-	}
-}
+fun main(args: Array<String>) {
+	fromApplication<MyApplication>().with(MyContainersConfiguration::class).run(*args)
+}

spring-boot-project/spring-boot/src/main/kotlin/org/springframework/boot/SpringApplicationExtensions.kt

@@ -17,6 +17,11 @@
 package org.springframework.boot
 
 import org.springframework.context.ConfigurableApplicationContext
+import org.springframework.util.Assert
+import org.springframework.util.ClassUtils
+import org.springframework.util.ReflectionUtils
+import kotlin.reflect.KClass
+import kotlin.reflect.KType
 
 /**
  * Top-level function acting as a Kotlin shortcut allowing to write
@@ -40,3 +45,34 @@ inline fun <reified T : Any> runApplication(vararg args: String): ConfigurableAp
  */
 inline fun <reified T : Any> runApplication(vararg args: String, init: SpringApplication.() -> Unit): ConfigurableApplicationContext =
 		SpringApplication(T::class.java).apply(init).run(*args)
+
+/**
+ * Top-level function acting as a Kotlin shortcut allowing to write
+ * `fromApplication<MyApplication>().with(...)`. This method assumes that
+ * the `main` function is declared in the same file as `T`.
+ *
+ * @author Phillip Webb
+ * @since 3.1.1
+ */
+inline fun <reified T : Any> fromApplication(): SpringApplication.Augmented {
+	val type = T::class
+	val ktClassName = type.qualifiedName + "Kt"
+	try {
+		val ktClass = ClassUtils.resolveClassName(ktClassName, type.java.classLoader)
+		val mainMethod = ReflectionUtils.findMethod(ktClass, "main", Array<String>::class.java)
+		Assert.notNull(mainMethod, "Unable to find main method")
+		return SpringApplication.from { ReflectionUtils.invokeMethod(mainMethod!!, null, it) }
+	} catch (ex: Exception) {
+		throw IllegalStateException("Unable to use 'fromApplication' with " + type.qualifiedName)
+	}
+}
+
+/**
+ * Extension function that allows `SpringApplication.Augmented.with` to work with Kotlin classes.
+ *
+ * @author Phillip Webb
+ * @since 3.1.1
+ */
+fun SpringApplication.Augmented.with(type: KClass<*>): SpringApplication.Augmented {
+	return this.with(type.java)!!
+}

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

@@ -16,9 +16,11 @@
 package org.springframework.boot
 
 import org.assertj.core.api.Assertions.assertThat
+import org.assertj.core.api.Assertions.assertThatIllegalStateException
 import org.junit.jupiter.api.Test
 
 import org.springframework.beans.factory.getBean
+import org.springframework.boot.kotlinsample.TestKotlinApplication
 import org.springframework.boot.web.servlet.server.MockServletWebServerFactory
 import org.springframework.context.annotation.Bean
 import org.springframework.context.annotation.Configuration
@@ -67,6 +69,18 @@ class SpringApplicationExtensionsTests {
 		assertThat(environment).isEqualTo(context.environment)
 	}
 
+	@Test
+	fun `Kotlin fromApplication() top level function`() {
+		val context = fromApplication<TestKotlinApplication>().with(ExampleWebConfig::class).run().applicationContext
+		assertThat(context.getBean<MockServletWebServerFactory>()).isNotNull
+	}
+
+	@Test
+	fun `Kotlin fromApplication() top level function when no main`() {
+		assertThatIllegalStateException().isThrownBy { fromApplication<ExampleWebConfig>().run() }
+			.withMessage("Unable to use 'fromApplication' with org.springframework.boot.SpringApplicationExtensionsTests.ExampleWebConfig")
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	internal open class ExampleWebConfig {
 

spring-boot-project/spring-boot/src/test/kotlin/org/springframework/boot/kotlinsample/TestKotlinApplication.kt

@@ -0,0 +1,11 @@
+package org.springframework.boot.kotlinsample
+
+import org.springframework.boot.runApplication
+import org.springframework.context.annotation.Configuration
+
+@Configuration(proxyBeanMethods = false)
+open class TestKotlinApplication
+
+fun main(args: Array<String>) {
+	runApplication<TestKotlinApplication>(*args)
+}