Add spring boot example for mill configuration compatible with spring initializr for a maven kotlin project (#6014)

## Motivation

Related to #6006

Getting a "learning path" on how to setup spring boot projects in
increasing complexity. The aim of the documentation is to follow the
spring boot learning path and map it to mill.

## In this PR

This is the first small step of getting a kotlin project as downloaded
from spring boot's [initializr](https://start.spring.io/) project to
work with mill . While spring boot examples exist in mill, this
differentiates by

- Project structure is the same as initializr
- Dependencies are up to date (and thus resulting in Junit5 failing
without the platform version override) - I can also do these updates in
the existing examples if you'd like
- Added an extra mill command to demonstrate how to build a jar and run
the spring boot server
- Documentation scaffolding (see below)

### Documentation Scaffolding

This PR adds a special section for spring boot in both Java and Kotlin.
The aim is to add more examples in increasing complexity as they appear
[here](https://spring.io/projects/spring-boot)

<img width="1409" height="982" alt="image"
src="https://github.com/user-attachments/assets/05ede119-a999-4e1c-8c2f-49014a1496f4"
/>

The examples an in a separate example/springboot so it will be easier to
the reader and to the tester (e.g. for directory grouping without
affecting other javalib/kotlinlib examples, port isolation)

## Manual Tests

### Running the assembly jar

<img width="1762" height="409" alt="image"
src="https://github.com/user-attachments/assets/1f896198-066b-4b9d-9af7-34686b885a41"
/>

<img width="277" height="124" alt="image"
src="https://github.com/user-attachments/assets/8a65915a-25ac-4cdd-8638-f1358694cdd7"
/>


## Follow up PRs (plan)

- More examples
- More docs

Author: vaslabs

.github/workflows/run-tests.yml

@@ -150,6 +150,9 @@ jobs:
           - java-version: 21
             millargs: "example.javalib.__.local.daemon"
 
+          - java-version: 21
+            millargs: "example.springboot.__.local.daemon"
+
           - java-version: 17
             millargs: "'example.androidlib.__.local.daemon'"
             setup-android: true

example/package.mill

@@ -46,6 +46,10 @@ object `package` extends Module {
     object script extends Cross[ExampleCrossModuleKotlin](build.listCross)
     object web extends Cross[ExampleCrossModuleKotlin](build.listCross)
   }
+  object springboot extends Module {
+    object java extends Cross[ExampleCrossModule](build.listCross)
+    object kotlin extends Cross[ExampleCrossModule](build.listCross)
+  }
   object scalalib extends Module {
     object basic extends Cross[ExampleCrossModule](build.listCross)
     object module extends Cross[ExampleCrossModule](build.listCross)
@@ -135,6 +139,7 @@ object `package` extends Module {
   trait ExampleCrossModuleAndroid extends ExampleCrossModule {
     def testExclusive = true
   }
+
   trait ExampleCrossModuleJava extends ExampleCrossModule {
 
     def upstreamCross(s: String) = s match {

example/springboot/java/1-web-initializr/build.mill

@@ -0,0 +1,49 @@
+package build
+
+import mill.*
+import mill.javalib.*
+
+object `package` extends MavenModule {
+
+  def mvnDeps = Seq(mvn"org.springframework.boot:spring-boot-starter-web:3.5.6")
+
+  object test extends MavenTests, TestModule.Junit5 {
+
+    override def junitPlatformVersion: T[String] = "6.0.0"
+
+    def mvnDeps =
+      Seq(mvn"org.springframework.boot:spring-boot-starter-test:3.5.6")
+  }
+}
+
+// This example demonstrates how to configure mill for a simple initializr project,
+// using https://start.spring.io/[https://start.spring.io/] by choosing:
+//
+//* Project: maven
+//* Language: Java
+//* Dependencies: Spring Web,
+//
+// The structure of this project follows the MavenModule structure (i.e. the same structure
+// as generated from the initialzr tool).
+//
+// For more information visit https://spring.io/quickstart[https://spring.io/quickstart]
+
+/** Usage
+
+> ./mill test
+...Test com.example.demo.DemoApplicationTests#contextLoads() finished...
+
+> ./mill runBackground
+
+> curl http://localhost:8096/hello
+...Hello, World!...
+
+> curl http://localhost:8096/hello?name=Mill
+...Hello, Mill!...
+
+> ./mill clean runBackground
+
+> ./mill show assembly
+.../out/assembly.dest/out.jar...
+
+*/

example/springboot/java/1-web-initializr/src/main/java/com/example/demo/DemoApplication.java

@@ -0,0 +1,21 @@
+package com.example.demo;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.bind.annotation.RestController;
+
+@SpringBootApplication
+@RestController
+public class DemoApplication {
+
+  public static void main(String[] args) {
+    SpringApplication.run(DemoApplication.class, args);
+  }
+
+  @GetMapping("/hello")
+  public String hello(@RequestParam(value = "name", defaultValue = "World") String name) {
+    return String.format("Hello, %s!", name);
+  }
+}

example/springboot/java/1-web-initializr/src/main/resources/application.properties

@@ -0,0 +1,2 @@
+spring.application.name=demo
+server.port=8096

example/springboot/java/1-web-initializr/src/test/java/com/example/demo/DemoApplicationTests.java

@@ -0,0 +1,11 @@
+package com.example.demo;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.boot.test.context.SpringBootTest;
+
+@SpringBootTest
+class DemoApplicationTests {
+
+  @Test
+  void contextLoads() {}
+}

example/springboot/kotlin/1-web-initializr/build.mill

@@ -0,0 +1,63 @@
+package build
+
+import mill.*
+import mill.javalib.*
+import mill.kotlinlib.KotlinMavenModule
+
+object `package` extends KotlinMavenModule {
+
+  def kotlinVersion = "1.9.25"
+
+  override def kotlincOptions: T[Seq[String]] = super.kotlincOptions() ++ Seq("-jvm-target", "17")
+
+  def mvnDeps = Seq(
+    mvn"com.fasterxml.jackson.module:jackson-module-kotlin:2.19.2",
+    mvn"org.jetbrains.kotlin:kotlin-reflect:1.9.25",
+    mvn"org.jetbrains.kotlin:kotlin-stdlib:1.9.25",
+    mvn"org.springframework.boot:spring-boot-starter-web:3.5.6"
+  )
+
+  object test extends KotlinMavenTests, TestModule.Junit5 {
+
+    override def junitPlatformVersion: T[String] = "6.0.0"
+
+    def mvnDeps = Seq(
+      mvn"org.jetbrains.kotlin:kotlin-test-junit5:1.9.25",
+      mvn"org.springframework.boot:spring-boot-starter-test:3.5.6"
+    )
+
+  }
+}
+
+// This example demonstrates how to configure mill for a simple initializr project,
+// using https://start.spring.io/[https://start.spring.io/] by choosing:
+//
+//* Project: maven
+//* Language: Kotlin
+//* Dependencies: Spring Web,
+//
+// The structure of this project follows the MavenModule structure (i.e. the same structure
+// as generated from the initialzr tool).
+//
+// For more information visit https://spring.io/quickstart[quickstart]
+// and https://spring.io/guides/tutorials/spring-boot-kotlin[spring-boot-kotlin]
+
+/** Usage
+
+> ./mill test
+...Test com.example.demokotlin.DemoKotlinApplicationTests#contextLoads() finished...
+
+> ./mill runBackground
+
+> curl http://localhost:9096/hello
+...Hello, World!...
+
+> curl http://localhost:9096/hello?name=Mill
+...Hello, Mill!...
+
+> ./mill clean runBackground
+
+> ./mill show assembly
+.../out/assembly.dest/out.jar...
+
+*/

example/springboot/kotlin/1-web-initializr/src/main/kotlin/com/example/demokotlin/DemoKotlinApplication.kt

@@ -0,0 +1,18 @@
+package com.example.demokotlin
+
+import org.springframework.boot.autoconfigure.SpringBootApplication
+import org.springframework.boot.runApplication
+import org.springframework.web.bind.annotation.GetMapping
+import org.springframework.web.bind.annotation.RequestParam
+import org.springframework.web.bind.annotation.RestController
+
+@SpringBootApplication
+@RestController
+class DemoKotlinApplication {
+    @GetMapping("/hello")
+    fun hello(@RequestParam(value = "name", defaultValue = "World") name: String?): String? = String.format("Hello, %s!", name)
+}
+
+fun main(args: Array<String>) {
+    runApplication<DemoKotlinApplication>(*args)
+}

example/springboot/kotlin/1-web-initializr/src/main/resources/application.properties

@@ -0,0 +1,2 @@
+spring.application.name=demo-kotlin
+server.port=9096

example/springboot/kotlin/1-web-initializr/src/test/kotlin/com/example/demokotlin/DemoKotlinApplicationTests.kt

@@ -0,0 +1,12 @@
+package com.example.demokotlin
+
+import org.junit.jupiter.api.Test
+import org.springframework.boot.test.context.SpringBootTest
+
+@SpringBootTest
+class DemoKotlinApplicationTests {
+
+    @Test
+    fun contextLoads() {
+    }
+}