Spring with Kotorin ―― 1. SPRING INITIALIZR

Overview / Description

Java EE graduated and officially enrolled in Spring: laughing: That's why I'm going to live a Spring life both publicly and privately: clap:

Now, back to the main subject, I think many of you are familiar with the relationship between Kotlin and Spring. Starting with Spring 5.0, Kotlin support has been taken in the core part of the framework.

Introducing Kotlin support in Spring Framework 5.0

Kotlin As a subject, I will touch on Spring in Kotlin's manners.

First, use SPRING INITIALIZR to create a template for Spring project and compare Java / Kotlin.

By the way, ** Kotlin 1.3.0 ** was just released the other day (2018/10/30): clap :: clap: There are updates such as ** Coroutines ** becoming official, but that's another story.

Assumptions / Environment

Runtime version

Kotlin : 1.3.0
SpringBoot : 2.1.0.RELEASE

By the way, the default Kotlin version was ** 1.2.70 ** (as of 2018/11/1) when generated by SPRING INITIALIZR.

Spring Dependencies

Development environment

OS : Mac
IDE : IntelliJ IDEA
Build : Gradle

Procedure / Explanation

Import the template generated by SPRING INITIALIZR into IntelliJ and use it. As mentioned earlier, the Kotlin version has changed from ** 1.2.70 ** to ** 1.3.0 **.

SPRING INITIALIZR

SPRING INITIALIZR is an online tool that literally creates a template for your Spring project and makes it easy to get started. The operation is very simple, just select the following items from the screen.

--Project type: [Maven / Gradle] --Development language: [Java / Kotlin / Groovy] --Spring Boot version: <img width = "168" alt = "Spring Initializr 2018-11-02 00-13-42.png " src = "https://qiita-image-store.s3.amazonaws.com/0" / 127983 / da4999bc-d61c-360b-6f69-898595e990d9.png "> --Group: Root package name --Artifact: Project name --Dependencies: Dependent components --This time ** Web ** is added

Spring Initializr 2018-11-02 00-03-21.png

Click ** Generate Project ** for both Java / Kotlin to generate and download the Spring project template. Import the template into the IDE and use it.

This Java project

Spring Initializr 2018-11-01 21-50-43.png

This Kotlin project

Spring Initializr 2018-11-01 21-51-57.png

Compare build.gradle

Let's compare the build.gradle for the Java project and the build.gradle for the Kotlin project described below.

Of course, you can see that the description contents and format of both are basically the same. The differences are the following two points.

--Applicable plugin --Dependent libraries

For plugins, the Kotlin project applies the Kotlin plugin instead of the Java plugin.

apply plugin: 'kotlin'
apply plugin: 'kotlin-spring'

Similarly, for dependent libraries, Kotlin libraries have been added.

implementation('com.fasterxml.jackson.module:jackson-module-kotlin')
implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
implementation("org.jetbrains.kotlin:kotlin-reflect")

The fact that the standard library I'm adding is * jdk * instead of * jre * is another story. In any case, specify ** jdk **.

Java

buildscript {
	ext {
		springBootVersion = '2.1.0.RELEASE'
	}
	repositories {
		mavenCentral()
	}
	dependencies {
		classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
	}
}

apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'org.springframework.boot'
apply plugin: 'io.spring.dependency-management'

group = 'io.pivotal.syanagihara'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = 1.8

repositories {
	mavenCentral()
}


dependencies {
	implementation('org.springframework.boot:spring-boot-starter-web')
	testImplementation('org.springframework.boot:spring-boot-starter-test')
}

Kotlin

buildscript {
	ext {
		kotlinVersion = '1.3.0'
		springBootVersion = '2.1.0.RELEASE'
	}
	repositories {
		mavenCentral()
	}
	dependencies {
		classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
		classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:${kotlinVersion}")
		classpath("org.jetbrains.kotlin:kotlin-allopen:${kotlinVersion}")
	}
}

apply plugin: 'kotlin'
apply plugin: 'kotlin-spring'
apply plugin: 'eclipse'
apply plugin: 'org.springframework.boot'
apply plugin: 'io.spring.dependency-management'

group = 'io.pivotal.syanagihara'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = 1.8
compileKotlin {
	kotlinOptions {
		freeCompilerArgs = ["-Xjsr305=strict"]
		jvmTarget = "1.8"
	}
}
compileTestKotlin {
	kotlinOptions {
		freeCompilerArgs = ["-Xjsr305=strict"]
		jvmTarget = "1.8"
	}
}

repositories {
	mavenCentral()
}


dependencies {
	implementation('org.springframework.boot:spring-boot-starter-web')
	implementation('com.fasterxml.jackson.module:jackson-module-kotlin')
	implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
	implementation("org.jetbrains.kotlin:kotlin-reflect")
	testImplementation('org.springframework.boot:spring-boot-starter-test')
}

Comparison of Main class

The comparison of Main class is exactly the same in Java / Kotlin. The only difference is whether the format of the Main method is Java style or Kotlin Kotlin style.

public static void main(String[] args) {
	SpringApplication.run(SimpleApplication.class, args);
}

fun main(args: Array<String>) {
    runApplication<SimpleApplication>(*args)
}

Array

In Kotlin, arrays in Java do not exist. Therefore, we will use an Array object to represent the array.

The difference between Java arrays and Kotlin arrays is not so much the way they are defined, but the nature.

--Java array: covariant --Does not consider inheritance relationships between type parameters --Kotlin array: immutable --Consider inheritance relationships between type parameters

For example, Kotlin cannot assign Array used in the parameter of Main method to Array . You can keep the type safe and avoid the risk of run-time errors.

Array expansion

There is a description of the following parameters that cannot be seen in Java.

*args

This is the format used when expanding and using the elements of an array.