Make Fatjar by changing the main class with Gradle

Purpose

There was a library that could only be used in Java, and there was an existing application that implemented it in Kotlin. This time I wanted to call some of the features from a Python batch and use them, so Change the main class of Kotlin application to make a jar, I wanted to hit Kotlin with a java command from Python.

It seems that if you use Gradle well, you can change the main class and make Fatjar. I will challenge

environment

Kotlin JDK 8 java8 jdk14 Gradle 6.4.1

Construction

I tried to create a simple project to try

app
├── gradle
│   └── wrapper
│       └── gradle-wrapper.properties
├── gradlew
├── gradlew.bat
├── settings.gradle
├── build.gradle
└── src
    ├── main
    │   ├── kotlin
    │   │   └── automated_data_create
    │   │       ├── App.kt
    │   │       ├── App2.kt
    │   │       └── model
    │   │           └── Osushi.kt
    │   └── resources
    └── test
        ├── kotlin
        │   └── automated_data_create
        │       └── AppTest.kt
        └── resources

`app/src/main/kotlin/automated_data_create/App.kt`


/*
 * This Kotlin source file was generated by the Gradle 'init' task.
 */
package automated_data_create

import automated_data_create.model.Osushi

class App {
    val greeting: String
        get() {
            return "Hello world."
        }

}

fun main(args: Array<String>) {
    println(App().greeting)
    val osushi = Osushi("Tuna", 100)
    println("I have selected this.: " + osushi.name + " -> " + "￥" + osushi.price)
}

`app/src/main/kotlin/automated_data_create/App2.kt`


/*
 * This Kotlin source file was generated by the Gradle 'init' task.
 */
package automated_data_create

import automated_data_create.model.Osushi


class App2 {
    val greeting: String
        get() {
            return "Hello world."
        }

}

fun main(args: Array<String>) {
    println(App2().greeting)
    val osushi = Osushi("Sea urchin", 200)
    println("I have selected this.: " + osushi.name + " -> " + "￥" + osushi.price)
}

`app/build.gradle`


/*
 * This file was generated by the Gradle 'init' task.
 *
 * This generated file contains a sample Kotlin application project to get you started.
 */

plugins {
    // Apply the Kotlin JVM plugin to add support for Kotlin.
    id 'org.jetbrains.kotlin.jvm' version '1.3.61'

    // Apply the application plugin to add support for building a CLI application.
    id 'application'
    id 'com.github.johnrengelman.shadow' version '5.0.0'
    id 'java'
}

repositories {
    // Use jcenter for resolving dependencies.
    // You can declare any Maven/Ivy/file repository here.
    jcenter()
}

dependencies {
    // Align versions of all Kotlin components
    implementation platform('org.jetbrains.kotlin:kotlin-bom')

    // Use the Kotlin JDK 8 standard library.
    implementation 'org.jetbrains.kotlin:kotlin-stdlib-jdk8'

    // Use the Kotlin test library.
    testImplementation 'org.jetbrains.kotlin:kotlin-test'

    // Use the Kotlin JUnit integration.
    testImplementation 'org.jetbrains.kotlin:kotlin-test-junit'
}

application {
    // Define the main class for the application.
    mainClassName = 'automated_data_create.AppKt'
}

Create a jar file with the main class as it is

You can build the project with the following command

$ ./gradlew shadowJar
# or
$ gradle shadowJar

In this case, since the AppKt file is specified in mainClassName in the app / build.gradle file, App.kt runs as the main class.

Command to execute the built jar

$ java -jar xxx-1.0-SNAPSHOT-all.jar
Hello world.
I have selected this.:Tuna-> ￥100

Build by changing the main class to another class

Add the build task to your gradle.build file.

`app/build.gradle`


:
application {
    // Define the main class for the application.
    mainClassName = 'automated_data_create.AppKt'
}
#Add below from here ↓
task customFatJar2(type:Jar) {
    //Main class settings
    manifest {
        attributes 'Main-Class': 'automated_data_create.App2Kt'
    }
    baseName = 'second'
    //Add dependent libraries
    from { configurations.compileClasspath.collect { it.isDirectory() ? it : zipTree(it) } }


    with jar
}

Here, the main class is specified as App2.Kt. Specify any jar file name for baseName.

Note that how to add dependent libraries depends on the version of gradle.

compile: Get the compile set in dependencies as a list.

However, it is not recommended to use compile after Gradle 3.4.

compileClasspath: Get the dependencies registered in implementation etc. in a list.

From gradle 4.9 or later

Reference: How to use compile and implementation properly 　　　　https://qiita.com/opengl-8080/items/6ad642e0b016465891de It was easy to see what cofigrations.compile and configurations.compileClasspath looked like 　　　　https://qiita.com/MirrgieRiana/items/d3271f6979c1451207a6

--Execute gradle task

$ ./gradlew customFatJar2
or
$ gradle customFatJar2

Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.4.1/userguide/command_line_interface.html#sec:command_line_warnings


BUILD SUCCESSFUL in 1s

--Jar execution

$ java -jar build/libs/second.jar
Hello world.
I have selected this.:Sea urchin-> ￥200

At the end

Now that you can hit Kotlin applications with java commands, All we have to do now is run it from Python! By the way, from Python I plan to run it using the subprocess library. Please let me know if there is a better way!