Gradle 可以在没有 main() 的情况下执行 TornadoFX 应用程序，而 IntelliJ 不能……为什么？

Question

I've just set up a simple Hello World project with the following code in Kotlin + TornadoFX with IntelliJ IDEA Community. For some reason, if I type gradlew run at the command line, it manages to run the app by loading the MainApp class directly, but IntelliJ insists "class MainApp has no main() method", refusing to launch the app unless I explicitly write a main() function. Why is this? How can I (or can I at all) get IntelliJ to run the app the same way gradle is managing?

Here's the Kotlin source file

import tornadofx.*

// Define application with main View `Main`
class MainApp : App(Main::class)

// Define the view to display
class Main : View() {

    // override the root view with our container with the label within
    override val root = hbox {
        // use the tornado kotlin dsl to add a label and set the text
        label {
            text = "Hello World"
        }
    }
}

And here's build.gradle.kts

plugins {
    kotlin("jvm") version "1.3.72"
    application
    id("org.openjfx.javafxplugin") version "0.0.8"
}

javafx {
    // Declare the javafx modules we need to use
    modules("javafx.controls")
}

group = "org.example"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
}

dependencies {
    implementation(kotlin("stdlib-jdk8"))
    implementation("no.tornado:tornadofx:1.7.20")
}

application {
    mainClassName = "MainApp"
}

tasks {
    compileKotlin {
        kotlinOptions.jvmTarget = "1.8"
    }
    compileTestKotlin {
        kotlinOptions.jvmTarget = "1.8"
    }
}

Answer 1

After downloading your test project there are a couple of things I see:

1. Your run configuration for "MainApp" is not using gradle but is using an Intellij managed project configured to build and run the application.
2. MainApp.kt does not have a "main" function for Intellij to run

I'm not an expert on Intellij or gradle so I can't explain why you can launch with gradle without main function but can't do so from Intellij. I expect that gradle can somehow inject a main function that can execute the JavaFX Application class defined by your MainApp (TornadoFX App class extends JavaFX Application class).

To get this working, you can either setup a run configuration to use gradle or add the main function to MyApp.kt and update the MainApp run configuration.

Use Gradle to Run from Intellij

1. Click the gradle tab on the right hand side of Intellij
2. Expand the kotlin-gradle configuration for Tasks -> application
   Note: You may need to click the "refresh" button on the top left of the gradle panel to refresh the list of available tasks.
3. Double click "run" and the gradle run task will be executed
4. When you do this, Intellij will automatically add a new configuration "test-project [run]"

Screenshot of Gradle Panel

Note: It is still a good idea to add the main function as described below and update your build.gradle.kts to use "MainAppKt". This will give you some flexibility in launching your application with Java 9+ without requiring module definitions (ie non-modular appplication)

Use Intellij to Run

1. Add the following code to your MainApp.kt

fun main(args: Array<String>) {
    launch<MainApp>(args)
}

2. Edit your existing "MainApp" run configuration and change the Main class to "MainAppKt"
3. Click Ok to save and you can now run the "MainApp" configuration

Last note : If you are using Java 9+, you need to use a snapshot build of TornadoFX otherwise, you will run into problems.

Add the snapshot repository to your respoitories section with

maven("https://oss.sonatype.org/content/repositories/snapshots")

And update the TornadoFX version to "2.0.0-SNAPSHOT"

implementation("no.tornado:tornadofx:2.0.0-SNAPSHOT")

Old Answer

You are missing the mainClassName setting in your build.gradle.kts:

application {
    mainClassName = "com.example.demo.app.MyApp"
}

Some notes:

• If you have a main function that launches your app such as below, then you need to append "Kt" to the main class name where the function is located since a new class is created by kotlin to hold the static main function for the JVM. For example, if in the MyApp.kt file I had the main function below, I would need to use "com.example.demo.app.MyAppKt" in order to call the main function to run the application.

fun main(args: Array<String>) {
    launch<MyApp>(args)
}

• The syntax for the mainClassName is different depending on the gradle version and the language. I normally use groovy so I had to look it up for kts. The current gradle version 6.4 uses a different syntax than 6.3. The syntax I provided is for 6.3. I'm not sure when the change was made so you may need to verify the correct syntax for your gradle version.