Document application plugin's need for its mainClassName property to be set

[rubenaranamorera]

The documentation states that:

By default, the executable archive’s main class will be configured automatically by looking for a class with a public static void main(String[]) method in directories on the task’s classpath.

But trying a simple example project we are unable to get this autodetected, forcing us to define mainClassName. Is there something I'm missing?

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:2.0.4.RELEASE")
    }
}

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

group = 'com.example'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = 1.8

repositories {
    mavenCentral()
}


dependencies {
    compile('org.springframework.boot:spring-boot-starter')
    testCompile('org.springframework.boot:spring-boot-starter-test')
}

$ ./gradlew build
> Task :startScripts FAILED

FAILURE: Build failed with an exception.

* What went wrong:
A problem was found with the configuration of task ':startScripts'.
> No value has been specified for property 'mainClassName'.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 2s
7 actionable tasks: 3 executed, 4 up-to-date

[snicoll]

The important bits that's not described here is that the application plugin was added.

[philwebb]

It would be nice if we could add a convention so that the same finder logic is used with the application plugin. If that's not possible we'll need to at least document the restriction.

[wilkinsona]

I'm not sure that this is a bug.

The application plugin's own documentation notes that "the only mandatory configuration for the plugin is the specification of the main class (i.e. entry point) of the application" and it's a task from the application plugin that's failing. The documentation quoted above is specifically referring to the bootJar and bootWar tasks. We also already document that, when the application plugin is applied, various other tasks (bootJar, bootWar, and bootRun) will then use its mainClassName property as a convention.

If we start using the found main class as a convention for the mainClassName property we'll either have a cycle between this plugin and the application plugin, or we'll have to change the current behaviour and stop using the mainClassName property from the application plugin as a convention for bootJar, bootWar, and bootRun.

In short, I think we should add a note to the documentation here to draw the user's attention to the fact that the mainClassName property is mandatory when using the application plugin.