Gradle to execute Java class (without modifying build.gradle)

There is no direct equivalent to mvn exec:java in gradle, you need to either apply the application plugin or have a JavaExec task.

application plugin

Activate the plugin:

plugins {
    id 'application'
    ...
}

Configure it as follows:

application {
    mainClassName = project.hasProperty("mainClass") ? project.getProperty("mainClass") : "NULL"
}

On the command line, write

$ gradle -PmainClass=Boo run

JavaExec task

Define a task, let's say execute:

task execute(type:JavaExec) {
   main = project.hasProperty("mainClass") ? getProperty("mainClass") : "NULL"
   classpath = sourceSets.main.runtimeClasspath
}

To run, write gradle -PmainClass=Boo execute. You get

$ gradle -PmainClass=Boo execute
:compileJava
:compileGroovy UP-TO-DATE
:processResources UP-TO-DATE
:classes
:execute
I am BOO!

mainClass is a property passed in dynamically at command line. classpath is set to pickup the latest classes.

If you do not pass in the mainClass property, both of the approaches fail as expected.

$ gradle execute

FAILURE: Build failed with an exception.

* Where:
Build file 'xxxx/build.gradle' line: 4

* What went wrong:
A problem occurred evaluating root project 'Foo'.
> Could not find property 'mainClass' on task ':execute'.

You just need to use the Gradle Application plugin:

apply plugin:'application'
mainClassName = "org.gradle.sample.Main"

And then simply gradle run.

As Teresa points out, you can also configure mainClassName as a system property and run with a command line argument.

Expanding on First Zero's answer, I'm guess you want something where you can also run gradle build without errors.

Both gradle build and gradle -PmainClass=foo runApp work with this:

task runApp(type:JavaExec) {
    classpath = sourceSets.main.runtimeClasspath

    main = project.hasProperty("mainClass") ? project.getProperty("mainClass") : "package.MyDefaultMain"
}

where you set your default main class.

You can parameterise it and pass gradle clean build -Pprokey=goodbye

task choiceMyMainClass(type: JavaExec) {
     group = "Execution"
    description = "Run Option main class with JavaExecTask"
    classpath = sourceSets.main.runtimeClasspath

    if (project.hasProperty('prokey')){
        if (prokey == 'hello'){
            main = 'com.sam.home.HelloWorld'
        } 
        else if (prokey == 'goodbye'){
            main = 'com.sam.home.GoodBye'
        }
    } else {
            println 'Invalid value is enterrd';

       // println 'Invalid value is enterrd'+ project.prokey;
    }