build.gradle

apply plugin: 'java'

repositories {
    mavenCentral()
}

dependencies {
    testCompile 'junit:junit:4.+'
}

sourceSets {
    main {
        java {
            srcDir '.'
            exclude 'Test*.java'
        }
    }
    test {
        java {
            srcDir '.'
            include 'Test*.java'
        }
    }
}

test {
    testLogging {
        events 'passed', 'skipped', 'failed', 'standardOut', 'standardError'
    }
}

task run(type:JavaExec) {
    main = project.hasProperty('main') ? project.getProperty('main') : 'TryOneJar'
    classpath = sourceSets.main.runtimeClasspath
}

TryOneJar.java

public class TryOneJar {
    public static void main(String[] args) {
        System.out.println("One-JAR");
    }
}