Okay, So if you have simple java project which has dir/file structure as below

Step-1: create file name build.gradle in project root directory as below

Step-2: add following gradle script in build.gradle
apply plugin: 'java'
apply plugin: 'eclipse'
archivesBaseName="someJar"
version = '1.0-SNAPSHOT'
repositories {
mavenCentral()
}
jar {
manifest {
attributes 'Main-Class': 'com.test.Run'
}
}
dependencies {
compile 'log4j:log4j:1.2.16'
}
Step-3: close or delete from eclipse this project and import it again while selecting Gradle project as below
Now your project hierarchy will look like this. (Also note its added Gradle Dependencies while importing project)

Step-4: Create source foler src/main/java in project and move your all packages in that source folder.

Step-5 last but not the least: Cheers 🙂
So now your Simple Java project has converted into Gradle project!

