Adding a sourceSet-only dependency?

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1

I have a simple project with a custom sourceSet.

src
├── main
│   ├── java
│   │   
│   └── resources
├── example
│   ├── java
│   │   
│   └── resources
│       
└── test
    ├── java
    └── resources

I want to add dependencies only to the example sourceSet. Is this not possible? I have the below build file:

sourceSets {
    example
}

dependencies {
    exampleCompile 'org.apache.commons:commons-lang3:3.7'
    // compile 'org.apache.commons:commons-lang3:3.7'
}

As it turns out, if I uncomment the compile 'org.apache.commons:commons-lang3:3.7' line, the commons lang dependency becomes available in the example sourceSet. I don’t seem to be able set this dependency to be only available on the example sourceSet without having to define the compile dependency as well. In fact, once I define a compile dependency, specifying it at exampleCompile level becomes unnecessary.

I think I am missing a big piece of knowledge here. Appreciate any help.