How do I add a new sourceset to Gradle?

If you're using

• Gradle 5.x, have a look at Documentation Section "Testing Java > Configuring integration tests Example 14 and 15 for details (both for Groovy and Kotlin DSL, either which one you prefer)
  
  • alt: "current" Gradle doc link at 2, but might defer in future, you should have a look at if examples changes)
• for Gradle 4 have a look at ancient version 3 which is close near to what @Spina posted in 2012

To get IntelliJ to recognize custom sourceset as test sources root:

plugin {
    idea
}

idea {
    module {
        testSourceDirs = testSourceDirs + sourceSets["intTest"].allJava.srcDirs
        testResourceDirs = testResourceDirs + sourceSets["intTest"].resources.srcDirs
    }
}

I gather the documentation wasn't great back in 2012 when this question was asked, but for anyone reading this in 2020+: There's now a whole section in the docs about how to add a source set for integration tests. You really should read it instead of copy/pasting code snippets here and banging your head against the wall trying to figure out why an answer from 2012-2016 doesn't quite work.

The answer is most likely simple but more nuanced than you may think, and the exact code you'll need is likely to be different from the code I'll need. For example, do you want your integration tests to use the same dependencies as your unit tests?

I'm new to Gradle, using Gradle 6.0.1 JUnit 4.12. Here's what I came up with to solve this problem.

apply plugin: 'java'
repositories { jcenter() }

dependencies {
    testImplementation 'junit:junit:4.12'
}

sourceSets {
  main {
    java {
       srcDirs = ['src']
    }
  }
  test {
    java {
      srcDirs = ['tests']
    }
  }
}

Notice that the main source and test source is referenced separately, one under main and one under test.

The testImplementation item under dependencies is only used for compiling the source in test. If your main code actually had a dependency on JUnit, then you would also specify implementation under dependencies.

I had to specify the repositories section to get this to work, I doubt that is the best/only way.