Add another java source directory to gradle script

I hava an example java project package

package com.example.testing;

with such file tree

app
|
  src->com->example->testing->Main.java

and a gradle script:

apply plugin: 'java'
apply plugin: 'application'

sourceSets {
    main {
        java {
            srcDirs 'src'
        }
    }


}

sourceSets.main.output.classesDir = file("classes")
mainClassName = 'com.example.testing.Main'

defaultTasks 'compileJava', 'run'

Now I want to add some module to this project and my folders will be something like this

app
|
  src1->com->example->testing->Main.java

  src2->com->another_example->another_testing->Library.java

How do I add new source code to gradle script?

I agree with @JB Nizet about respecting standard conventions. If you still insist on being an Anarchist though:

You already have src declared in your sourceset, why not add src1 and src2 as well? You can add them to the same sourceset, or define a sourceset per module if you want.

sourceSets {
    main {
        java {
            srcDirs 'src'
            srcDirs 'src1'
            srcDirs 'src2'
        }
    }
 }

To reference files outside the project, see this answer.