Groovy mocking static method. This is a fairly good example of how to correctly mock any groovy object without destroying your all regression.

Groovy mocking static method

/* 
 * Groovy Test Mocking explains.
 *
 * A modification of metaClass static method apply to the class inside the JVM.
 * You run your test with fork mode enabled, each test is isolated from another at the leve of a class or method.
 * However you can delete the metaClass behavior by 
 *  MyClass.metaClass = null
 *
 * Must know
 *  - The metaClass method signature is very important. The documentation does not higlight that the method signature must match exactely:
 * e.g.: Arrays.sort(String[] arrays) <==> Arrays.metaClass.'static'.sort = { String[] a -> // do something    }
 *  
 * Requierement :
 * - Groovy 2.1.6
 * - JUnit 4.1x 
 */

import org.junit.After
import org.junit.Before
import org.junit.Test

class ArraysTest
{
  @Before
  void setUp()
  {}

  @After
  void tearDown()
  {
    // You should reset metaClass between test
    //Arrays.metaClass = null
  }

  @Test
  void basic()
  {
    String[] things = [ 'dog', 'ant', 'bee', 'cat' ]
    // non-mocked first to prove it works normally
    Arrays.sort(things)
    assert ["ant", "bee", "cat", "dog"] == things

    // Replace sort default behaviors
    Arrays.metaClass.'static'.sort = { String[] a ->
      a[0] = 'ewe'
      a[1] = 'fox'
    }

    // Sort again
    things = [ 'dog', 'ant', 'bee', 'cat' ]
    Arrays.sort(things)

    // item 0 & 1 have changed
    assert ["ewe", "fox", "bee", "cat"] == things
  }

  @Test
  void sideEffect()
  {
    // This test is affect by the sort behavior
    // Sort again
    String[] things = [ 'dog', 'ant', 'bee', 'cat' ]
    Arrays.sort(things)

    // item 0 & 1 have changed
    assert ["ewe", "fox", "bee", "cat"] == things

    Arrays.metaClass = null
    things = [ 'dog', 'ant', 'bee', 'cat' ]
    Arrays.sort(things)

    // item 0 & 1 have changed
    assert ["ant", "bee", "cat", "dog"] == things
  }
}