Parameter specified as non null is null

Note: As of Mockito 5.0 release, the inline mock maker is the default and the limitations here don't apply.

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Mockito often returns null when calling methods like any(), eq() etcetera. Passing these instances to methods that are not properly mocked, can cause NullPointerExceptions:

open class Foo {
  fun bar(s: String) { }
}

@Test
fun myTest() {
  whenever(foo.bar(any())) // This fails, since null is passed to bar.
}

Since the bar method is not marked open, it is final and Mockito cannot mock it. As a consequence, the method is executed when calling bar(any()). Since Kotlin inserts null-checks, an NPE is thrown.

To solve this, mark bar as open or use mock-maker-inline.
(warning: inline mocking has a ~3x performance penalty)

Another way of solving this is by defining the mock with the following syntax:

open class Foo {
  fun bar(s: String) { }
  fun baz(s: String): String { return "original" }
}

@Test
fun myTest() {
  // this fails
  // whenever(foo.baz(any())).thenReturn("mocked")

  // these work
  doReturn("mocked").whenever(foo).baz(any())
  doNothing().whenever(foo).bar(any())
}