How to mock internally constructed instance with mockk.io?

Question

I am writing unit test case in mockk and Junit5 for a static method defined in companion object of FileUtility class.

the method is as below,

class FileUtility {

    companion object {
        fun getFileObject(fileName: String): File {
            require(!StringUtils.isBlank(fileName)) { "Expecting a valid filePath" }
            val file = File(fileName)
            if (file.isHidden)
                throw llegalArgumentException()
            return file
        }
    }

}

the unit test case is as below,

@Test
fun `get file object test throws exception when file path is hidden`() {
    val filePath = "filepath"
    val file = mockk<File>()
    every { file.isHidden } returns true
    assertThrows(IllegalArgumentException::class.java) {
        getFileObject(filePath)
    }
    verify { file.isHidden}
}

getting the following error,

Expected java.lang.Exception to be thrown, but nothing was thrown.

Also, the verify { file.isHidden} line is not working, its giving the following error.

java.lang.AssertionError: Verification failed: call 1 of 1: File(#1).isHidden()) was not called

Answer 1

The function you are testing instantiates its own instance of a File. It is not using the mocked instance you created.

For this type of test you need to mock the constructor so that any instantiated instance of the class is mocked. You can read more here https://mockk.io/#constructor-mocks but here is your example (with a different assertion library):

@Test
fun `get file object test throws exception when file path is hidden`() {
    val filePath = "filepath"
    mockkConstructor(File::class)
    every { anyConstructed<File>().isHidden } returns true
    assertThat{
        getFileObject(filePath)
    }.isFailure().isInstanceOf(IllegalArgumentException::class)
    verify { anyConstructed<File>().isHidden}
}