Mockito verify fails when method takes a function as argument - scala

Question

I have a Scala test which uses Mockito to verify that certain DataFrame transformations are invoked. I broke it down to this simple problematic example
import org.apache.spark.sql.DataFrame
import org.scalatest.funsuite.AnyFunSuite
import org.apache.spark.sql.functions._
import org.mockito.{Mockito, MockitoSugar}
class SimpleTest extends AnyFunSuite{
def withGreeting(df: DataFrame):DataFrame = {
df.withColumn("greeting", lit("hello"))
}
test("sample test") {
val mockDF = MockitoSugar.mock[DataFrame]
val mockDF2 = MockitoSugar.mock[DataFrame]
MockitoSugar.doReturn(mockDF2).when(mockDF).transform(withGreeting)
mockDF.transform(withGreeting)
val orderVerifier = Mockito.inOrder(mockDF)
orderVerifier.verify(mockDF).transform(withGreeting)
}
}
I'm trying to assert that the transform was called on my mockDF, but it fails with
Argument(s) are different! Wanted:
dataset.transform(<function1>);
-> at org.apache.spark.sql.Dataset.transform(Dataset.scala:2182)
Actual invocations have different arguments:
dataset.transform(<function1>);
Why would the verify fail in this case?

Answer 1

You need to save lambda expression argument for transform as val for correct testing and pass it to all transform calls:
def withGreeting(df: DataFrame):DataFrame = {
df.withColumn("greeting", lit("hello"))
}
test("sample test") {
val mockDF = MockitoSugar.mock[DataFrame]
val mockDF2 = MockitoSugar.mock[DataFrame]
val withGreetingExpression = df => withGreeting(df)
MockitoSugar.doReturn(mockDF2).when(mockDF).transform(withGreetingExpression)
mockDF.transform(withGreetingExpression)
val orderVerifier = Mockito.inOrder(mockDF)
orderVerifier.verify(mockDF).transform(withGreetingExpression)
}
Mockito requires to provide same (or equal) arguments to the mocked functions calls. When you are passing lambda expression without saving each call transform(withGreeting) creates new object Function[DataFrame, DataFrame]
transform(withGreeting)
is the same as:
transform(new Function[DataFrame, DataFrame] {
override def apply(df: DataFrame): DataFrame = withGreeting(df)
})
And they aren't equal to each other - this is the cause of error message:
Argument(s) are different!
For example, try to execute:
println(((df: DataFrame) => withGreeting(df)) == ((df: DataFrame) => withGreeting(df))) //false
You can read more about objects equality in java (in the scala it's same):
wikibooks
javaworld.com