Introduce `@FieldSource` for parameterized tests

[ktkopone]

Overview

Introduce @FieldSource argument source for parameterized tests.

The feature is analogous to the existing @MethodSource support, except that the value of a named field is used as the source of arguments instead of the value returned by a factory method.

See #2014 (comment) and #2014 (comment) for examples.

Deliverables

• Introduce @FieldSource and FieldArgumentsProvider.
  • Reject fields with Stream values.
  • Reject fields with Iterator values.
  • Support Supplier<T> fields where T is a type supported for @MethodSource.
  • Write unit and integration tests for preconditions, edge cases, etc.
  • Provide examples, etc. in class-level Javadoc for @FieldSource.
• Document in the User Guide.
  • Provide examples for the User Guide.
• Document in the Release Notes.

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Original Issue Description

I understand that for memory/performance you probably want to recommend everyone use a Stream-returning method and @MethodSource, but given that an @ArraySource would be easier to use without having to convert all test data into methods, this would be a nice-to-have.

[marcphilipp]

Have you tried @ValueSource?

[sbrannen]

A @MethodSource factory method doesn't have to return a Stream. It can also return an array. So why don't you just do that?

Also, how would an @ArraySource annotation be used?

Would it reference the name of a field that has an array type?

If not, then @ValueSource likely already meets your needs as @marcphilipp already pointed out.

[ktkopone]

Apologies, I had a longer writeup with context but tried to pair it down. In my case I have an array variable of strings that I want to reuse as data for multiple parameterized tests (hence I don't want to just hardcode it in the @ValueSource strings parameter). If there's a way to pass an array variable to @ValueSource without running into Attribute value must be constant, I don't know it.

It's good to know that @MethodSource can return arrays. However, it is still a pain point that I'll have to write a custom method just to return data which I have stored in array vars. I could skip on having the array variable at all but I like being able to define all my test data together as properties of a TestData class, and I'm sure there exist scenarios where there would be good reasons to store in arrays rather than only being returned via methods.

So given that in many use cases the data will live in an array field, it would be convenient if the user could pass this via @ArraySource(arrayVar) without having to write a dummy method for each field.

[sbrannen]

Based on your feedback, what you are effectively requesting is a @FieldSource annotation analogous to the existing @MethodSource. In other words, there is nothing specific in your request that pertains only to arrays. I will therefore update the title of the issue accordingly.

[sbrannen]

Tentatively slated for the 5.6 backlog for team discussion

[marcphilipp]

In the meantime, this might be a little heavy on the ceremony but works:

@Retention(RetentionPolicy.RUNTIME)
@ValueSource(strings = { "racecar", "radar", "able was I ere I saw elba" })
@interface MyArraySource {}

@ParameterizedTest
@MyArraySource
void palindromes(String candidate) {
	assertTrue(StringUtils.isPalindrome(candidate));
}