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What is the FastCSV migration in JUnit 6 and how does it affect @CsvSource and @CsvFileSource?

JUnit 6 replaced the univocity-parsers library (which had stopped receiving maintenance) with FastCSV for parsing CSV data in @CsvSource and @CsvFileSource. FastCSV is actively maintained, has better performance, and is stricter about malformed input.

@CsvSource/@CsvFileSource changes in JUnit 6
AspectJUnit 5 (univocity-parsers)JUnit 6 (FastCSV)
MaintenanceUnmaintained libraryActively maintained
Strict parsingSilently accepted some malformed CSVStrict: throws exception on malformed input
Extra chars after quotesAllowed silently: 'foo'INVALIDException thrown - no extra chars after closing quote
lineSeparator attributePresent in @CsvFileSourceRemoved: line separator auto-detected (\r, \n, \r\n)
Header fieldsignoreLeadingAndTrailingWhitespace did not apply to headersNow applies to headers too
commentCharacterNot configurable (# caused conflicts)New commentCharacter attribute added; default still #
// JUnit 5: this was silently accepted (malformed CSV)
@CsvSource({"'foo'INVALID,'bar'"})

// JUnit 6: throws exception -- "extra characters after closing quote"
// Fix: remove the extra characters
@CsvSource({"'foo','bar'"})

// JUnit 5: @CsvFileSource with explicit line separator
@CsvFileSource(resources = "/data.csv", lineSeparator = "\n")
// JUnit 6: lineSeparator removed -- auto-detected
@CsvFileSource(resources = "/data.csv")
// Auto-detection handles \r, \n, and \r\n correctly

// New in JUnit 6: configure commentCharacter
// Previously # was hardcoded and conflicted with some delimiters
@CsvSource(
    value = {"1, 2, 3", "# this is a comment"},
    commentCharacter = '#'   // can now be customised or disabled
)
void additionTest(int a, int b, int expected) { ... }

Why did JUnit 6 replace univocity-parsers with FastCSV for CSV parsing?
What happens in JUnit 6 if @CsvSource data contains extra characters after a closing quote, such as 'foo'INVALID?

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