SONARJAVA-6198 S1451 should accept an empty headerFormat as the absence of any line

[NoemieBenard]

Modify S1451 to recognize an empty headerFormat as the absence of any line.

[hashicorp-vault-sonar-prod]

SONARJAVA-6198

[sonar-review-alpha]

Summary

What changed: Modified FileHeaderCheck to recognize an empty headerFormat configuration as "no header required" rather than crashing or splitting an empty string. When headerFormat is empty, the check now sets expectedLines to an empty array, allowing any file (with or without blank lines at the start) to pass.

Why: This handles an edge case where users configure the rule with an empty value to disable header validation. Previously, the code didn't explicitly handle this case, leading to undefined behavior. The fix makes the intent clear: empty format = no header needed.

What reviewers should know

Where to start: Review the logic change in FileHeaderCheck.java (the empty check and split logic).

Key points for reviewers:

• The fix is defensive: checks if headerFormat.isEmpty() before calling split, avoiding potential issues with splitting an empty string
• Two new test files validate the behavior: Class4 (with leading blank line) and Class5 (without), both should pass with empty headerFormat
• Minor cleanup: also fixed whitespace in if( → if ( on the same line (lines 81-82)
• The getHeaderFormat() method still needs protection when called with empty format (charAt(0) call), but that's only called when headerFormat is explicitly set to a non-empty regex pattern, so it's safe

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[sonar-review-alpha]

LGTM! ✅

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