Why Does My Java DateTimeFormatterBuilder Unit Test Fail, But Runtime Execution Succeeds?

Java DateTimeFormatterBuilder Encountering Issues in Unit Test Context

In a Java application, a unit test for DateTimeFormatterBuilder is failing while functioning correctly at runtime. The test method employs a formatter with the pattern dd-MMM-yyyy HH:mm to parse a string input.

The failing string value in question is "25-May-2018 11:10." When executed at runtime, the formatter successfully parses this string. However, the same string causes the unit test to fail.

Cause:

The crux of the issue lies in the lack of explicit locale specification for the formatter used in the unit test. Java's DateTimeFormatter relies on the JVM's default locale, which may not always align with the expected month names. Consequently, the test environment's default locale could differ from the runtime environment, leading to parsing discrepancies.

Solution:

To resolve this problem, explicitly set the locale of the DateTimeFormatterBuilder to Locale.ENGLISH. By doing so, the formatter will consistently interpret the month name as English, ensuring consistency both at runtime and in unit tests.

Here's the modified code with the locale explicitly set:

DateTimeFormatter DATE_TIME_FORMAT = new DateTimeFormatterBuilder()
    .parseCaseInsensitive()
    .appendPattern("dd-MMM-yyyy HH:mm")
    .toFormatter(Locale.ENGLISH);

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