Why Does Java 8\'s `java.time` Package Fail to Parse Fraction-of-Second Values in Strict Mode?

Is java.time Failing to Parse Fraction-of-Second?

With the initial release of Java 8, parsing fraction-of-second using the java.time package may encounter an exception. Consider this code:

<code class="java">String input = "2011120312345655";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyyMMddHHmmssSS");
LocalDateTime localDateTime = LocalDateTime.parse(input, formatter);</code>

This code fails with an exception:

java.time.format.DateTimeParseException

The documentation states that strict mode requires the same number of format characters as input digits. However, the code includes "SS" for fraction-of-second, which contradicts the number of digits in the input.

To address this issue, a workaround was suggested:

<code class="java">DateTimeFormatter dtf =
  new DateTimeFormatterBuilder()
  .appendPattern("yyyyMMddHHmmss")
  .appendValue(ChronoField.MILLI_OF_SECOND, 3)
  .toFormatter();</code>

However, this workaround does not fully cover the use case of only two "S" pattern symbols. Other fields like MICRO_OF_SECOND (six SSSSSS) or NANO_OF_SECOND (nine SSSSSSSSS) must be used.

According to the documentation:

"Fraction: Outputs the nano-of-second field as a fraction-of-second. (...) When parsing in strict mode, the number of parsed digits must match the count of pattern letters."

This implies that "S" represents any fraction of second, not just milliseconds.

Resolution in Java 9

As of Java 9, the issue has been resolved. The initial release of Java 8 encountered challenges in implementing adjacent value parsing for fractional parts. This limitation hindered the interpretation of fields in text that spanned multiple fields without clear boundaries.

However, in Java 9, the JSR-310 parser gained the ability to parse fractional parts in adjacent value parsing. This improvement addresses the challenges faced in earlier versions and provides enhanced flexibility when working with fraction-of-second values.

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