Why Does java.time Fail to Parse Fraction-of-Second Despite Documentation?

java.time Unable to Parse Fraction-of-Second

Despite adhering to the documentation, parsing fraction-of-second with java.time encounters issues. This article explores the problem and provides alternative solutions until the issue is resolved.

The Issue

When using DateTimeFormatter to parse a string with fraction-of-second (represented by "SS"), an exception is thrown. This behavior contradicts the documentation's claim that strict mode matches the number of format characters to input digits.

Possible Workarounds

Until Java 9 (or later) addresses this problem, several workarounds are available:

• Use DateTimeFormatterBuilder with appendValue():

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

Note: This approach may not handle cases with only two pattern symbols (e.g., "SS").

• Use SimpleDateFormat:

<code class="java">String input = "2011120312345655";
SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMddHHmmssSS");
Date d = sdf.parse(input);</code>

However, this workaround can also misinterpret fraction-of-second values.

• Use 3rd-Party Libraries:

External libraries like Joda-Time or Time4J support fraction-of-second parsing. For example, using Joda-Time:

<code class="java">String input = "2011120312345655";
DateTimeFormatter dtf = DateTimeFormat.forPattern("yyyyMMddHHmmssSS");
System.out.println(dtf.parseLocalDateTime(input));</code>

Hacky Solutions:

One could manually extract the fractional part from the input string and apply adjustments, but this approach is prone to errors.

Conclusion

Parsing fraction-of-second values using java.time currently has limitations. Until Java 9 addresses this issue, users can employ the workarounds discussed above. It is also worth noting that SimpleDateFormat may also misinterpret fraction-of-second values. For definitive results, it is recommended to use external libraries like Joda-Time or Time4J.

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