Monday, January 07, 2019

Hard-won JDK offset knowedge

It took far more research time than I expected. The goal: Output an OffsetDateTime with offset for Zulu (OTC) timezone as +00:00.

I have a project where a 3rd-party JSON exchange expected timestamps in the format 01-02-03T04:05:06+00:00. We're using Jackson in a Java project. All the default configuration I could find, and trying all the "knobs" on Jackson I could find, led to: 01-02-03T04:05:06Z. Interesting, as any non-0 offset for timezone produced: 01-02-03T04:05:06+07:00 rather than a timezone abbreviation: Zero offset is special.

Finally, circling back to the JDK javadocs yet again, I spotted what I had overlooked many times before:

Offset X and x: This formats the offset based on the number of pattern letters. One letter outputs just the hour, such as '+01', unless the minute is non-zero in which case the minute is also output, such as '+0130'. Two letters outputs the hour and minute, without a colon, such as '+0130'. Three letters outputs the hour and minute, with a colon, such as '+01:30'. Four letters outputs the hour and minute and optional second, without a colon, such as '+013015'. Five letters outputs the hour and minute and optional second, with a colon, such as '+01:30:15'. Six or more letters throws IllegalArgumentException. Pattern letter 'X' (upper case) will output 'Z' when the offset to be output would be zero, whereas pattern letter 'x' (lower case) will output '+00', '+0000', or '+00:00'.

The key is to use lowercase 'x' in the format specification. So my problem with Jackson became:

    @JsonFormat(pattern = "yyyy-MM-dd'T'HH:mm:ssxxx")
    private final OffsetDateTime someOffsetDateTime;

And the result is the desired, 01-02-03T04:05:06+00:00.

Now I can return to more interesting problems.

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