Zero garbage serialization of ISO timestamps

[felixbarny]

Quoting the Javadoc:

/**
 * This class serializes an epoch timestamp in milliseconds to a ISO 8601 date time sting,
 * for example {@code 1970-01-01T00:00:00.000Z}
 * <p>
 * The main advantage of this class is that is able to serialize the timestamp in a garbage free way,
 * i.e. without object allocations and that it is faster than {@link java.text.DateFormat#format(Date)}.
 * </p>
 * <p>
 * The most complex part when formatting a ISO date is to determine the actual year,
 * month and date as you have to account for leap years.
 * Leveraging the fact that for a whole day this stays the same
 * and that the agent only serializes the current timestamp and not arbitrary ones,
 * we offload this task to {@link java.text.DateFormat#format(Date)} and cache the result.
 * So we only have to serialize the time part of the ISO timestamp which is easy
 * as a day has exactly {@code 1000 * 60 * 60 * 24} milliseconds.
 * Also, we don't have to worry about leap seconds when dealing with the epoch timestamp.
 * </p>
 * <p>
 * Note: this class is not thread safe.
 * As serializing the payloads is done in a single thread,
 * this is no problem though.
 * </p>
 */

[codecov-io]

Codecov Report

Merging #77 into master will increase coverage by 0.42%.
The diff coverage is 97.91%.

@@             Coverage Diff              @@
##             master      #77      +/-   ##
============================================
+ Coverage     76.01%   76.43%   +0.42%     
- Complexity      724      737      +13     
============================================
  Files            78       79       +1     
  Lines          2643     2674      +31     
  Branches        240      241       +1     
============================================
+ Hits           2009     2044      +35     
+ Misses          501      497       -4     
  Partials        133      133

Impacted Files	Coverage Δ	Complexity Δ
...lastic/apm/report/serialize/DslJsonSerializer.java	84.75% <100%> (+0.1%)	78 <1> (+1)	⬆️
...o/elastic/apm/report/serialize/DateSerializer.java	100% <100%> (ø)	12 <12> (?)
...a/co/elastic/apm/impl/transaction/Transaction.java	76.34% <100%> (+2.87%)	28 <0> (ø)	⬇️
...n/java/co/elastic/apm/impl/error/ErrorCapture.java	59.09% <50%> (-1.78%)	8 <1> (ø)

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