ppkarwasz commented on PR #3789:
URL: https://github.com/apache/logging-log4j2/pull/3789#issuecomment-3066741660
> @ppkarwasz, consider users having their custom instant formatters, some
might even be relying on `{Fixed,Fast}DateFormat`. They will get excited about
`NamedInstantPattern`, will try to use it, and it won't work. They will check
the docs, have their _"Aha!"_ moment, enable legacy formatters, only to see
that it still does not work. What do you think about this issue?
Sure! Here's a clearer and more polished version of your answer. It keeps
your technical points but improves the flow, tone, and readability:
If you're suggesting that `NamedInstantPattern.getPattern()` should return
either the DTF-style or legacy pattern depending on the value of the
`log4j2.instantFormatter` config property — I’d consider that an anti-pattern.
For example, code like:
```java
DateTimeFormatter formatter =
DateTimeFormatter.ofPattern(NamedInstantPattern.getPattern());
formatter.format(Instant.now());
```
would break if a user enables legacy formatters by setting
`log4j2.instantFormatter=legacy`. That kind of hidden behavior change
undermines the method’s predictability and makes it harder to use safely.
If someone wants to use the named patterns with `FastDateFormat`, they’ll
need to explicitly convert them. Here's an example of how that could be done:
```java
public static String convertToLegacyFormat(final String pattern) {
// Replace each consecutive 'S' beyond the first three with literal zeros
final Matcher matcher = Pattern.compile("(?<=SSS)S+").matcher(pattern);
final StringBuilder sb = new StringBuilder();
while (matcher.find()) {
matcher.appendReplacement(sb, "'" +
"0".repeat(matcher.group().length()) + "'");
}
matcher.appendTail(sb);
// Replace 'x' with 'X'
return sb.toString().replace('x', 'X');
}
```
This keeps the behavior explicit and avoids magic behind the scenes.
If someone wants to use the named pattern with `FixedDateFormat` they should
consider using something more standard.
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