ashr123 commented on code in PR #3789:
URL: https://github.com/apache/logging-log4j2/pull/3789#discussion_r2194921907


##########
log4j-core/src/main/java/org/apache/logging/log4j/core/pattern/DatePatternConverter.java:
##########
@@ -109,84 +109,11 @@ private static String readPattern(@Nullable final 
String[] options) {
      * @since 2.25.0
      */
     static String decodeNamedPattern(final String pattern) {
-
-        // If legacy formatters are enabled, we need to produce output aimed 
for `FixedDateFormat` and `FastDateFormat`.
-        // Otherwise, we need to produce output aimed for `DateTimeFormatter`.
-        // In conclusion, we need to check if legacy formatters enabled and 
apply following transformations.
-        //
-        //                               | Microseconds | Nanoseconds | 
Time-zone
-        // 
------------------------------+--------------+-------------+-----------
-        // Legacy formatter directive    | nnnnnn       | nnnnnnnnn   | X, XX, 
XXX
-        // `DateTimeFormatter` directive | SSSSSS       | SSSSSSSSS   | x, xx, 
xxx
-        //
-        // Enabling legacy formatters mean that user requests the pattern to 
be formatted using deprecated
-        // `FixedDateFormat` and `FastDateFormat`.
-        // These two have, let's not say _bogus_, but an _interesting_ way of 
handling certain pattern directives:
-        //
-        // - They say they adhere to `SimpleDateFormat` specification, but use 
`n` directive.
-        //   `n` is neither defined by `SimpleDateFormat`, nor 
`SimpleDateFormat` supports sub-millisecond precisions.
-        //   `n` is probably manually introduced by Log4j to support 
sub-millisecond precisions.
-        //
-        // - `n` denotes nano-of-second for `DateTimeFormatter`.
-        //   In Java 17, `n` and `N` (nano-of-day) always output nanosecond 
precision.
-        //   This is independent of how many times they occur consequently.
-        //   Yet legacy formatters use repeated `n` to denote sub-milliseconds 
precision of certain length.
-        //   This doesn't work for `DateTimeFormatter`, which needs
-        //
-        //   - `SSSSSS` for 6-digit microsecond precision
-        //   - `SSSSSSSSS` for 9-digit nanosecond precision
-        //
-        // - Legacy formatters use `X`, `XX,` and `XXX` to choose between 
`+00`, `+0000`, or `+00:00`.
-        //   This is the correct behaviour for `SimpleDateFormat`.
-        //   Though `X` in `DateTimeFormatter` produces `Z` for zero-offset.
-        //   To avoid the `Z` output, one needs to use `x` with 
`DateTimeFormatter`.
-        final boolean compat = 
InstantPatternFormatter.LEGACY_FORMATTERS_ENABLED;
-
-        switch (pattern) {
-            case "ABSOLUTE":
-                return "HH:mm:ss,SSS";
-            case "ABSOLUTE_MICROS":
-                return "HH:mm:ss," + (compat ? "nnnnnn" : "SSSSSS");
-            case "ABSOLUTE_NANOS":
-                return "HH:mm:ss," + (compat ? "nnnnnnnnn" : "SSSSSSSSS");
-            case "ABSOLUTE_PERIOD":
-                return "HH:mm:ss.SSS";
-            case "COMPACT":
-                return "yyyyMMddHHmmssSSS";
-            case "DATE":
-                return "dd MMM yyyy HH:mm:ss,SSS";
-            case "DATE_PERIOD":
-                return "dd MMM yyyy HH:mm:ss.SSS";
-            case "DEFAULT":
-                return "yyyy-MM-dd HH:mm:ss,SSS";
-            case "DEFAULT_MICROS":
-                return "yyyy-MM-dd HH:mm:ss," + (compat ? "nnnnnn" : "SSSSSS");
-            case "DEFAULT_NANOS":
-                return "yyyy-MM-dd HH:mm:ss," + (compat ? "nnnnnnnnn" : 
"SSSSSSSSS");
-            case "DEFAULT_PERIOD":
-                return "yyyy-MM-dd HH:mm:ss.SSS";
-            case "ISO8601_BASIC":
-                return "yyyyMMdd'T'HHmmss,SSS";
-            case "ISO8601_BASIC_PERIOD":
-                return "yyyyMMdd'T'HHmmss.SSS";
-            case "ISO8601":
-                return "yyyy-MM-dd'T'HH:mm:ss,SSS";
-            case "ISO8601_OFFSET_DATE_TIME_HH":
-                return "yyyy-MM-dd'T'HH:mm:ss,SSS" + (compat ? "X" : "x");
-            case "ISO8601_OFFSET_DATE_TIME_HHMM":
-                return "yyyy-MM-dd'T'HH:mm:ss,SSS" + (compat ? "XX" : "xx");
-            case "ISO8601_OFFSET_DATE_TIME_HHCMM":
-                return "yyyy-MM-dd'T'HH:mm:ss,SSS" + (compat ? "XXX" : "xxx");
-            case "ISO8601_PERIOD":
-                return "yyyy-MM-dd'T'HH:mm:ss.SSS";
-            case "ISO8601_PERIOD_MICROS":
-                return "yyyy-MM-dd'T'HH:mm:ss." + (compat ? "nnnnnn" : 
"SSSSSS");
-            case "US_MONTH_DAY_YEAR2_TIME":
-                return "dd/MM/yy HH:mm:ss.SSS";
-            case "US_MONTH_DAY_YEAR4_TIME":
-                return "dd/MM/yyyy HH:mm:ss.SSS";
+        try {
+            return NamedDatePattern.valueOf(pattern).getPattern();
+        } catch (IllegalArgumentException ignored) {
+            return pattern;
         }

Review Comment:
   @ppkarwasz I agree with you completely, I'll in case  of 
`InstantPatternFormatter.LEGACY_FORMATTERS_ENABLED` is `true`, I'll use 
`FixedDateFormat.FixedFormat#getPattern`, is that ok?
   Of course, `NamedInstantPattern` will contain only the non-legacy patterns



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