- PatternLayoutBase creates a new StringBuilder for every event it must serialize causing garbage and  unnecessary pressure on the GC. Try to reuse a StringBuilder per thread.
- Filtering of empty fields is getting values multiple times from the event which becomes expensive when the value is computed by a PatternLayout. Use Jackson's FilteringJsonGenerator instead.
- Make use of the most appropriate JsonGenerator#write() method instead of using writeObject() for "simple" value types. Using writeObject() delegates to the underlying ObjectCode which is slower and produces lot more garbage.
- Refactor the code to make it easier to read and understand.

DelegatingNodeWriter is private and used only by OmitEmptyFieldWriter. Suppress the class and move the delegate field directly into the sub-class.

…s instead of logging an ERROR status

AbstractJsonPatternParser now throws a JsonPatternException in case of invalid pattern. It is up to the AbstractPatternLayoutJsonProvider to catch the exception, log an error status and decide how it should behave.

Invalid PatternLayout formats are now detected and throw a JsonPatternException as well. Detection is done by configuring the PatternLayout instances with a custom Logback context that throws an exception when an ERROR status is logged.

The former implementation used to ignore fields with an invalid pattern layout while keeping the other. This results in a partial JSON output with only the valid fields. This behaviour is now changed and the provider reverts to producing nothing in case of bad configuration.

These changes address issue #686.

… patterns instead of logging an ERROR status"

This reverts commit 414f482.
This changes will be committed in a separate branch.

…s instead of logging an ERROR status

AbstractJsonPatternParser now throws a JsonPatternException in case of invalid pattern. It is up to the AbstractPatternLayoutJsonProvider to catch the exception, log an error status and decide how it should behave.

Invalid PatternLayout formats are now detected and throw a JsonPatternException as well. Detection is done by configuring the PatternLayout instances with a custom Logback context that throws an exception when an ERROR status is logged.

The former implementation used to ignore fields with an invalid pattern layout while keeping the other. This results in a partial JSON output with only the valid fields. This behaviour is now changed and the provider reverts to producing nothing in case of bad configuration.

These changes address issue #686.

… convention

- Make 'operation' a standard java Function accepting a String as input.
- Build the LayoutValueGetter outside of the operation factory