public final class CaseConverterResolver
extends Object
implements EventResolver

Converts the case of string values.

Configuration

 config                = case , input , [ locale ] , [ errorHandlingStrategy ]
 input                 = JSON
 case                  = "case" -> ( "upper" | "lower" )
 locale                = "locale" -> (
                             language                                   |
                           ( language , "_" , country )                 |
                           ( language , "_" , country , "_" , variant )
                         )
 errorHandlingStrategy = "errorHandlingStrategy" -> (
                           "fail"    |
                           "pass"    |
                           "replace"
                         )
 replacement           = "replacement" -> JSON
 

input can be any available template value; e.g., a JSON literal, a lookup string, an object pointing to another resolver.

Unless provided, locale points to the one returned by JsonTemplateLayoutDefaults.getLocale(), which is configured by log4j.layout.jsonTemplate.locale system property and by default set to the default system locale.

errorHandlingStrategy determines the behavior when either the input doesn't resolve to a string value or case conversion throws an exception:

• fail propagates the failure
• pass causes the resolved value to be passed as is
• replace suppresses the failure and replaces it with the replacement, which is set to null by default

errorHandlingStrategy is set to replace by default.

Most of the time JSON logs are persisted to a storage solution (e.g., Elasticsearch) that keeps a statically-typed index on fields. Hence, if a field is always expected to be of type string, using non-string replacements or pass in errorHandlingStrategy might result in type incompatibility issues at the storage level.

Unless the input value is passed intact or replaced, case conversion is not garbage-free.

Examples

Convert the resolved log level strings to upper-case:

 {
   "$resolver": "caseConverter",
   "case": "upper",
   "input": {
     "$resolver": "level",
     "field": "name"
   }
 }
 

Convert the resolved USER environment variable to lower-case using nl_NL locale:

 {
   "$resolver": "caseConverter",
   "case": "lower",
   "locale": "nl_NL",
   "input": "${env:USER}"
 }
 

Convert the resolved sessionId thread context data (MDC) to lower-case:

 {
   "$resolver": "caseConverter",
   "case": "lower",
   "input": {
     "$resolver": "mdc",
     "key": "sessionId"
   }
 }
 

Above, if sessionId MDC resolves to a, say, number, case conversion will fail. Since errorHandlingStrategy is set to replace and replacement is set to null by default, the resolved value will be null. One can suppress this behavior and let the resolved sessionId number be left as is:

 {
   "$resolver": "caseConverter",
   "case": "lower",
   "input": {
     "$resolver": "mdc",
     "key": "sessionId"
   },
   "errorHandlingStrategy": "pass"
 }
 

or replace it with a custom string:

 {
   "$resolver": "caseConverter",
   "case": "lower",
   "input": {
     "$resolver": "mdc",
     "key": "sessionId"
   },
   "errorHandlingStrategy": "replace"
   "replacement": "unknown"
 }