LoggingEventCompositeJsonEncoder - how omit nestedField if empty?

[dtanner]

I'm using a LoggingEventCompositeJsonEncoder as configured in this logback.xml example:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
      <providers>
        <message/>
        <loggerName>
          <fieldName>loggerName</fieldName>
        </loggerName>
        <nestedField>
          <fieldName>thrown</fieldName>
          <providers>
            <throwableClassName>
              <fieldName>name</fieldName>
            </throwableClassName>
            <stackTrace>
              <fieldName>extendedStackTrace</fieldName>
              <throwableConverter class="net.logstash.logback.stacktrace.ShortenedThrowableConverter">
              </throwableConverter>
            </stackTrace>
          </providers>
        </nestedField>
      </providers>
    </encoder>
  </appender>

  <root level="INFO">
    <appender-ref ref="STDOUT"/>
  </root>
</configuration>

And it's producing logs like this:

{
  "message": "Starting app",
  "loggerName": "my.api.App",
  "thrown": {}
}

I only want the thrown field output when a Throwable exists though. Is there a way to accomplish this using the provided classes in logstash-logback-encoder? If not, is there a suggestion on where a custom class/method would need to be overridden?

[brenuart]

Hi Dan,

There is unfortunately no built-in way to accomplish what you describe. The PatternJsonProvider is the only provider with the ability to omit empty fields (cfr omitEmptyField property). We have plan to make this feature available to all providers and are working on it... expect this feature to be included in one of the up coming releases.

In the meantime you can decorate the Jackson JsonGenerator yourself to make it filter empty fields. The following sample JsonGeneratorDecorator should do the job:

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.filter.FilteringGeneratorDelegate;
import com.fasterxml.jackson.core.filter.TokenFilter;
import com.fasterxml.jackson.core.filter.TokenFilter.Inclusion;
import net.logstash.logback.decorate.JsonGeneratorDecorator;
import net.logstash.logback.util.StringUtils;

public class OmitEmptyFieldJsonDecorator implements JsonGeneratorDecorator {

	@Override
	public JsonGenerator decorate(JsonGenerator generator) {
		return new FilteringGeneratorDelegate(
				generator, 
				NullExcludingTokenFilter.INSTANCE, 
				Inclusion.INCLUDE_ALL_AND_PATH, 
				true /* multiple matches */);
	}
	
    private static class NullExcludingTokenFilter extends TokenFilter {
        private static final NullExcludingTokenFilter INSTANCE = new NullExcludingTokenFilter();

        @Override
        public boolean includeNull() {
            return false;
        }
        
        @Override
        public boolean includeString(String value) {
            return !StringUtils.isEmpty(value);
        }
    }
}

Then simply register the decorator in your logback configuration file as follows:

<appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
	<encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
		<jsonGeneratorDecorator class="OmitEmptyFieldJsonDecorator" />
		<providers>
			...
		<providers>
...

Note that this will filter all properties and and won't be limited to your "nested" property. Properties with a value satisfying one of the following conditions will be removed (i.e. not included in the final output):

• null value
• empty string
• empty (json) array
• empty (json) object

Hope this helps. Do not hesitate to ask for more information if needed.

[dtanner]

Sounds good - I'll try that. Thank you!