Is is possible to customize the JSON output from JsonEncoder?

[ecerulm]

When I use ch.qos.logback.classic.encoder.JsonEncoder I get output like this

{"sequenceNumber":0,"timestamp":1758485334555,"nanoseconds":555853000,"level":"DEBUG","threadName":"main","loggerName":"jsonEncoderLogger","context":{"name":"default","birthdate":1758485334487,"properties":{}},"mdc": {},"message":"Performing some operations...","throwable":null}
{"sequenceNumber":0,"timestamp":1758485334555,"nanoseconds":555876000,"level":"INFO","threadName":"main","loggerName":"jsonEncoderLogger","context":{"name":"default","birthdate":1758485334487,"properties":{}},"mdc": {},"message":"Operations completed successfully","throwable":null}

From the JsonEncoder docs I know I can remove fields by passing <withXXXX>false</withXXXX>:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<appender name="jsonEncoder" class="ch.qos.logback.core.ConsoleAppender">
    <encoder class="ch.qos.logback.classic.encoder.JsonEncoder">
       <withSequenceNumber>false</withSequenceNumber>
    </encoder>
</appender>

<logger name="jsonEncoderLogger" level="TRACE">
    <appender-ref ref="jsonEncoder" />
</logger>
</configuration>

But what about other transformations, I need my logs to conform to Google GKE Logging agent: special JSON fields so;

• severity instead of level
• timestamp.seconds instead of timestamp
• timestamp.nanos instead of nanoseconds

Is that possible with plain logback? or do I need some other library that provides another "JsonEncoder" with those capabilities?
I would appreciate any pointer to "trusted" and "sound" alternative encoder that produce JSON and allows the transformations mentioned.

[ceki]

Hello @ecerulm,

Have you considered subclassing JsonEncoder? The JsonEncoder class is pretty straight forward and should not be too difficult to adapt to your needs.

[ecerulm]

I gave a try at subclassing ch.qos.logback.classic.encoder.JsonEncoder , I ended up with this GoogleStructuredLoggingEncoder.java

which produces the following output

{"timestampSeconds": 1758571889,"timestampNanos": 74338000,"severity": "DEBUG","message": "Hello world, AgyZLR"}

But I must say that it doesn't look like ch.qos.logback.classic.encoder.JsonEncoder is NOT designed with subclassing in mind. Lots of things are private or protected like OPEN_OBJ, etc and the appendLoggerContext(), appendMap(),etc plus the jsonEscape() , etc aso not reusable.

So I other words it feels like I need to create a new Encoder to have this custom JSON that complies with GKE logging agent expectations.

In my case, I just wanted to have a quick an easy solution to modify the logback.xml of an existing application (OpenMetadata) to force it to output structured logs compatible with the GKE logging agent. So, implementing a custom Encoder seems a bit too much for that. I would have been nice to have more customization via configuration file but after your comment and reading the JsonEncoder source code I see that is not possible.

I also explored using net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder from logstash-logback-encoder but I run into problems formatting the %date as epoch seconds as described in #968 (for which I created a PR #969)

@ceki , although it won't fix my problem now, I wonder if I prepare a PR to make JsonEncoder accept a googleLoggingFormat options that will modify the JSON output to include timestampSeconds, timestampNanos, severity and message(as FormattedMessage), has that any possibility of getting merged or you want to keep JsonEncoder the way it is.

[ecerulm]

For reference in case anyone else is targeting GKE logging infrastructure, I wrote this basic custom Encoder that uses jackson's JsonGenerator to produce the JSON with the special fields required by GKE logging agent (time(RFC3339), timestamp.seconds, timestamp.nanos, timestampSeconds, timestampNanos, severity, message):

https://github.com/ecerulm/logback-custom-json-encoder-google-logging/blob/main/src/main/java/com/rubenlaguna/GoogleStructuredLoggingJacksonEncoder.java

The output of mvn compile exec:java:

{"time":"2025-09-23T07:20:58.363508Z","timestamp":{"seconds":1758612058,"nanos":363508000},"timestampSeconds":1758612058,"timestampNanos":363508000,"severity":"INFO","message":"Hello World d2AhvR, iteration: 0"}
{"time":"2025-09-23T07:20:58.365794Z","timestamp":{"seconds":1758612058,"nanos":365794000},"timestampSeconds":1758612058,"timestampNanos":365794000,"severity":"INFO","message":"Hello World fxXb6G, iteration: 1"}

[ceki]

@ecerulm How about GoogleStructuredLoggingEncoder extending JsonEncoder by modifying the base class where needed?

@ceki , although it won't fix my problem now, I wonder if I prepare a PR to make JsonEncoder accept a googleLoggingFormat options that will modify the JSON output to include timestampSeconds, timestampNanos, severity and message(as FormattedMessage), has that any possibility of getting merged or you want to keep JsonEncoder the way it is.

Sounds good and very similar to my proposal above.

[ecerulm]

Then I'll prepare a PR with a new ch.qos.logback.classic.encoder.GoogleStructuredLoggingEncoder

[ecerulm]

#972