Logback and JsonLayout: can't pass custom fields - spring

Question

I have this configuration in my logback.xml into a Spring Web Application (NO Spring Boot).
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<appender name="Console" class="ch.qos.logback.core.ConsoleAppender">
<encoder class="ch.qos.logback.core.encoder.LayoutWrappingEncoder">
<layout class="ch.qos.logback.contrib.json.classic.JsonLayout">
<timestampFormat>yyyy-MM-dd'T'HH:mm:ss.SSSX</timestampFormat>
<timestampFormatTimezoneId>Etc/UTC</timestampFormatTimezoneId>
<jsonFormatter class="ch.qos.logback.contrib.jackson.JacksonJsonFormatter">
<prettyPrint>true</prettyPrint>
</jsonFormatter>
</layout>
<customFields>{"appname":"foobar"}</customFields>
</encoder>
</appender>
<!-- LOG everything at INFO level -->
<root level="INFO">
<appender-ref ref="Console" />
</root>
</configuration>
The JSON layout works fine but custom fields as "appname": "foobar" are not printed:
{
"timestamp" : "2020-06-10T14:55:25.534Z",
"level" : "INFO",
"thread" : "Catalina-utility-1",
"logger" : "org.springframework.web.servlet.DispatcherServlet",
"message" : "FrameworkServlet 'dispatcher': initialization completed in 72 ms",
"context" : "default"
}
What am I doing wrong?
SOLUTION
I was using the wrong libraries for my needs:
logback-jackson
logback-json-classic
Because of the fact that I need to process logs through Logstash I've corrected my configuration like this:
pom.xml
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.2.3</version>
</dependency>
<dependency>
<groupId>net.logstash.logback</groupId>
<artifactId>logstash-logback-encoder</artifactId>
<version>6.4</version>
</dependency>
logback.xml
<appender name="Console" class="ch.qos.logback.core.ConsoleAppender">
<encoder class="net.logstash.logback.encoder.LogstashEncoder">
<customFields>{"customer":"X", "appname":"Y", "environment":"dev"}</customFields>
</encoder>
</appender>
and now It works fine.

Answer 1

I just stumbled this question because I had the same problem, and I found a solution, with logback-jackson and logback-json-classic.
Option 1: Per-Thread via Mapped Diagnostic Context (MDC)
SLF4j's Mapped Diagnostic Context is a per-thread key-value store that we can use to write custom structured data to the log output.
MDC.put("customKey", "customValue");
Logback's JsonLayout will automatically print this value under a special mdc JSON object without any further configuration.
{ [...], "mdc": {"customKey", "customValue"}}
Note that the MDC is constructed per thread and if it is empty, no mdc field is printed to the log output.
Option 2: Global (for all threads)
If you want custom fields to appear at the JSON output's root, you need to create a custom, but simple Layout class that extends JsonLayout. JsonLayout provides us with a addCustomDataToJsonMap we can override.
package com.mypackage;
import ch.qos.logback.contrib.json.classic.JsonLayout;
public class CustomJsonLayout extends JsonLayout {
#Override
protected void addCustomDataToJsonMap(Map<String, Object> map, ILoggingEvent event) {
map.put("customKey", "customValue");
}
}
Now, you just need to tell Logback to use CustomJsonLayout instead of JsonLayout in your logback.xml file and keep the rest the same.
<layout class="com.mypackage.CustomJsonLayout">
...
</layout>
Now, any log message will have the following output:
{ ..., "customKey": "customValue"}