JSON Logging with named fields in Java?

Question

Here is the Golang version of what I want to do in Java:

package main

import (
    "log/slog"
    "os"
)

func main() {
    logLevel := new(slog.LevelVar) // Info by default
    logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: logLevel}))
    slog.SetDefault(logger)

    slog.Info("this is a test info message",
        slog.Bool("my-flag", true),
        slog.Int("my-int", 123),
        slog.Float64("my-float64", 3.14159),
        slog.String("my-name", "abcdefg"))
}

That generates:

{"time":"2024-04-05T16:54:14.028207-05:00","level":"INFO","msg":"this is a test info message","my-flag":true,"my-int":123,"my-float64":3.14159,"my-name":"abcdefg"}

In Java, I'm aware of three popular logging APIs:

System.Logger
slf4j
the log4j api org.apache.logging.log4j.Logger.

AFAIK, none of these three support named fields like Golang slog does. Is that correct? Am I missing something? It seems like this should be easier.

I tried writing examples in all three logging apis.

stackoverflow.com/q/60281901/112968

knittl
– knittl

2024-04-05 23:01:05 +00:00
Commented Apr 5, 2024 at 23:01 — knittl
– knittl, Commented Apr 5, 2024 at 23:01
baeldung.com/java-structured-logging

knittl
– knittl

2024-04-05 23:02:18 +00:00
Commented Apr 5, 2024 at 23:02 — knittl
– knittl, Commented Apr 5, 2024 at 23:02
stackoverflow.com/q/37430644/112968

knittl
– knittl

2024-04-05 23:03:18 +00:00
Commented Apr 5, 2024 at 23:03 — knittl
– knittl, Commented Apr 5, 2024 at 23:03

tgdavies · Accepted Answer · 2024-04-09 00:21:51Z

If you use logback, for instance, you can use ch.qos.logback.classic.encoder.JsonEncoder to log JSON objects:

You need this configuration in logback.xml:

configuration>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder class="ch.qos.logback.classic.encoder.JsonEncoder"/>
    </appender>
    <root level="debug">
        <appender-ref ref="STDOUT" />
    </root>
</configuration>

And then the code:

package com.example.so;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class Eg {
    public static void main(String[] args) {
        Logger l = LoggerFactory.getLogger("test");

        l.atInfo().addKeyValue("my-flag", true)
                .addKeyValue("my-int", 123)
                .addKeyValue("my-double", Math.PI)
                .addKeyValue("my-name", "abcdefg")
                .log();
    }
}

produces:

{
  "sequenceNumber": 0,
  "timestamp": 1712366291199,
  "nanoseconds": 199243000,
  "level": "INFO",
  "threadName": "main",
  "loggerName": "test",
  "context": {
    "name": "default",
    "birthdate": 1712366291102,
    "properties": {}
  },
  "mdc": {},
  "kvpList": [
    {
      "my-flag": "true"
    },
    {
      "my-int": "123"
    },
    {
      "my-double": "3.141592653589793"
    },
    {
      "my-name": "abcdefg"
    }
  ],
  "message": "null",
  "throwable": null
}

I'm getting the required dependencies via Spring Boot:

[INFO] +- org.springframework.boot:spring-boot-starter-web:jar:3.2.0-RC1:compile
[INFO] |  +- org.springframework.boot:spring-boot-starter:jar:3.2.0-RC1:compile
[INFO] |  |  +- org.springframework.boot:spring-boot-starter-logging:jar:3.2.0-RC1:compile
[INFO] |  |  |  +- ch.qos.logback:logback-classic:jar:1.4.11:compile
[INFO] |  |  |  |  \- ch.qos.logback:logback-core:jar:1.4.11:compile
[INFO] |  |  |  +- org.apache.logging.log4j:log4j-to-slf4j:jar:2.21.0:compile
[INFO] |  |  |  |  \- org.apache.logging.log4j:log4j-api:jar:2.21.0:compile
[INFO] |  |  |  \- org.slf4j:jul-to-slf4j:jar:2.0.9:compile

This solution logs all values as JSON strings, even if there's a primitive representation available. The class ch.qos.logback.classic.encoder.JsonEncoder doesn't lend itself to extension, but you can copy it and modify it to log numbers and booleans naturally. (The code below works but has not been tested much)

Add two methods:

    private void appenderMemberUnquotedValue(StringBuilder sb, String key, String value) {
        sb.append(QUOTE).append(key).append(QUOTE_COL).append(value);
    }

    String toJson(Object o) {
        return switch (o) {
            case String s -> "\"" + jsonEscape(s) + "\"";
            case Boolean b -> b.toString();
            case Number n -> n.toString();
            case null -> "null";
            default -> "\"" + jsonEscapedToString(o) + "\"";
        };
    }

And modify one line in this method:

    private void appendKeyValuePairs(StringBuilder sb, ILoggingEvent event) {
        List<KeyValuePair> kvpList = event.getKeyValuePairs();
        if (kvpList == null || kvpList.isEmpty())
            return;

        sb.append(QUOTE).append(KEY_VALUE_PAIRS_ATTR_NAME).append(QUOTE_COL).append(SP).append(OPEN_ARRAY);
        final int len = kvpList.size();
        for (int i = 0; i < len; i++) {
            if (i != 0)
                sb.append(VALUE_SEPARATOR);
            KeyValuePair kvp = kvpList.get(i);
            sb.append(OPEN_OBJ);
//             appenderMember(sb, jsonEscapedToString(kvp.key), jsonEscapedToString(kvp.value)); // OLD

            appenderMemberUnquotedValue(sb, jsonEscapedToString(kvp.key), toJson(kvp.value)); // NEW
            sb.append(CLOSE_OBJ);
        }
        sb.append(CLOSE_ARRAY);
        sb.append(VALUE_SEPARATOR);
    }

that's great. One flaw is the extra key-values are all logged as JSON strings, and not JSON booleans or JSON numerics.

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