Print Logs Output as JSON String in Spring Boot Java — Log4j2

Assumptions:

You already have spring boot project. If you don’t, take the tutorial — https://spring.io/guides/gs/spring-boot/

If you have spring boot project, your pom.xml would be something like the following:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>
   <parent>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-parent</artifactId>
      <version>2.2.6.RELEASE</version>
      <relativePath/> 
   </parent>
   <groupId>com.vb.math</groupId>
   <artifactId>learnit</artifactId>
   <version>0.0.1-SNAPSHOT</version>
   <name>learnit</name>
   <description>Demo project for Spring Boot</description>

   <properties>
      <java.version>1.8</java.version>
   </properties>

   <dependencies>
      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-actuator</artifactId>
      </dependency>
      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-jdbc</artifactId>
      </dependency>
      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-web</artifactId>
      </dependency>

Spring boot comes default packaged with logging. Refer here — https://docs.spring.io/spring-boot/docs/2.1.x/reference/html/boot-features-logging.html

The format of the default log is:

INFO 45469 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/7.0.52
2019-03-05 10:57:51.253  INFO 45469 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2019-03-05 10:57:51.253  INFO 45469 --- [ost-startStop-1] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1358 ms
2019-03-05 10:57:51.698  INFO 45469 --- [ost-startStop-1] o.s.b.c.e.ServletRegistrationBean        : Mapping servlet: 'dispatcherServlet' to [/]
2019-03-05 10:57:51.702  INFO 45469 --- [ost-startStop-1] o.s.b.c.embedded.FilterRegistrationBean

You want JSON log out put.

We will achieve that using log4j2.

Step 1:

Include the log4j2 spring boot dependency and exclude the any version conflicts of including common libs at multiple places.

<!-- New Dependencies and Exclusions -->
<dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.10.3</version>
    <exclusions>
            <exclusion>
                <groupId>com.fasterxml.jackson.core</groupId>
                <artifactId>jackson-core</artifactId>
            </exclusion>
            <exclusion>
                <groupId>com.fasterxml.jackson.core</groupId>
                <artifactId>jackson-annotations</artifactId>
            </exclusion>
     </exclusions>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.10.3</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-annotations</artifactId>
    <version>2.10.3</version>
</dependency>
<!-- Exclusions in case you need -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <exclusions>
            <exclusion>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-starter-logging</artifactId>
            </exclusion>
        </exclusions>
</dependency>
         
         
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-jdbc</artifactId>
    <exclusions>
            <exclusion>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-starter-logging</artifactId>
            </exclusion>
        </exclusions>
</dependency>

Step 2:

If you have been using any other logs please check and remove those also using exclusions as above. Check dependencies using the following and remove them:

mvn dependency:tree -Dincludes=*log*

Step 3:

Create a file with name log4j2-spring.xml with the following content and add to your resources folder.

<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
    <Appenders>
        <Console name="Console" target="SYSTEM_OUT">
            <JsonLayout complete="false"  compact="true" eventEol="true"></JsonLayout>
            <PatternLayout
                        pattern="%style{%d{ISO8601}}{black} %highlight{%-5level }[%style{%t}{bright,blue}] %style{%C{1.}}{bright,yellow}: %msg%n%throwable" />
        </Console>
    </Appenders>
    <Loggers>
        <!-- LOG everything at INFO level -->
        <Root level="info">
            <AppenderRef ref="Console" />
        </Root>
        <Logger name="com.vb" level="trace"></Logger>
    </Loggers>
</Configuration>

You have to replace the italics. These are the pattern of the log and your packages where you want your logger to be available.

Step 4:

Use your logger.

import org.slf4j.Logger;
import org.slf4j.LoggerFactory; 
// Instantiate your logger
private static Logger logger - LoggerFactory.getLogger(MyClassName.class);
//Print the logs
logger.debug(" Debug Message");
logger.error(" Error Message");

The output would be something like the following:

{
  "thread": "main",
  "level": "INFO",
  "loggerName": "com.vb.math.learnit.LearnitApplication",
  "message": "Started LearnitApplication in 2.508 seconds (JVM running for 4.586)",
  "endOfBatch": false,
  "loggerFqcn": "org.apache.commons.logging.LogAdapter$Log4jLog",
  "instant": {
    "epochSecond": 1597070439,
    "nanoOfSecond": 853000000
  },
  "threadId": 1,
  "threadPriority": 5
}