Logging in Spring Boot

By default Spring Boot includes logback and slf4j dependencies, so any application can use SLF4J libraries for logging. Default configuration is to log output in console.
Code reference: https://github.com/GlueCoders/springboot-guide/releases/tag/logging

Output to file

To output in a file add following line to the application.properties in src/main/resources folder.

logging.file=custom.log

Log file rotate automatically after 10mb is reached.

Log Levels

To control log levels, it can be done by adding package controls in application.properties as shown below.

logging.level.root=WARN
logging.level.org.springframework.web=INFO
logging.level.org.gluecodders.library=DEBUG

Log Patterns

logging.pattern.file for logging pattern for file appenders. Similarly logging.pattern.console for logging pattern for console appenders.

Log4j2

Let’s include log4j2 in the project and look at the available configurations.

POM Dependency

   <dependency>
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter</artifactId>
       <version>${spring.version}</version>
       <exclusions>
            <exclusion>
               <groupId>org.springframework.boot</groupId>
               <artifactId>spring-boot-starter-logging</artifactId>
           </exclusion>
       </exclusions>
   </dependency>
   <dependency>
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter-log4j2</artifactId>
       <version>${spring.version}</version>
   </dependency>

Basic Configuration

Log4j2 supports xml, yaml, json and properties configuration. For verbosity let’s explore XML configuration.

For simple console output below xml will work fine.

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
    <Appenders>
        <Console name="Console" target="SYSTEM_OUT">
            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
        </Console>
    </Appenders>
    <Loggers>
        <logger name="org.gluecoders.library" level="debug" additivity="false">
            <AppenderRef ref="Console"/>
        </logger>
        <Root level="info">
            <AppenderRef ref="Console"/>
        </Root>
    </Loggers>
</Configuration>

This configuration defines two loggers one is RootLogger and other is for application’s package org.gluecoders.library. Only console appender is declared here, so all the logs will be logged to console output.

JSON Layout Configuration

<Appenders>
   <Console name="Console" target="SYSTEM_OUT">
      <JsonLayout compact="true" eventEol="true"/>
   </Console>
</Appenders>  

This will log statements in json format which might be useful if the log output goes to analytics platform like Elasticsearch.

File Appender Configuration

<File  name="File" fileName="app.log">
   <JsonLayout compact="true" eventEol="true"/>
</File>

Rolling File Appendr Configuration

<RollingFile name="Rolling" fileName="app.log" filePattern="target/rolling1/app.%i.log.gz">
   <JsonLayout compact="true" eventEol="true"/>
   <SizeBasedTriggeringPolicy size="500" />
</RollingFile>

Configuring logging for different packages

<logger name="org.springframework" level="error" additivity="false">
   <AppenderRef ref="Console"/>
</logger>

To control logging of third party libraries and other packages, different logger configurations can be added as shown above.

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