SivaLabs

Logging is a common and important requirement for running applications in production. Spring Boot provides great support for application logging out of the box and offers various customization options. In this tutorial, you will learn how to implement logging in your Spring Boot application using Logback and Log4j2.

Spring Boot Default Logging Support

When you create a Spring Boot application with any starter added, they have a dependency on spring-boot-starter which in turn depends on spring-boot-starter-logging which transitively add logback dependencies for logging.

Spring Boot default logging configuration is already configured in spring-boot-starter with CONSOLE appender. You can view the default configuration files (defaults.xml, base.xml, console-appender.xml, file-appender.xml) packaged in spring-boot.jar under the package org/springframework/boot/logging/logback.

So, by default Spring Boot is configured with Slf4j and Logback for logging and you can start logging in your application as follows:

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

@Service
class CustomerService {
    private static final Logger log = LoggerFactory.getLogger(CustomerService.class);
    
    public Customer findById(Long id) {
        log.info("Fetching customer by id: {}", id);
        ...
    }
}

This will produce a INFO level log entry in your CONSOLE similar to the following:

2023-07-26T19:44:04.492+05:30  INFO 69881 --- [nio-8080-exec-1] c.s.demo.CustomerService      : Fetching customer by id: : 1

Customizing Log levels

By default, Spring Boot is configured with root log level set to INFO. You can configure the log level for root or for any specific package in src/main/resources/application.properties as follows:

logging.level.root=WARN
logging.level.com.sivalabs=DEBUG
logging.level.org.springframework=INFO
logging.level.org.hibernate.validator=DEBUG

Enabling File Appender

Spring Boot already configured RollingFileAppender, but is disabled by default. You can enable File Appender by simply configuring either logging.file.name or logging.file.path property in application.properties as follows:

# relative to the current directory
logging.file.name=product-service.log

# absolute path
logging.file.name=/var/log/product-service.log

# write logs to /var/log/product-service/spring.log file
logging.file.path=/var/log/product-service

Note that when you configure logging.file.name or logging.file.path properties, both Console and File Appenders are enabled. If you want to disable Console appender and enable only File appender, or you want to have more control over logback configuration, you can create src/main/resources/logback.xml and configure the logging as per your needs.

Custom Logback configuration

If the Spring Boot logging customization options via logging.* properties are not sufficient, you can create your own src/main/resources/logback.xml file and Spring Boot will configure logging according to this configuration.

However, Spring Boot provides some logback extensions which helps you to use advanced configuration options. In order to use these extensions, create the configuration file with name logback-spring.xml instead of logback.xml.

A sample logback-spring.xml file looks as follows:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <include resource="org/springframework/boot/logging/logback/defaults.xml" />
    <include resource="org/springframework/boot/logging/logback/console-appender.xml" />

    <root level="INFO">
        <appender-ref ref="FILE" />
    </root>
    <logger name="org.springframework" level="WARN"/>
    <logger name="com.sivalabs" level="DEBUG"/>

</configuration>

Spring Boot Logback extensions comes handy if you want to use any Spring Boot property or customize logging based on Spring profiles.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <springProperty scope="context" 
                    name="logstashHost" 
                    source="logstash.host"
                    defaultValue="localhost"/>
    <include resource="org/springframework/boot/logging/logback/defaults.xml" />

    <springProfile name="docker">
        <include resource="org/springframework/boot/logging/logback/file-appender.xml" />
        <property name="LOG_FILE" value="/var/logs/myapp.log"/>

        <appender name="logstash" class="net.logstash.logback.appender.LogstashTcpSocketAppender">
            <destination>${logstashHost}</destination>
            ...
            ...
        </appender>
        
        <root level="INFO">
            <appender-ref ref="FILE" />
            <appender-ref ref="logstash" />
        </root>
    </springProfile>
    
    <springProfile name="!docker">
        <include resource="org/springframework/boot/logging/logback/console-appender.xml" />
        <root level="INFO">
            <appender-ref ref="CONSOLE" />
        </root>
    </springProfile>
    
    <logger name="com.sivalabs" level="DEBUG"/>
    <logger name="org.springframework" level="INFO"/>

</configuration>

In the above configuration, we have enabled appenders based on Spring profile using <springProfile name="...">. If docker profile is enabled then we are using FILE, Logstash appenders. If the docker profile is not activated then we are using CONSOLE appender only. Also, notice that we are using <springProperty …/> to use the logstash.host property value to specify the Logstash host.

Using Log4j2 instead of Logback

If you prefer to use Log4j2 instead of Logback, you can exclude spring-boot-starter-logging from spring-boot-starter and include spring-boot-starter-log4j2 in pom.xml as follows.

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</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-log4j2</artifactId>
</dependency>

In case you are using Gradle, you can configure to use log4j2 instead of logback as follows:

configurations.all {
	exclude group: 'org.springframework.boot', module: 'spring-boot-starter-logging'
}

dependencies {
    implementation('org.springframework.boot:spring-boot-starter-log4j2')
    ...
    ...
}

Spring Boot by default includes log4j2.xml configuration file in org/springframework/boot/logging/log4j2/log4j2.xml. But if you want to provide a customized configuration, then you can create src/main/resources/log4j2-spring.xml as follows:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
    <Properties>
        <Property name="LOG_PATTERN">
            %d{yyyy-MM-dd HH:mm:ss.SSS} %-5p %c{1}:%L - %m%n
        </Property>
    </Properties>
    <Appenders>
        <Console name="Console" target="SYSTEM_OUT" follow="true">
            <PatternLayout pattern="${LOG_PATTERN}"/>
        </Console>
    </Appenders>
    <Loggers>
        <Logger name="com.sivalabs" level="debug" additivity="false">
            <AppenderRef ref="Console" />
        </Logger>

        <Root level="info">
            <AppenderRef ref="Console" />
        </Root>
    </Loggers>
</Configuration>

If you use slf4j API to create logger objects, then you don’t have to make any code changes. The slf4j API will delegate the actual logging to log4j2.

Summary

Spring Boot makes it easy to implement logging by providing the default configuration with customization options. In this tutorial, we have learned about Spring Boot default logging support using Slf4J and Logback and how to customize it. Then we have explored how to use Log4j2 instead of default Logback implementation.