Spring Boot Logging Example

By default, if you use the starters, Logback is used for logging

https://docs.spring.io/spring-boot/reference/features/logging.html

Table of contents:

• 1. Project Directory
• 2. Spring Boot Logging Dependencies
• 3. Spring Boot Logging using Logback
  • 3.1. Log Example
  • 3.2 Log variables
• 4. Log to File
  • 4.1 File Rotation
• 5. Log Levels
• 6. Native logback.xml
• 7. logback-spring.xml
  • 7.1 Spring Boot Profile-Specific Logging
• 8. The order of application.properties, logback.xml, and logback-spring.xml?
• 9. So, which one for Spring Boot Logback?
• 10. Download Source Code
• 11. References

1. Project Directory

2. Spring Boot Logging Dependencies

In Spring Boot, the dependency spring-boot-starter-logging includes the logging frameworks. Since many Spring Boot starters include the spring-boot-starter-logging automatically, we unlikely need to add it manually.

For example, adding the dependency web spring-boot-starter-web will automatically include the spring-boot-starter-logging.

mvn dependency:tree

[INFO] +- org.springframework.boot:spring-boot-starter-web:jar:3.2.2:compile
[INFO] |  +- org.springframework.boot:spring-boot-starter:jar:3.2.2:compile
[INFO] |  |  +- org.springframework.boot:spring-boot:jar:3.2.2:compile
[INFO] |  |  +- org.springframework.boot:spring-boot-autoconfigure:jar:3.2.2:compile
[INFO] |  |  +- org.springframework.boot:spring-boot-starter-logging:jar:3.2.2:compile
[INFO] |  |  |  +- ch.qos.logback:logback-classic:jar:1.4.14:compile
[INFO] |  |  |  |  \- ch.qos.logback:logback-core:jar:1.4.14:compile
[INFO] |  |  |  +- org.apache.logging.log4j:log4j-to-slf4j:jar:2.21.1:compile
[INFO] |  |  |  |  \- org.apache.logging.log4j:log4j-api:jar:2.21.1:compile
[INFO] |  |  |  \- org.slf4j:jul-to-slf4j:jar:2.0.11:compile
[INFO] |  |  +- jakarta.annotation:jakarta.annotation-api:jar:2.1.1:compile
[INFO] |  |  \- org.yaml:snakeyaml:jar:2.2:compile

<?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
         http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <artifactId>spring-boot-slf4j</artifactId>
    <packaging>jar</packaging>
    <name>Spring Boot Logging SLF4j</name>
    <version>1.0</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.2.2</version>
        <relativePath/> <!-- lookup parent from repository, not local -->
    </parent>

    <properties>
        <java.version>17</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.11.0</version>
                <configuration>
                    <source>${java.version}</source>
                    <target>${java.version}</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

3. Spring Boot Logging using Logback

By default, Spring Boot uses Logback for the logging, and the loggers are pre-configured to use console output with optional file output.

3.1. Log Example

We write a simple Spring Boot REST endpoint / and review the default logging to the console output.

package com.mkyong;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloController {

    // Get the SLF4J logger interface, default Logback, a SLF4J implementation
    private static final Logger logger = LoggerFactory.getLogger(HelloController.class);

    @GetMapping("/")
    public String hello() {

        // log to console (default)

        logger.debug("Debug level - Hello Logback");

        logger.info("Info level - Hello Logback");
        
        logger.error("Error level - Hello Logback");

        return "Hello SLF4J";
    }

}

package com.mkyong;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class MainApplication {

	public static void main(String[] args) {
		SpringApplication.run(MainApplication.class, args);
	}

}

Run the Spring Boot application.

mvn spring-boot:run 

Test the Spring Boot endpoints /.

curl localhost:8080

Review the default log format in the console output:

2024-01-29T18:36:08.564+08:00  INFO 19361 --- [nio-8080-exec-1] com.mkyong.HelloController  : Info level - Hello Logback
2024-01-29T18:36:08.564+08:00 ERROR 19361 --- [nio-8080-exec-1] com.mkyong.HelloController  : Error level - Hello Logback

3.2 Log variables

We can use {} to log the variables.

package com.mkyong;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloController {

    // Get the SLF4J logger interface, default Logback, a SLF4J implementation
    private static final Logger logger = LoggerFactory.getLogger(HelloController.class);

    // Log with variable
    @GetMapping("/hello/{name}")
    String find(@PathVariable String name) {

        logger.debug("Debug level - Hello Logback {}", name);

        logger.info("Info level - Hello Logback {}", name);
        
        logger.error("Error level - Hello Logback {}", name);

        return "Hello SLF4J" + name;

    }

}

Run the Spring Boot application and access the the endpoints /hello/{name}.

curl localhost:8080/hello/mkyong

Output.

2024-01-30T13:24:20.682+08:00  INFO  ... com.mkyong.HelloController : Info level - Hello Logback mkyong
2024-01-30T13:24:20.684+08:00 ERROR  ... com.mkyong.HelloController : Error level - Hello Logback mkyong

4. Log to File

By default, Spring Boot does not write logs to files; If we want to write to log files, we need to provide logging.file.name or logging.file.path property in the application.properties.

P.S. Log files rotate when they reach 10 MB

logging.file.name=logs/app.log

4.1 File Rotation

For Logback, we can configure the file rotation directly in the application.properties.

The configuration file below will rotate or roll based on file size and date time.

logging.file.name=logs/app.log

#The filename pattern used to create log archives.
logging.logback.rollingpolicy.file-name-pattern=logs/%d{yyyy-MM, aux}/app.%d{yyyy-MM-dd}.%i.log

#The maximum size of log file before it is archived.
logging.logback.rollingpolicy.max-file-size=100MB

#The maximum amount of size log archives can take before being deleted.
logging.logback.rollingpolicy.total-size-cap=10GB

#The maximum number of archive log files to keep (defaults to 7).
logging.logback.rollingpolicy.max-history=10

5. Log Levels

We can define the log levels in the application.properties.

Please review the log levels below and their defined order.

TRACE
DEBUG
INFO
WARN
ERROR
OFF

• When we set the log level to INFO (default), it logs the INFO, WARN, and ERROR log events.
• When we set the log level to DEBUG, it logs the DEBUG, INFO, WARN, and ERROR log events.
• When we set the log level to ERROR, it logs only the ERROR log events.

    // log event DEBUG
    logger.debug("Debug level - Hello Logback {}", name);

    // log event INFO
    logger.info("Info level - Hello Logback {}", name);
    
    // log event ERROR
    logger.error("Error level - Hello Logback {}", name);

Spring Boot Logback supports package-level logging.

# root level
logging.level.root=error

# package level logging
logging.level.org.springframework.web=debug
logging.level.org.hibernate=error
logging.level.com.mkyong=error

For example, when we set the logging.level.org.springframework.web=debug, it logs the log events DEBUG, INFO, WARN and ERROR under the package org.springframework.web.

6. Native logback.xml

Spring Boot supports the native [logback.xml(https://logback.qos.ch/manual/index.html) configuration file; put the logback.xml in the resources folder, and Spring Boot will auto-detect it.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>

    <property name="HOME_LOG" value="logs/app.log"/>

    <appender name="FILE-ROLLING" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <file>${HOME_LOG}</file>

        <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
            <fileNamePattern>logs/%d{yyyy-MM, aux}/app.%d{yyyy-MM-dd}.%i.log</fileNamePattern>
            <!-- each archived file, size max 10MB -->
            <maxFileSize>10MB</maxFileSize>
            <!-- total size of all archive files, if total size > 20GB, it will delete old archived file -->
            <totalSizeCap>20GB</totalSizeCap>
            <!-- 60 days to keep -->
            <maxHistory>60</maxHistory>
        </rollingPolicy>

        <encoder>
            <pattern>%d %p %c{1.} [%t] %m%n</pattern>
        </encoder>
    </appender>

    <logger name="com.mkyong" level="debug" additivity="false">
        <appender-ref ref="FILE-ROLLING"/>
    </logger>

    <root level="error">
        <appender-ref ref="FILE-ROLLING"/>
    </root>

</configuration>

7. logback-spring.xml

For Spring Boot Logbak extensions, use the logback-spring.xml configuration file. Create logback-spring.xml and put it under the resources folder.

7.1 Spring Boot Profile-Specific Logging

Below is a Spring Boot profile-specific logging example:

• If no active profile (default), or active profiles are dev, staging, or test, logs to the console.
• If the profile is production, logs to rolling files.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <include resource="org/springframework/boot/logging/logback/defaults.xml" />
    
    <springProfile name="default | dev | staging | test">
        <include resource="org/springframework/boot/logging/logback/console-appender.xml"/>
        <root level="INFO">
            <appender-ref ref="CONSOLE"/>
        </root>
    </springProfile>

    <springProfile name="production">
        <appender name="FILE-ROLLING" class="ch.qos.logback.core.rolling.RollingFileAppender">
            <file>/Users/mkyong/logs/prod.log</file>

            <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
                <fileNamePattern>/Users/mkyong/logs/%d{yyyy-MM, aux}/prod.%d{yyyy-MM-dd}.%i.log</fileNamePattern>
                <!-- each archived file, size max 10MB -->
                <maxFileSize>10MB</maxFileSize>
                <!-- total size of all archive files, if total size > 20GB, it will delete old archived file -->
                <totalSizeCap>20GB</totalSizeCap>
                <!-- 60 days to keep -->
                <maxHistory>60</maxHistory>
            </rollingPolicy>

            <encoder>
                <pattern>%d %p %c [%t] %m%n</pattern>
            </encoder>
        </appender>

        <logger name="org.springframework.web" level="DEBUG"/>
        <logger name="com.mkyong" level="DEBUG"/>
        <root level="ERROR">
            <appender-ref ref="FILE-ROLLING"/>
        </root>
    </springProfile>

</configuration>

8. The order of application.properties, logback.xml, and logback-spring.xml?

In Spring Boot, we can use different configuration files, such as application.properties, logback.xml, and logback-spring.xml.

Spring Boot defined the order like this:

1. logback-spring.xml has the highest priority; it supports Spring Profiles.
2. If the logback-spring.xml is absent, Spring Boot looks for logback.xml, the standard configuration file Logback uses.
3. Both application.properties and application.yml are used for general Spring Boot configuration. If logback-spring.xml or logback.xml are present, they take precedence, and the settings in application.properties or application.yml supplement or override certain logging configurations (like log levels).

9. So, which one for Spring Boot Logback?

• Use logback-spring.xml to support Spring Profiles or the profile-specific logging features.
• Use logback.xml if there is no need to use the Spring profile.
• Use application.properties for a simple console or file logging.

10. Download Source Code

11. References

• Spring Boot Logging
• Logback Project
• SLF4J Logback Tutorial
• logback.xml Examples
• Logback – RollingFileAppender
• common application properties