Mastering Java Logging: SLF4J, Logback, and Spring Boot Configuration Guide

1. Logging Frameworks

When developing a large system, you may start with simple System.out.println but soon need a logging framework to record runtime information. Common Java logging frameworks include JUL, JCL, JBoss-logging, Logback, Log4j, Log4j2, and SLF4J.

日志门面（日志的抽象层）

日志实现

JCL（Jakarta Commons Logging） SLF4J（Simple Logging Facade for Java）

jboss-logging

Log4j JUL（java.util.logging） Log4j2

Logback

Select a logging facade (e.g., SLF4J) on the left and a concrete implementation (e.g., Logback) on the right.

2. Using SLF4J

Import the SLF4J API and a concrete implementation such as Logback, then obtain a logger via LoggerFactory.getLogger(...) and log at various levels.

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class HelloWorld {
    public static void main(String[] args) {
        Logger logger = LoggerFactory.getLogger(HelloWorld.class);
        logger.info("Hello World");
    }
}

Each implementation has its own configuration file; after switching to SLF4J, the configuration file belongs to the chosen implementation.

3. Spring Boot Logging Relationship

Spring Boot uses SLF4J and Logback by default. Adding spring-boot-starter-logging brings these dependencies. Spring Boot also replaces other logging APIs (Commons-Logging, JBoss-logging, etc.) with SLF4J.

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

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

Key points:

Spring Boot logs via SLF4J + Logback.

Other logging frameworks are automatically routed through SLF4J.

Custom logging configuration can be provided via logback-spring.xml, logback.xml, log4j2-spring.xml, etc.

4. Customizing Log Output

Spring Boot’s default configuration logs to the console at INFO level. You can change the level, file location, and pattern using properties such as logging.level.*, logging.file, logging.path, logging.pattern.console, and logging.pattern.file.

logging.level.com.atguigu=trace
logging.path=/spring/log
logging.pattern.console=%d{yyyy-MM-dd} [%thread] %-5level %logger{50} - %msg%n
logging.pattern.file=%d{yyyy-MM-dd} === [%thread] === %-5level === %logger{50} ==== %msg%n

logging.file

logging.path

Example

Description

(none)

(none)

Console only

my.log

(none)

my.log

Write to my.log file

(none)

/var/log

/var/log

Write to spring.log in the specified directory

5. Switching Logging Implementations

To replace Logback with Log4j2, exclude the default logging starter and add spring-boot-starter-log4j2 as a dependency.

<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-log4j2</artifactId>
</dependency>