How to set Log4j2 level from command line - spring

Question

I build a Spring Boot Web Service application and I am using Log4j2.
I want to set log level by passing something like: --log4j2.level=DEBUG - How can I do that?
I didn't found any documentation for it.
I know that I can set the log4j configuration file by --logging.config=/path/to/file but this is not what I want.
My pom.xml:
<?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>2.2.4.RELEASE</version>
<relativePath/>
</parent>
<groupId>...</groupId>
<artifactId>...</artifactId>
<version>0.0.1-SNAPSHOT</version>
<name>...</name>
<description>...</description>
<properties>
<java.version>1.8</java.version>
</properties>
<dependencies>
<dependency>
<groupId>com.fasterxml.jackson.dataformat</groupId>
<artifactId>jackson-dataformat-yaml</artifactId>
<version>2.9.8</version>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.9.8</version>
</dependency>
<dependency>
<groupId>org.yaml</groupId>
<artifactId>snakeyaml</artifactId>
<version>1.25</version>
</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>io.swagger</groupId>
<artifactId>swagger-annotations</artifactId>
<version>1.6.1</version>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.18.12</version>
<scope>provided</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>
</project>
My main class looks like:
#SpringBootApplication
#Log4j2
public class MyApplication {
public static void main(String[] args) {
log.debug("Test something");
SpringApplication.run(HlsManipulatorApplication.class, args);
}
}
Thanks for help!

Answer 1

Spring Boot applications initialize logging at least 3 times. The first is in SpringApplication when it creates a logger, which is literally the first thing that happens. If you are using Log4j 2 this will cause it to use its normal mechanism to find a log4j2.xml, log4j2.properties or log4j2.yml file. Spring boot provides a default configuration but as you know you can override it. If you do you can specify the root logger as
<Root level="${sys:root_log_level:-INFO}">
...
</Root>
This will allow you to use -Droot_log_level=DEBUG on the command line. If the system property is not configured it will default to INFO.
Note that you can also specify an alternate location for the logging configuration by a) specifying the location of the file in bootstrap.yml as
logging:
config: log4j2-myapp.xml
and then b) in a file named log4j2.application.properties as
log4j.configurationFile=log4j-mmyapp.xml
Doing this will cause the same configuration to be used each time Log4j reconfigures, including the initial configuration. You can see an example of this in Log4j's sample Spring Boot application where it retrieves the logging configuration from Spring Cloud Config.
Of course, you can utilize Spring's mechanism to set the logging level but that will not impact the first time logging is configured since Spring hasn't done anything at that point.
I should also point out that Log4j comes with a SpringLookup that can be used in Spring applications to reference spring configuration variables in the log4j configuration. Of course, the variables won't have a value the first time logging is configured since the Spring environment won't have been crea