Logback support variable substitution functionality in the configuration file.
How to define a variable in logback.xml file?
<property name="myVar" value="valueOfTheVar" />
How to refer the value of variable myVar?
${myVar}
Example
<configuration scan="true" scanPeriod="15 seconds">
<property name="LOG_FILES_HOME" value="/Users/Shared/logback" />
<appender name="FILE" class="ch.qos.logback.core.FileAppender">
<file>${LOG_FILES_HOME}/myApp.log</file>
<encoder>
<pattern>%date %level [%thread] %logger{10} [%file:%line] %msg%n
</pattern>
</encoder>
</appender>
.......
.......
</configuration>
As you see in the above snippet, I set the variable "LOG_FILES_HOME" and using it in FILE appender.
Let’s see it with an example.
Step 1: Create new maven project ‘logback-variable-substitution’.
Step 2: Update pom.xml with maven dependencies.
pom.xml
<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>
<groupId>com.sample.app</groupId>
<artifactId>logback-variable-substitution</artifactId>
<version>1</version>
<dependencies>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.2.10</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-core</artifactId>
<version>1.2.10</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.36</version>
</dependency>
</dependencies>
</project>
Step 3: Create logback.xml file under src/main/resources folder.
logback.xml
<configuration scan="true" scanPeriod="15 seconds">
<property name="LOG_FILES_HOME" value="/Users/Shared/logback" />
<appender name="FILE" class="ch.qos.logback.core.FileAppender">
<file>${LOG_FILES_HOME}/myApp.log</file>
<encoder>
<pattern>%date %level [%thread] %logger{10} [%file:%line] %msg%n
</pattern>
</encoder>
</appender>
<appender name="STDOUT"
class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
</pattern>
</encoder>
</appender>
<logger name="com.sample.app" level="WARN">
<appender-ref ref="FILE" />
</logger>
<root level="INFO">
<appender-ref ref="STDOUT" />
</root>
</configuration>
Step 4: Define HelloWorld class.
HelloWorld.java
package com.sample.app;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class HelloWorld {
private static Logger logger = LoggerFactory.getLogger(HelloWorld.class);
public static void main(String[] args) {
logger.trace("Trace message");
logger.debug("Debug message");
logger.info("Info message");
logger.warn("Warning message");
logger.error("Error message");
}
}
Total project structure looks like below.
Run HelloWorld.java, you will see below messages in console.
15:46:38.568 [main] WARN com.sample.app.HelloWorld - Warning message 15:46:38.570 [main] ERROR com.sample.app.HelloWorld - Error message
You can observe the same in myApp.log file.
$pwd
/Users/Shared/logback
$
$cat myApp.log
2022-02-22 15:46:38,568 WARN [main] c.s.a.HelloWorld [HelloWorld.java:14] Warning message
2022-02-22 15:46:38,570 ERROR [main] c.s.a.HelloWorld [HelloWorld.java:15] Error message
You can even set the variable by setting the system property like below.
java -DLOG_FILES_HOME="/Users/Shared/logback" HelloWorld
You can download complete working application from below link.
https://github.com/harikrishna553/java-libs/tree/master/logback/logback-variable-substitution
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