How to define a variable in Logback with a default value and override it?

Question

I would like to define a property/variable in Logback (1.2.1) that:

• Has a default value
• Can be overriden via a Java command-line option

Basically, during development, I would like Maven to be invoking the maven-surefire-plugin with something like this:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <configuration>
        <systemPropertyVariables>
            <log.dir>${project.build.directory}/logs</log.dir>
        </systemPropertyVariables>
    </configuration>
</plugin>

(I am sure that the above is working fine, because I have other properties being passed in for the tests this way and those work as expected).

At the moment, I have the following logback.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration>
<configuration scan="true" scanPeriod="30 seconds" debug="false">

    <property name="log.dir" value="."/>

    <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <file>${log.dir}/logs/my.log</file>
        <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
            <fileNamePattern>strongbox_%d{yyyy-MM-dd}.%i.log</fileNamePattern>

            <fileNamePattern>strongbox-%d{yyyy-MM-dd}.%i.log</fileNamePattern>
            <maxFileSize>128MB</maxFileSize>
            <maxHistory>31</maxHistory>
            <totalSizeCap>1GB</totalSizeCap>

            <maxHistory>30</maxHistory>
        </rollingPolicy>

        <encoder>
            <charset>UTF-8</charset>
            <pattern>%d{HH:mm:ss.SSS dd-MM-yyyy} | %-5.5p | %-20.20t | %-50.50logger{50} | %m%n</pattern>
        </encoder>
    </appender>
    ...
</configuration>

The -Dlog.dir=foo/logs, it is simply being ignored and the log file is being produced in the current directory. What am I doing wrong here? Does it need a scope? Does it need an if condition to be set up?

Answer 1

If you need a variable, say called log.file.root, which should default to a value app-logs, define it as:

<property name="LOG_ROOT" value="${log.file.root:-app-logs}" />

Then use ${LOG_ROOT} wherever you need to.

This variable can be overridden via the command line by:

-Dlog.file.root=/home/user/logs

Reference: https://logback.qos.ch/manual/configuration.html -> "Default values for variables"

Answer 2

You specify the property for the maven-surefire-plugin.
So the property will be bound only for this plugin execution :

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <configuration>
        <systemPropertyVariables>
            <log.dir>${project.build.directory}/logs</log.dir>
        </systemPropertyVariables>
    </configuration>
</plugin>

In your case, you want that the property be bound whatever the executed plugin. Not only for test executions.

So you should use properties declared in the <build> tag of the pom.

<build>
      ...
  <properties>
     <log.dir>${project.build.directory}/logs</log.dir>
  </properties>
      ...
</build>

In order that the Maven property used in the logback configuration be replaced by the actual value computed from the pom, you have to enable the Maven resources filtering :

  ...
  <resource>
    <directory>src/main/resources</directory>
    <filtering>true</filtering>
  </resource>
  ...

And the Logback configuration has to be located inside the src/main/resources folder of course (this folder or a child).