how to define logback variables/properties before logback auto-load logback.xml?

Question

My company has an environment management tool that enables you to look up properties from the environment programmatically in Java. I want to leverage this tool to configure logback. For example, suppose I have a logback.xml as follows (the file appender section in particular):

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <!-- console appender -->
    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{yyyy-MM-dd/HH:mm:ss.SSS} [%thread] %-5level %logger{20}: %msg%n</pattern>
        </encoder>
    </appender>

    <!-- file appender -->
    <appender name="FILE" class="ch.qos.logback.core.FileAppender">
        <file>${LOG_FILE:-/default/log/file/path</file>
        <encoder>
            <pattern>%d{yyyy-MM-dd/HH:mm:ss.SSS} [%thread] %-5level %logger{20}: %msg%n</pattern>
        </encoder>
    </appender>

    <root level="DEBUG">
        <appender-ref ref="CONSOLE" />
        <appender-ref ref="FILE" />
    </root>
</configuration>

So, in this case, I want to look up the LOG_FILE property from the environment (or OS, if you will), and pass it to logback before logback loads the logback.xml so that it will know the value of LOG_FILE. So, how can I achieve that? BTW, I know how to define a file appender programmatically, but that's not what I want here.

Thank you very much.

Answer 1

Define a property in logback.xml and load it into the "context":

<property scope="context" name="logfolder" value="${location.of.the.log.folder}" />

Then define your appender referencing the property:

<appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>${logfolder}/logfile.log</file>
    <append>true</append>
    <encoder>
        <pattern>[%d{ISO8601}] [%p] [%t] [%c] [%m]%n</pattern>
    </encoder>
</appender>

From the documentation:

A property with context scope is inserted into the context and lasts as long as the context or until it is cleared. Once defined, a property in context scope is part of the context. As such, it is available in all logging events, including those sent to remote hosts via serialization.

So the default scope, which is "local" may well be sufficient.