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

After quite a bit of scratching my head, I am settling with the following solution.

First, put logback.xml outside classpath so that logback will not automatically load anything.

Second, add the settings from the environment to system properties so that logback can look them up when parsing logback.xml.

Third, programmatically configure logback in the application code. (The official logback documentation has one nice example of that. )

Done.