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Ninja uses the Logback via slf4j as logging library.

Configuration of Logback inside Ninja

Usually you want to use different logging settings when running in test, dev or on production.

There are three main ways how you can configure Logback.

#1 Using logback.xml

By default Logback will look for a file called logback.xml in the root of your application. If it finds one it will intialize the logging system accordingly. This approach is okay for simple setups.

#2 Using Java system property to specify link to logback.xml

Logback evaluates a Java system property named logback.configurationFile. This approach is handy if you are using Ninja’s standalone mode:

java -Dlogback.configurationFile=/srv/conf/logback.xml -jar ninja-application.jar

This allows you to use one logging configuration for all your instances. More about that approach here:

#3 Using application.conf to switch configurations

If you cannot or do not want to use external Java system properties you can use application.conf and the key logback.configurationFile to specify the location of the logging file. Ninja will pick up the file and configure Logback accordingly.

# An example for application.conf based configuration of logback
%prod.logback.configurationFile=logback_prod.xml   # will be used in production
%dev.logback.configurationFile=logback_dev.xml     # will be used in dev mode

Ninja will look for specified files in three places in the following order:

  • On the classpath
  • On the filesystem
  • and if the “file” is a http url it will try to load the configuration from there.
There is one tiny drawback with using application.conf to configure logging: Logging can start before Ninja starts up. Think of Jetty starting up and once startup is finished Ninja begins to load. Jetty already might write some log messages, but these are not configured by Ninja's applications.conf. In that early phase it might be the case that Logback uses a default configuration that prints out messages to System.out. Using the Java system property -Dlogback.configurationFile to configure logging will always override all settings in application.conf. This also configures logging from the very beginning.

Quick intro to Logback

Ninja provides you with a simple foundation to use Logback. And this is perfect, because logback has everything you need to configure logging - even for the largest systems you can image.

The best way to configure Logback is to follow the excellent guide at:

But just in case you were wondering how such a logback.xml file can look like:


  <appender name="FILE" class="ch.qos.logback.core.FileAppender">

      <pattern>%date %level [%thread] %logger{10} [%file:%line] %msg%n</pattern>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">

  <root level="debug">
    <appender-ref ref="FILE" />
    <appender-ref ref="STDOUT" />

By the way - you can also write logback configuration files in Groovy.