# Chapter 3: Logback configuration & Joran

In symbols one observes an advantage in discovery which is greatest when they express the exact nature of a thing briefly and, as it were, picture it; then indeed the labor of thought is wonderfully diminished.

—GOTTFRIED WILHELM LEIBNIZ

Joran stands for a cold north-west wind which, every now and then, blows forcefully on Lake Geneva. Located right in the middle of Europe, the Geneva lake happens to be the continent's largest sweet water reserve.

In the first part, we start by presenting ways for configuring logback, with many example configuration scripts. In the second part, we present Joran, a generic configuration framework, which you can put into use in order to configure your own applications.

## Configuration in logback

Inserting log requests into the application code requires a fair amount of planning and effort. Observation shows that approximately four percent of code is dedicated to logging. Consequently, even moderately sized applications will contain thousands of logging statements embedded within its code. Given their number, we need tools to manage these log statements.

Logback can be configured either programmatically or with configuration a script (expressed in XML format). By the way, existing log4j users can convert their log4j.properties files to logback.xml using our PropertiesTranslator web-application.

Let us begin by discussing the initialization steps that logback follows to try to configure itself:

1. Logback tries to find a file called logback-test.xml in the classpath.

2. If no such file is found, it checks for the file logback.xml.

3. In case neither file is found, logback configures itself automatically using the `BasicConfigurator` which will cause logging output to be directed on the console.

The third and last step is meant to provide a default (but very basic) logging functionnality in the absence of a configuration file.

Assuming the logback-test.xml file is placed under src/test/resources folder, Maven will ensure that it won't be included in the artifact produced. Thus, you can use a different configuration file, namely logback-test.xml during testing, and another file, namely, logback.xml, in production. The same principle applies by analogy for Ant.

### Automatically configuring logback

The simplest way to configure logback is by letting logback fallback to its default configuration. Let us give a taste of how this is done in an imaginary application called `MyApp1`.

Example 3.: Simple example of `BasicConfigurator` usage (logback-examples/src/main/java/chapter3/MyApp1.java)

    package chapter3;

    import org.slf4j.Logger;
    import org.slf4j.LoggerFactory;

    public class MyApp1 {
      final static Logger logger = LoggerFactory.getLogger(MyApp1.class);

      public static void main(String[] args) {
        logger.info("Entering application.");

        Foo foo = new Foo();
        foo.doIt();
        logger.info("Exiting application.");
      }
    }

This class defines a static logger variable. It then instantiates a Foo object. The Foo class is listed below:

Example 3.: Small class doing logging (logback-examples/src/main/java/chapter3/Foo.java)

    package chapter3;

    import org.slf4j.Logger;
    import org.slf4j.LoggerFactory;

    public class Foo {
      static final Logger logger = LoggerFactory.getLogger(Foo.class);

      public void doIt() {
        logger.debug("Did it again!");
      }
    }

In order to run the examples in this chapter, you need to make sure that certain jar files are present on the classpath. Please refer to the setup page for further details.

Assuming the configuration files logback-test.xml or logback.xml could not be found, logback will default to a minimal configuration mentioned earlier. This configuration is hardwired to attaching a `ConsoleAppender` to the root logger. The output is formatted using a `PatternLayout` set to the pattern %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n. Moreover, by default the root logger is assigned to the `DEBUG` level.

Thus, the output of the command java chapter3.MyApp1 should be similar to:

    16:06:09.031 [main] INFO  chapter3.MyApp1 - Entering application.
    16:06:09.046 [main] DEBUG chapter3.Foo - Did it again!
    16:06:09.046 [main] INFO  chapter3.MyApp1 - Exiting application.

The `MyApp1` application links to logback via calls `org.slf4j.LoggerFactory` and `org.slf4j.Logger` classes, retrieve the loggers it wishes to use, and chugs on. Note that the only dependence of the `Foo` class on logback are through `org.slf4j.LoggerFactory` and `org.slf4j.Logger` imports. Except code that configures logback (if such code exists) client code does not need to depend on logback. Given that SLF4J permits the use of any implementation under its abstraction layer, it is rather easy to migrate large bodies of code from one logging system to another.

### Automatic configuration with logback-test.xml or logback.xml

As mentioned earlier, logback will try to configure itself using the files logback-test.xml or logback.xml if found on the classpath. Here is a configuration file equivalent to the one established by `BasicConfigrator` we've just seen.

Example 3.: Basic configuration file (logback-examples/src/main/java/chapter3/sample0.xml)

    <configuration>

      <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</Pattern>
        </layout>
      </appender>

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

After you have renamed sample0.xml as logback.xml (or logback-test.xml) place it into a directory accesible from the classpath. Running the MyApp1 application should give identical results to its previous run.

If and only if there are errors during the parsing of the configuration file, logback will automatically print status data on the console. In the absence of errors, if you still wish to inspect logback's internal status, then you can instruct logback to print status data by invoking the `print()` of the `StatusPrinter` class. The MyApp2 application shown below is identical to MyApp1 except the addition of two lines of code for printing internal status data.

Example 3.: Print logback's internal status information (logback-examples/src/main/java/chapter3/MyApp2.java)

      public static void main(String[] args) {
        logger.info("Entering application.");
        // print logback's internal status
        LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory();
        StatusPrinter.print(lc);
        ...
      }

If everything goes well, you should see the following output on the console

    17:44:58,578 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Found resource [logback-test.xml]
    17:44:58,671 |-INFO in ch.qos.logback.classic.joran.action.ConfigurationAction - debug attribute not set
    17:44:58,671 |-INFO in ch.qos.logback.core.joran.action.AppenderAction - About to instantiate appender of type [ch.qos.logback.core.ConsoleAppender]
    17:44:58,687 |-INFO in ch.qos.logback.core.joran.action.AppenderAction - Naming appender as [STDOUT]
    17:44:58,812 |-INFO in ch.qos.logback.core.joran.action.AppenderAction - Popping appender named [STDOUT] from the object stack
    17:44:58,812 |-INFO in ch.qos.logback.classic.joran.action.LevelAction - root level set to DEBUG
    17:44:58,812 |-INFO in ch.qos.logback.core.joran.action.AppenderRefAction - Attaching appender named [STDOUT] to Logger[root]

    17:44:58.828 [main] INFO  chapter3.MyApp2 - Entering application.
    17:44:58.828 [main] DEBUG chapter3.Foo - Did it again!
    17:44:58.828 [main] INFO  chapter3.MyApp2 - Exiting application.

At the end of this output, you can recognize the lines that were printed in the previous example. You should also notice the logback's internal messages, a.k.a. `Status` objects, which allow convient access to logback's internal state.

Instead of invoking `StatusPrinter` programmatically from your code, you can instruct the configuration file to dump status data, even in the absence of errors. To achieve this, you need to set the debug attribute of the configuration element, i.e. the top-most element in the configuration file, as shown below. Please note that this debug attribute relates only to the status data. It does not affect logback's configuration otherwise, in particuler with respect to logger levels. (Put differently, no, the root logger will not be set to `DEBUG`.)

Example 3.: Basic configuration file using debug mode (logback-examples/src/main/java/chapter3/sample1.xml)

    <configuration debug="true">

      <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</Pattern>
        </layout>
      </appender>

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

Setting the `debug` attribute within the <configuration> element will output status information, under the assumption that

1. the configuration file is found
2. the configuration file is well-formed XML.

If any of these two conditions is not fulfilled, the Joran cannot interpret `debug` attribute since the configuration file cannot be read. If the configuration file is found but is ill-formed, then logback will detect the error condition and automatically print its internal status on the console. However, if the configration file cannot be found, since this is not necessarily an error condition, logback will not automatically print its status data. Programmatically invoking `StatusPrinter.print()`, as as in MyApp2 application above, ensures that status information is always printed.

### Invoking `JoranConfigurator` directly

Logback relies on a configuration library called Joran which is part of logback-core. Logback's default configuration mechanism invokes JoranConfigurator on the default configuration files it finds on the claspath. For whatever reason if you wish to override logback's default configuration meachanism, you can do so by invoking JoranConfigurator directly. The next application, MyApp3, invokes JoranConfirator on a configuration file passed as parameter.

Example 3.: Invoking `JoranConfigurator` directly (logback-examples/src/main/java/chapter3/MyApp3.java)

    package chapter3;

    import org.slf4j.Logger;
    import org.slf4j.LoggerFactory;

    import ch.qos.logback.classic.LoggerContext;
    import ch.qos.logback.classic.joran.JoranConfigurator;
    import ch.qos.logback.core.joran.spi.JoranException;
    import ch.qos.logback.core.util.StatusPrinter;

    public class MyApp3 {
      final static Logger logger = LoggerFactory.getLogger(MyApp3.class);

      public static void main(String[] args) {
        // assume logback is in use
        LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory();

        try {
          JoranConfigurator configurator = new JoranConfigurator();
          configurator.setContext(lc);
          // the context was probably already configured by default configuration
          // rules
          lc.shutdownAndReset();
          configurator.doConfigure(args[0]);
        } catch (JoranException je) {
           je.printStackTrace();
        }
        StatusPrinter.printIfErrorsOccured(lc);

        logger.info("Entering application.");

        Foo foo = new Foo();
        foo.doIt();
        logger.info("Exiting application.");
      }
    }

This application fetches the `LoggerContext` currently in effect, creates a new `JoranConfigurator`, sets the context on which it will operate, resets the logger context, and then finally asks the configurator to configure the context using configuration file passed as parameter to the application. Internal status data is printed in case errors occur.

## Configuration file Syntax

To obtain these different logging behaviors we do not need to recompile code. You can easily configure logback so as to disable logging for certain parts of your application, or direct output to a UNIX Syslog daemon, to a database, to a log visualizer, or forward logging events to a remote logback server, which would log according to local server policy, for example by forwarding the log event to a second logback server.

The remainder of this section presents the syntax of configuration files.

As shall become clear, the syntax of logback configuration files is extremely flexible. As such, it is not possible specify the allowed syntax with a DTD file or an XML Schema. Nevertheles, the very basic structure of configration can be desribed as, <configuration> element, followed by zero or more <appender> elements, followed by by zero or more <logger> elements, followed by at most one <root> element. The following diagram illustrates this basic structure.

#### Configuring Loggers, or the `<logger>` element

A logger is configured using the `logger` element. A logger element takes exactly one mandatory name atttribute, an optional level attribute, and an optional aditivity attribute, which admits the values true or false. The value of the level attribute can be one of the case-insensitive strings TRACE, DEBUG, INFO, WARN, ERROR, ALL or OFF. The special case-insensitive value INHERITED, or its synonym NULL, will force the level of the logger to be inherited from higher up in the hierarchy. This comes in handy in case you set the level of a logger and later decide that it should inherit its level.

The logger element may contain zero or more appender-ref elements; each appender thus referenced is added to the named logger. It is important to keep mind that each named logger that is declared with a <logger element first has all its appenders removed and only then are the referenced appenders attached to it. In particular, if there are no appender references, then the named logger will lose all its appenders.

#### Configuring the roor logger, or the `<root>` element

The <root> element configures the root logger. It admits a single attribute, namely the level attribute. It does not admit any other attributes because the additivity flag does not apply to the root logger. Moreover, since the root logger is already named, it does not admit a name attribute either. The value of the level attribute can be set to one of the case-insensitive strings TRACE, DEBUG, INFO, WARN, ERROR, ALL or OFF. Note that the level of the root logger cannot be set to INHERITED or NULL.

The <root> element admits zero or more <appender-ref> elements. Similar to the <logger element, declaring a <root element will have the effect of first closing and then detaching all its current appenders and only subsequently will referenced appenders, if any, will be added. In particular, if it has no appender references, then the root logger will lose all its appenders.

#### Example

Setting the level of a logger or root logger is as simple as declaring it and setting its level, as the next example illustrates. Suppose we are no longer interested in seeing any DEBUG messages from any component belonging to the "chapter3" package. The following configuration file shows how to achieve that.

Example 3.: Setting the level of a logger (logback-examples/src/main/java/chapter3/sample2.xml)

    <configuration>

      <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</Pattern>
        </layout>
      </appender>

      <logger name="chapter3" level="INFO"/>

      <!-- Strictly speaking, the level attribute is not necessary since -->
      <!-- the level of the root level is set to DEBUG by default.       -->
      <root level="DEBUG">
        <appender-ref ref="STDOUT" />
      </root>

    </configuration>

This new configuration will yield the following output, when invoked with the MyApp3 application.

    17:34:07.578 [main] INFO  chapter3.MyApp3 - Entering application.
    17:34:07.578 [main] INFO  chapter3.MyApp3 - Exiting application.

You can configure the levels of as many loggers as you wish. In the next configuration file, we set the level of the chapter3 logger to INFO but at the same time set the level of the chapter3.Foo logger to `DEBUG`.

Example 3.: Setting the level of multiple loggers (logback-examples/src/main/java/chapter3/sample3.xml)

    <configuration>

      <appender name="STDOUT"
        class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>
            %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
         </Pattern>
        </layout>
      </appender>

      <logger name="chapter3" level="INFO" />
      <logger name="chapter3.Foo" level="DEBUG" />

      <root level="DEBUG">
        <appender-ref ref="STDOUT" />
      </root>

    </configuration>

Running `MyApp3` with this configuration file will result in the following output on the console:

    17:39:27.593 [main] INFO  chapter3.MyApp3 - Entering application.
    17:39:27.593 [main] DEBUG chapter3.Foo - Did it again!
    17:39:27.593 [main] INFO  chapter3.MyApp3 - Exiting application.

The table below list the loggers and their levels, after `JoranConfigurator` has configured logback with the sample3.xml configuration file.

Logger name | Assigned Level | Effective Level
--- | --- | ---
root | `DEBUG` | `DEBUG`
chapter3 | `INFO` | `INFO`
chapter3.MyApp3 | `null` | `INFO`
chapter3.Foo | `DEBUG` | `DEBUG`

It follows that the two logging statements of level `INFO` in the `MyApp3` class as well as the DEBUG messages in `Foo.doIt()` are all enabled. Note that the level of the root logger is always set to a non-null value, which is DEBUG by default.

Let us note that the basic-selection rule depends on the effective level of the logger being invoked, not the level of the logger where appenders are attached. Loback will first determine wheteher a logging statement is enabled or not, and if enabled, it will invoke the appenders found in the logger hierarchy, regardless of their level. The configuration file sample4.xml is a case in point:

Example 3.: Logger level sample (logback-examples/src/main/java/chapter3/sample4.xml)

    <configuration>

      <appender name="STDOUT"
       class="ch.qos.logback.core.ConsoleAppender">
       <layout class="ch.qos.logback.classic.PatternLayout">
         <Pattern>
            %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
          </Pattern>
        </layout>
      </appender>

      <logger name="chapter3" level="INFO" />

      <!-- turn OFF all logging (children can override) -->
      <root level="OFF">
        <appender-ref ref="STDOUT" />
      </root>

    </configuration>

The following table lists the loggers and their levels after applying the sample4.xml configuration file.

Logger name | Assigned Level | Effective Level
--- | --- | ---
root | `OFF` | `OFF`
chapter3 | `INFO` | `INFO`
chapter3.MyApp3 | `null` | `INFO`
chapter3.Foo | `null` | `INFO`

The ConsoleAppender named STDOUT, the only configured appender in sample4.xml, is attached to the root logger whose level is set to `OFF`. However, running MyApp3 with configuration script sample4.xml will yield:

    17:52:23.609 [main] INFO chapter3.MyApp3 - Entering application.
    17:52:23.609 [main] INFO chapter3.MyApp3 - Exiting application.

Thus, the level of the root logger has no apparent effect because the loggers in `chapter3.MyApp3` and `chapter3.Foo` classes are all enabled for the `INFO` level. As a side note, the chapter3 logger exists by virtue of its declaration in the configuration file - even if the Java source code does not directly refer to it.

#### Configuring Appenders

Appenders are configured using `<appender>` elements, taking two attributes name and class, both of which are mandatory. The name attribute specifies the name of the appender whereas the class attribute specifies the fully qualified name of the class of which the named appender will be an instance. The `<appender>` element may contain zero or one `<layout>` elements and zero or more `<filter>` elements. Appart from these two common elements, `<appender>` elements may contain any number of element corresponding to javabean properties of the appender class. Seamlessly supporting any property of a given logback component is one of the major strengths of Joran. The following diagram illustrates the common structure. Note that support for properties is not visible.

The `<layout>` element takes a mandatory class attribute specifying the fully qualified name of the class of which the associated layout should be an instance. Like the `<appender>` element, it may contain other elements corresponding to properties of the layout class.

Logging to multiple appenders is as easy as defining the various appenders and referencing them in a logger, as the next configuration file illustrates:

Example 3.: Multiple loggers (logback-examples/src/main/java/chapter3/multiple.xml)

    <configuration>

      <appender name="FILE" class="ch.qos.logback.core.FileAppender">
        <file>myApp.log</file>

        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%date %level [%thread] %logger{10} [%file:%line] %msg%n</Pattern>
        </layout>
      </appender>

      <appender name="STDOUT"
        class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%msg%n</Pattern>
        </layout>
      </appender>

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

This configuration scripts defines two appenders called FILE and STDOUT. The FILE appender logs to a file called myApp.log. The layout for this appender is a `PatternLayout` that outputs the date, level, thread name, logger name, file name and line number where the log request is located, the message and line separator character(s). The second appender called `STDOUT` outputs to the console. The layout for this appender outputs only the message string followed by a line separator.

The appenders are attached to the root logger by referencing them by name within an appender-ref element. Note that each appender has its own layout. Layouts are usually not designed to be shared by multiple appenders. As such, logback configuration files do not provide any syntactical means for sharing layouts.

By default, appenders are cumulative: a logger will log to the appenders attached to itself (if any) as well as all the appenders attached to its ancestors. Thus, attaching the same appender to multiple loggers will cause logging output to be duplicated.

Example 3.: Duplicate appender (logback-examples/src/main/java/chapter3/duplicate.xml)

    <configuration>

      <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</Pattern>
        </layout>
      </appender>

      <logger name="chapter3">
        <appender-ref ref="STDOUT" />
      </logger>

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

Running `MyApp3` with duplicate.xml will yield the following output:

    14:25:36.343 [main] INFO  chapter3.MyApp3 - Entering application.
    14:25:36.343 [main] INFO  chapter3.MyApp3 - Entering application.
    14:25:36.359 [main] DEBUG chapter3.Foo - Did it again!
    14:25:36.359 [main] DEBUG chapter3.Foo - Did it again!
    14:25:36.359 [main] INFO  chapter3.MyApp3 - Exiting application.
    14:25:36.359 [main] INFO  chapter3.MyApp3 - Exiting application.

Notice the duplicated output. The appender named STDOUT is attached to two loggers, to root and to chapter3. Since the root logger is the ancestor of all loggers and chapter3 is the parent of chapter3.MyApp3 and chapter3.Foo, logging request made with these two loggers will be output twice, once because STDOUT is attached to chapter3 and once because it is attached to root.

Appender additivity is not intended as a trap for new users. It is a quite convenient logback feature. For instance, you can configure logging such that log messages appear on the console (for all loggers in the system) while messages only from some specific set of loggers flow into a specific appender.

Example 3.: Multiple appender (logback-examples/src/main/java/chapter3/restricted.xml)

    <configuration>

      <appender name="FILE" class="ch.qos.logback.core.FileAppender">
        <file>myApp.log</file>
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%date %level [%thread] %logger{10} [%file:%line] %msg%n</Pattern>
        </layout>
      </appender>

      <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%msg%n</Pattern>
        </layout>
      </appender>

      <logger name="chapter3">
        <appender-ref ref="FILE" />
      </logger>

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

In this example, the console appender will log all the messages (for all loggers in the system) whereas only logging request originating from loggers chapter3 and below go into the myApp.log file.

#### Overriding the default cumulative behaviour

In case the default cumulative behavior turns out to be unsuitable for your needs, you can override it by setting the additivity flag to false. Thus, a branch in your logger tree may direct output to a set of appenders different than those of the rest of the tree.

Example 3.: Additivity flag (logback-examples/src/main/java/chapter3/additivityFlag.xml)

    <configuration>

      <appender name="FILE" class="ch.qos.logback.core.FileAppender">
        <file>foo.log</file>
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%date %level [%thread] %logger{10} [%file : %line] %msg%n</Pattern>
        </layout>
      </appender>

      <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%msg%n</Pattern>
        </layout>
      </appender>

      <logger name="chapter3.Foo" additivity="false">
        <appender-ref ref="FILE" />
      </logger>

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

This example, the appender named FILE is attached to the chapter3.Foo logger. Moreover, the chapter3.Foo logger has its additivity flag set to false such that its logging output will be sent to the appender named FILE but not to any appender attached higher in the hierarchy. Other loggers remain oblivious to the additivity setting of the chapter3.Foo logger. Running the `MyApp3` application with the additivityFlag.xml configuration file will output results on the console from the chapter3.MyApp3 logger. However, output from the chapter3.Foo logger will appear in the foo.log file and only in that file.

#### Variable substitution

In principle, variable substitution can occur at any point where a value can be specified. The syntax of variable substitution is similar to that of Unix shells. The string between an opening ${ and closing } is interpreted as a key. The value of the substituted variable can be defined in the configuration file itself, in an external properties file or as a system property. The corresponding value replaces ${aKey} sequence. For example, if java.home.dir system property is set to /home/xyz, then every occurrence of the sequence ${java.home.dir} will be interpreted as /home/xyz.

The next example shows a variable, a.k.a. a substitution property, declared the beginning of the configuration file. It is then used further down the file to specify the location of the output file.

Example 3.: Simple Variable substitution (logback-examples/src/main/java/chapter3/variableSubstitution1.xml)

    <configuration>

      <substitutionProperty name="USER_HOME" value="/home/sebastien" />

      <appender name="FILE"
        class="ch.qos.logback.core.FileAppender">
        <file>${USER_HOME}/myApp.log</file>
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%msg%n</Pattern>
        </layout>
      </appender>

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

The next example shows the use of a System property to achieve the same result. The property is not declared in the configuration file, thus logback will look for it in the System properties. Java system properties can be set on the command line.

java -DUSER_HOME="/home/sebastien" MyApp2

Example 3.: System Variable substitution (logback-examples/src/main/java/chapter3/variableSubstitution2.xml)

    <configuration>

      <appender name="FILE"
        class="ch.qos.logback.core.FileAppender">
        <file>${USER_HOME}/myApp.log</file>
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%msg%n</Pattern>
        </layout>
      </appender>

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

When multiple variables are needed, it may be more convenient to create a separate file that will contain all the variables. Here is how one can do such a setup.

Example 3.: Variable substitution using a separate file (logback-examples/src/main/java/chapter3/variableSubstitution3.xml)

    <configuration>

      <substitutionProperty file="variables1.properties" />

     <appender name="FILE"
        class="ch.qos.logback.core.FileAppender">
        <file>${USER_HOME}/myApp.log</file>
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%msg%n</Pattern>
        </layout>
      </appender>

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

This configuration file contains a reference to a file named variables1.properties. The variables contained in that file will be read defined within the context of the logback configuration file. Here is what the variable.properties file might look like.

Example 3.: Variable file (logback-examples/src/main/java/chapter3/variables1.properties)

    USER_HOME=/home/sebastien

Nested variabled subsitution is also supported. By nested, we mean that the value definition of a variable contains references to other variables. Suppose you wish to use variables to specify not only the destination directory but also the file name, and combine those two variables in a third variable called "destination". The properties file shown below gives an example.

Example 3.: Nested variable references (logback-examples/src/main/java/chapter3/variables2.properties)

    USER_HOME=/home/sebastien
    fileName=myApp.log
    destination=${USER_HOME}/${fileName}

Note that in the properties file above, "destination" is composed out of two other variables, namely "USER_HOME" and "fileName".

Example 3.: Variable substitution using a separate file (logback-examples/src/main/java/chapter3/variableSubstitution4.xml)

    <configuration>

      <substitutionProperty file="variables2.properties" />

      <appender name="FILE"
        class="ch.qos.logback.core.FileAppender">
        <file>${destination}</file>
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>%msg%n</Pattern>
        </layout>
      </appender>

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

### Default substitution values for variables

Under certain circumstances, it may be desirable for a variable to have a default value in case it is not declared or its value is null. As in the Bash shell, default values can be specified using the ":-" operator. For example, assuming aKey is not defined, `"${aKey:-golden}"` will be interpreted as "golden".

### File inclusion

Joran supports including parts of a configuration file from another file. This is done by declaring a `<include>` element, as shown below:

Example 3.: File include (logback-examples/src/main/java/chapter3/containingConfig.xml)

    <configuration>
      <include file="src/main/java/chapter3/includedConfig.xml"/>

      <root level="DEBUG">
        <appender-ref ref="includedConsole" />
      </root>

    </configuration>

The target file MUST have its elements nested inside an `<included>` element. For example, a `ConsoleAppender` could be declared as:

Example 3.: File include (logback-examples/src/main/java/chapter3/includedConfig.xml)

    <included>
      <appender name="includedConsole" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
          <Pattern>"%d - %m%n"</Pattern>
        </layout>
      </appender>
    </included>

The file to be included can be referenced as a file, as a URL or as a resource. To reference a file use the file attribute. To reference a URL use the url attribute. To reference a resource, use the resource attribute.