Chapter 7: Filters

In the preceding chapters, the basic selection rule, which lies at the heart of logback-classic, has been presented. In this chapter, additional filtering methods will be introduced.

Logback filters are based on ternary logic allowing them to be assembled or chained together to compose an arbitrarily complex filtering policy. They are largely inspired by Linux's iptables.

In logback-classic

Logback-classic offers two types of filters, regular filters and turbo filters.

Regular filters

Regular logback-classic filters extend the Filter abstract class which essentially consists of a single decide() method taking an ILoggingEvent instance as its parameter.

Filters are organized as an ordered list and are based on ternary logic. The decide(ILoggingEvent event) method of each filter is called in sequence. This method returns one of the FilterReply enumeration values, i.e. one of DENY, NEUTRAL or ACCEPT. If the value returned by decide() is DENY, then the log event is dropped immediately without consulting the remaining filters. If the value returned is NEUTRAL, then the next filter in the list is consulted. If there are no further filters to consult, then the logging event is processed normally. If the returned value is ACCEPT, then the logging event is processed immediately skipping the invocation of the remaining filters.

In logback-classic, filters can be added to Appender instances. By adding one or more filters to an appender, you can filter events by arbitrary criteria, such as the contents of the log message, the contents of the MDC, the time of day or any other part of the logging event.

Implementing your own Filter

Creating your own filter is easy. All you have to do is extend the Filter abstract class and implement the decide() method.

The SampleFilter class shown below provides an example. Its decide method returns ACCEPT for logging events containing the string "sample" in its message field. For other events, the value NEUTRAL is returned.

package chapters.filters;

import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.filter.Filter;
import ch.qos.logback.core.spi.FilterReply;

public class SampleFilter extends Filter<ILoggingEvent> {

  @Override
  public FilterReply decide(ILoggingEvent event) {    
    if (event.getMessage().contains("sample")) {
      return FilterReply.ACCEPT;
    } else {
      return FilterReply.NEUTRAL;
    }
  }
}

The configuration files shown next attaches a SampleFilter to a ConsoleAppender.

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

    <filter class="chapters.filters.SampleFilter" />

    <encoder>
      <pattern>
        %-4relative [%thread] %-5level %logger - %msg%n
      </pattern>
    </encoder>
  </appender>
	
  <root>
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

With the help of Joran, logback's configuration framework, specifying properties or sub-components to filters is also easy. After adding the corresponding setter method in the filter class, specify the value of the property in an xml element named after the property, nesting it within a <filter> element.

Often, the desired filter logic consists of two orthogonal parts, a match/mismatch test and a response depending on the match/mismatch. For example, for a given test, e.g. message equals "foobar", one filter might respond ACCEPT on match and NEUTRAL on mismatch, and another filter might respond NEUTRAL on match and DENY on mismatch.

Taking notice of this orthogonality, logback ships with the AbstractMatcherFilter class which provides a useful skeleton for specifying the appropriate response on match and on mismatch, with the help of two properties, named OnMatch and OnMismatch. Most of the regular filters included in logback are derived from AbstractMatcherFilter.

LevelFilter

LevelFilter filters events based on exact level matching. If the event's level is equal to the configured level, the filter accepts or denies the event, depending on the configuration of the onMatch and onMismatch properties. Here is a sample configuration file.

<configuration>
  <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
    <filter class="ch.qos.logback.classic.filter.LevelFilter">
      <level>INFO</level>
      <onMatch>ACCEPT</onMatch>
      <onMismatch>DENY</onMismatch>
    </filter>
    <encoder>
      <pattern>
        %-4relative [%thread] %-5level %logger{30} - %msg%n
      </pattern>
    </encoder>
  </appender>
  <root level="DEBUG">
    <appender-ref ref="CONSOLE" />
  </root>
</configuration>

ThresholdFilter

The ThresholdFilter filters events below the specified threshold. For events of level equal or above the threshold, ThresholdFilter will respond NEUTRAL when its decide() method is invoked. However, events with a level below the threshold will be denied. Here is a sample configuration file.

<configuration>
  <appender name="CONSOLE"
    class="ch.qos.logback.core.ConsoleAppender">
    <!-- deny all events with a level below INFO, that is TRACE and DEBUG -->
    <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
      <level>INFO</level>
    </filter>
    <encoder>
      <pattern>
        %-4relative [%thread] %-5level %logger{30} - %msg%n
      </pattern>
    </encoder>
  </appender>
  <root level="DEBUG">
    <appender-ref ref="CONSOLE" />
  </root>
</configuration>

EvaluatorFilter

EvaluatorFilter is a generic filter encapsulating an EventEvaluator. As the name suggests, an EventEvaluator evaluates whether a given criteria is met for a given event. On match and on mismatch, the hosting EvaluatorFilter will return the value specified by the onMatch or onMismatch properties respectively.

Note that EventEvaluator is an abstract class. You can implement your own event evaluation logic by sub-classing EventEvaluator.

GEventEvaluator

GEventEvaluator is a concrete EventEvaluator implementation taking arbitrary Groovy language boolean expressions as the evaluation criteria. We refer such Groovy language boolean expressions as "groovy evaluation expressions". Groovy evaluation expressions enable hitherto unprecedented flexibility in event filtering. GEventEvaluator requires the Groovy runtime. Please see the corresponding section of the setup document on adding the Groovy runtime to your class path.

Evaluation expressions are compiled on-the-fly during the interpretation of the configuration file. As a user, you do not need to worry about the actual plumbing. However, it is your responsibility to ensure that the groovy-language expression is valid.

The evaluation expression acts on the current logging event. Logback automatically inserts the current logging event of type ILoggingEvent as a variable referred to as 'event' as well as its shorthand referred to as 'e'. The variables TRACE, DEBUG, INFO, WARN and ERROR are also exported into the scope of the expression. Thus, "event.level == DEBUG" and "e.level == DEBUG" are equivalent and valid groovy expressions which will return true only if the current logging event's level is identical to DEBUG. For other comparison operators on levels, the level field should be converted to integer with the toInt() operator.

Here is a more complete example.

<configuration>
    
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <filter class="ch.qos.logback.core.filter.EvaluatorFilter">      
      <evaluator class="ch.qos.logback.classic.boolex.GEventEvaluator"> 
        <expression>
           e.level.toInt() >= WARN.toInt() &amp;&amp;  <!-- Stands for && in XML -->
           !(e.mdc?.get("req.userAgent") =~ /Googlebot|msnbot|Yahoo/ )
        </expression>
      </evaluator>
      <OnMismatch>DENY</OnMismatch>
      <OnMatch>NEUTRAL</OnMatch>
    </filter>
    <encoder>
      <pattern>
        %-4relative [%thread] %-5level %logger - %msg%n
      </pattern>
    </encoder>
  </appender>

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

The above filter will let events of level WARN and higher go through onto the console unless the error is generated by Web crawlers associated with Google, MSN or Yahoo. It does so by checking whether the MDC associated with the event contains a value for "req.userAgent" matching the /Googlebot|msnbot|Yahoo/ regular expression. Note that since the MDC map can be null, we are also using Groovy's safe dereferencing operator, that is the ?. operator. The equivalent logic would have been much longer if expressed in Java.

If you are wondering how the identifier for the user agent was inserted into the MDC under the 'req.userAgent' key, it behooves us to mention that logback ships with a servlet filter named MDCInsertingServletFilter designed for this purpose. It will be described in a later chapter.

JaninoEventEvaluator

Logback-classic ships with another concrete EventEvaluator implementation called JaninoEventEvaluator taking an arbitrary Java language block returning a boolean value as the evaluation criteria. We refer to such Java language boolean expressions as "evaluation expressions". Evaluation expressions enable great flexibility in event filtering. JaninoEventEvaluator requires the Janino library. Please see the corresponding section of the setup document. Compared to JaninoEventEvaluator, GEventEvaluator, by virtue of the Groovy language, is significantly more convenient to use, but JaninoEventEvaluator will usually run (much) faster for equivalent expressions.

Evaluation expressions are compiled on-the-fly during the interpretation of the configuration file. As a user, you do not need to worry about the actual plumbing. However, it is your responsibility to ensure that the Java language expression returns a boolean, i.e. that it evaluates to true or false.

The evaluation expression is evaluated on the current logging event. Logback-classic automatically exports various fields of the logging event as variables accessible from the evaluation expression. The case-sensitive names of these exported variables are listed below.

Here is a concrete example.

<configuration>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <filter class="ch.qos.logback.core.filter.EvaluatorFilter">      
      <evaluator> <!-- defaults to type ch.qos.logback.classic.boolex.JaninoEventEvaluator -->
        <expression>return message.contains("billing");</expression>
      </evaluator>
      <OnMismatch>NEUTRAL</OnMismatch>
      <OnMatch>DENY</OnMatch>
    </filter>
    <encoder>
      <pattern>
        %-4relative [%thread] %-5level %logger - %msg%n
      </pattern>
    </encoder>
  </appender>

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

The bold part in the above configuration file adds an EvaluatorFilter to a ConsoleAppender. An evaluator of type JaninoEventEvaluator is then injected into the EvaluatorFilter. In the absence of class attribute in the <evaluator> element specified by the user, Joran will infer a default type of JaninoEventEvaluator for the evaluator. This is one of the few occurrences where Joran implicitly infers the type of a component.

The expression element corresponds to the evaluation expression just discussed. The expression return message.contains("billing"); returns a boolean value. Notice that the message variable is exported automatically by JaninoEventEvaluator.

Given that the OnMismatch property is set to NEUTRAL and the OnMatch property set to DENY, this evaluator filter will drop all logging events whose message contains the string "billing".

The FilterEvents application issues ten logging requests, numbered 0 to 9. Let us first run FilterEvents class without any filters:

java chapters.filters.FilterEvents src/main/java/chapters/filters/basicConfiguration.xml

All requests will be displayed, as shown below:

0    [main] INFO  chapters.filters.FilterEvents - logging statement 0
0    [main] INFO  chapters.filters.FilterEvents - logging statement 1
0    [main] INFO  chapters.filters.FilterEvents - logging statement 2
0    [main] DEBUG chapters.filters.FilterEvents - logging statement 3
0    [main] INFO  chapters.filters.FilterEvents - logging statement 4
0    [main] INFO  chapters.filters.FilterEvents - logging statement 5
0    [main] ERROR chapters.filters.FilterEvents - billing statement 6
0    [main] INFO  chapters.filters.FilterEvents - logging statement 7
0    [main] INFO  chapters.filters.FilterEvents - logging statement 8
0    [main] INFO  chapters.filters.FilterEvents - logging statement 9

Suppose that we want to get rid of the "billing statement". The basicEventEvaluator.xml configuration file listed above filters messages containing the string "billing" which is precisely the desired outcome.

Running with basicEventEvaluator.xml:

java chapters.filters.FilterEvents src/main/java/chapters/filters/basicEventEvaluator.xml

we obtain:

0 [main] INFO chapters.filters.FilterEvents - logging statement 0 0 [main] INFO chapters.filters.FilterEvents - logging statement 1 0 [main] INFO chapters.filters.FilterEvents - logging statement 2 0 [main] DEBUG chapters.filters.FilterEvents - logging statement 3 0 [main] INFO chapters.filters.FilterEvents - logging statement 4 0 [main] INFO chapters.filters.FilterEvents - logging statement 5 0 [main] INFO chapters.filters.FilterEvents - logging statement 7 0 [main] INFO chapters.filters.FilterEvents - logging statement 8 0 [main] INFO chapters.filters.FilterEvents - logging statement 9

Evaluation expressions can be Java blocks. For example, the following is a valid expression.

<evaluator>
  <expression>
    if(logger.startsWith("org.apache.http"))
      return true;

    if(mdc == null || mdc.get("entity") == null)
      return false;

    String payee = (String) mdc.get("entity");

    if(logger.equals("org.apache.http.wire") &amp;&amp; <!-- & encoded as &amp; -->
        payee.contains("someSpecialValue") &amp;&amp;
        !message.contains("someSecret")) {
      return true;
    }

    return false;
  </expression>
</evaluator>

Matchers

While it is possible to do pattern matching by invoking the matches() method in the String class, this incurs the cost of compiling of a brand new Pattern object each time the filter is invoked. To eliminate this overhead, you can predefine one or more Matcher objects. Once a matcher is defined, it can be repeatedly referenced by name in the evaluator expression.

An example should clarify the point:

<configuration debug="true">

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <filter class="ch.qos.logback.core.filter.EvaluatorFilter">
      <evaluator>        
        <matcher>
          <Name>odd</Name>
          <!-- filter out odd numbered statements -->
          <regex>statement [13579]</regex>
        </matcher>
        
        <expression>odd.matches(formattedMessage)</expression>
      </evaluator>
      <OnMismatch>NEUTRAL</OnMismatch>
      <OnMatch>DENY</OnMatch>
    </filter>
    <encoder>
      <pattern>%-4relative [%thread] %-5level %logger - %msg%n</pattern>
    </encoder>
  </appender>

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

Running with evaluatorWithMatcher.xml:

java chapters.filters.FilterEvents src/main/java/chapters/filters/evaluatorWithMatcher.xml

we obtain:

260 [main] INFO chapters.filters.FilterEvents - logging statement 0 264 [main] INFO chapters.filters.FilterEvents - logging statement 2 264 [main] INFO chapters.filters.FilterEvents - logging statement 4 266 [main] ERROR chapters.filters.FilterEvents - billing statement 6 266 [main] INFO chapters.filters.FilterEvents - logging statement 8

If you need to define additional matchers, you can do so by adding further <matcher> elements.

TurboFilters

TurboFilter objects all extend the TurboFilter abstract class. Like the regular filters, they use ternary logic to return their evaluation of the logging event.

Overall, they work much like the previously mentioned filters. However, there are two main differences between Filter and TurboFilter objects.

TurboFilter objects are tied to the logging context. Hence, they are called not only when a given appender is used, but each and every time a logging request is issued. Their scope is wider than appender-attached filters.

More importantly, they are called before the LoggingEvent object creation. TurboFilter objects do not require the instantiation of a logging event to filter a logging request. As such, turbo filters are intended for high performance filtering of logging events, even before the events are created.

Implementing your own TurboFilter

To create your own TurboFilter component, just extend the TurboFilter abstract class. As previously, when implementing a customized filter object, developing a custom TurboFilter only asks that one implement the decide() method. In the next example, we create a slightly more complex filter:

package chapters.filters;

import org.slf4j.Marker;
import org.slf4j.MarkerFactory;

import ch.qos.logback.classic.Level;
import ch.qos.logback.classic.Logger;
import ch.qos.logback.classic.turbo.TurboFilter;
import ch.qos.logback.core.spi.FilterReply;

public class SampleTurboFilter extends TurboFilter {

  String marker;
  Marker markerToAccept;

  @Override
  public FilterReply decide(Marker marker, Logger logger, Level level,
      String format, Object[] params, Throwable t) {

    if (!isStarted()) {
      return FilterReply.NEUTRAL;
    }

    if ((markerToAccept.equals(marker))) {
      return FilterReply.ACCEPT;
    } else {
      return FilterReply.NEUTRAL;
    }
  }

  public String getMarker() {
    return marker;
  }

  public void setMarker(String markerStr) {
    this.marker = markerStr;
  }

  @Override
  public void start() {
    if (marker != null && marker.trim().length() > 0) {
      markerToAccept = MarkerFactory.getMarker(marker);
      super.start(); 
    }
  }
}

The TurboFilter above accepts events that contain a specific marker. If said marker is not found, then the filter passes the responsibility to the next filter in the chain.

To allow more flexibility, the marker that will be tested can be specified in the configuration file, hence the getter and setter methods. We also implemented the start() method, to check that the option has been specified during the configuration process.

Here is a sample configuration that makes use of our newly created TurboFilter.

<configuration>
  <turboFilter class="chapters.filters.SampleTurboFilter">
    <Marker>sample</Marker>
  </turboFilter>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>
        %-4relative [%thread] %-5level %logger - %msg%n
      </pattern>
    </encoder>
  </appender>

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

Logback classic ships with several TurboFilter classes ready for use. The MDCFilter checks the presence of a given value in the MDC whereas DynamicThresholdFilter allows filtering based on MDC key/level threshold associations. On the other hand, MarkerFilter checks for the presence of a specific marker associated with the logging request.

Here is a sample configuration, using both MDCFilter and MarkerFilter.

<configuration>

  <turboFilter class="ch.qos.logback.classic.turbo.MDCFilter">
    <MDCKey>username</MDCKey>
    <Value>sebastien</Value>
    <OnMatch>ACCEPT</OnMatch>
  </turboFilter>
	
  <turboFilter class="ch.qos.logback.classic.turbo.MarkerFilter">
    <Marker>billing</Marker>
    <OnMatch>DENY</OnMatch>
  </turboFilter>

  <appender name="console" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%date [%thread] %-5level %logger - %msg%n</pattern>
    </encoder>
  </appender>

  <root level="INFO">
    <appender-ref ref="console" />
  </root>  
</configuration>

You can see this configuration in action by issuing the following command:

java chapters.filters.FilterEvents src/main/java/chapters/filters/turboFilters.xml

As we've seen previously, the FilterEvents application issues 10 logging requests, numbered 0 to 9. Except for requests 3 and 6, all of the requests are of level INFO, the same level as the one assigned to the root logger. The 3rd request, is issued at the the DEBUG level, which is below the effective level. However, since the MDC key "username" is set to "sebastien" just before the 3rd request and removed just afterwards, the MDCFilter specifically accepts the request (and only that request). The 6th request, issued at the ERROR level, is marked as "billing". As such, it is denied by the MarkerFilter (the second turbo filter in the configuration).

Thus, the output of FilterEvents application configured with turboFilters.xml file shown above is:

2006-12-04 15:17:22,859 [main] INFO chapters.filters.FilterEvents - logging statement 0 2006-12-04 15:17:22,875 [main] INFO chapters.filters.FilterEvents - logging statement 1 2006-12-04 15:17:22,875 [main] INFO chapters.filters.FilterEvents - logging statement 2 2006-12-04 15:17:22,875 [main] DEBUG chapters.filters.FilterEvents - logging statement 3 2006-12-04 15:17:22,875 [main] INFO chapters.filters.FilterEvents - logging statement 4 2006-12-04 15:17:22,875 [main] INFO chapters.filters.FilterEvents - logging statement 5 2006-12-04 15:17:22,875 [main] INFO chapters.filters.FilterEvents - logging statement 7 2006-12-04 15:17:22,875 [main] INFO chapters.filters.FilterEvents - logging statement 8 2006-12-04 15:17:22,875 [main] INFO chapters.filters.FilterEvents - logging statement 9

One can see that the 3rd request, which should not be displayed if we only followed the overall INFO level, appears anyway, because it matched the first TurboFilter requirements and was accepted.

On the other hand, the 6th request, that is an ERROR level request should have been displayed. But it satisfied the second TurboFilter whose OnMatch option is set to DENY. Thus, the 6th request was not displayed.

DuplicateMessageFilter

The DuplicateMessageFilter merits a separate presentation. This filter detects duplicate messages, and beyond a certain number of repetitions, drops repeated messages.

To detect repetition, this filter uses simple String equality between messages. It does not detect messages which are very similar, varying only by few characters. For example, if you write:

logger.debug("Hello "+name0);
logger.debug("Hello "+name1);

Assuming name0 and name1 have different values, the two "Hello" messages will be considered as unrelated. Depending on user demand, future releases may check for string similarity, eliminating repetitions of similar but not identical messages.

Note that in case of parameterized logging, only the raw message is taken into consideration. For example, in the next two requests, the raw messages, i.e. "Hello {}.", are identical, and thus considered as repetitions.

logger.debug("Hello {}.", name0);
logger.debug("Hello {}.", name1);

The number of allowed repetitions can be specified by the AllowedRepetitions property. For example, if the property is set to 1, then the 2nd and subsequent occurrences of the same message will be dropped. Similarly, if the property is set to 2, then the 3rd and subsequent occurrences of the same message will be dropped. By default, the AllowedRepetitions property is set to 5.

In order to detect repetitions, this filter needs to keep references to old messages in an internal cache. The size of this cache is determined by the CacheSize property. By the default, this is set to 100.

<configuration>

  <turboFilter class="ch.qos.logback.classic.turbo.DuplicateMessageFilter"/>

  <appender name="console" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%date [%thread] %-5level %logger - %msg%n</pattern>
    </encoder>
  </appender>

  <root level="INFO">
    <appender-ref ref="console" />
  </root>  
</configuration>

Thus, the output for FilterEvents application configured with duplicateMessage.xml is:

2008-12-19 15:04:26,156 [main] INFO chapters.filters.FilterEvents - logging statement 0 2008-12-19 15:04:26,156 [main] INFO chapters.filters.FilterEvents - logging statement 1 2008-12-19 15:04:26,156 [main] INFO chapters.filters.FilterEvents - logging statement 2 2008-12-19 15:04:26,156 [main] INFO chapters.filters.FilterEvents - logging statement 4 2008-12-19 15:04:26,156 [main] INFO chapters.filters.FilterEvents - logging statement 5 2008-12-19 15:04:26,171 [main] ERROR chapters.filters.FilterEvents - billing statement 6

"logging statement 0" is the first occurrence of the message "logging statement {}". "logging statement 1" is the first repetition, "logging statement 2" is the second repetition. Interestingly enough, "logging statement 3" of level DEBUG, is the third repetition, even though it is later dropped by virtue of the basic selection rule. This can be explained by the fact that turbo filters are invoked before other types of filters, including the basic selection rule. Thus, DuplicateMessageFilter considers "logging statement 3" as a repetition, oblivious to the fact that it will be dropped further down in the processing chain. "logging statement 4" is the fourth repetition and "logging statement 5" the fifth. Statements 6 and beyond are dropped because only 5 repetitions are allowed by default.

In logback-access

Logback-access offers most of the features available with logback-classic. In particular, Filter objects are available and work in the same way as their logback-classic counterparts, with one notable difference. Instead of LoggingEvent instances logback-access filters act upon AccessEvent instances. At present time, logback-access ships with a limited number of filters described below. If you would like to suggest additional filters, please contact the logback-dev mailing list.

CountingFilter

With the help of CountingFilter class, logback-access can provide statistical data about access to the web-server. Upon initialization, CountingFilter registers itself as an MBean onto the platform's JMX server. You can then interrogate that MBean for statistical data, e.g. averages by minute, hour, day, week, or month. Other statistics such the count for the preceding week, day, hour or month as well as the total count are also available.

The following logback-access.xml configuration file declares a CountingFilter.

<configuration>
  <statusListener class="ch.qos.logback.core.status.OnConsoleStatusListener" />

  <filter class="ch.qos.logback.access.filter.CountingFilter">
    <name>countingFilter</name>
  </filter>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%h %l %u %t \"%r\" %s %b</pattern>
    </encoder>
  </appender>

  <appender-ref ref="STDOUT" />
</configuration>

You can examine the various statistics maintained by CountingFilter on your platform's JMX server via the jconsole application.

EvaluatorFilter

EvaluatorFilter is a generic filter encapsulating an EventEvaluator. As the name suggests, an EventEvaluator evaluates whether a given criteria is met for a given event. On match and on mismatch, the hosting EvaluatorFilter will return the value specified by the onMatch or onMismatch properties respectively. Note that EvaluatorFilter has been previously discussed in the context of logback-classic (see above). The present text is mostly a repetition of the previous discussion.

Note that EventEvaluator is an abstract class. You can implement your own event evaluation logic by sub-classing EventEvaluator. Logback-access ships with a concrete implementation named JaninoEventEvaluator. It takes arbitrary Java language boolean expressions as the evaluation criteria. We refer to such Java language blocks as "evaluation expressions". Evaluation expressions enable great flexibility in event filtering. JaninoEventEvaluator requires the Janino library. Please see the corresponding section of the setup document.

Evaluation expressions are compiled on-the-fly during the interpretation of the configuration file. As a user, you do not need to worry about the actual plumbing. However, it is your responsibility to ensure that the Java language expression returns a boolean, i.e. that it evaluates to true or false.

The evaluation expression is evaluated on the current access event. Logback-access automatically exports the current AccessEvent instance under the variable name event. You can read the various data associated with the HTTP request as well as the HTTP response via the event variable. Please refer to the AccessEvent class source code for the exact list.

The next logback-access configuration file illustrates filtering based on the 404 (Not Found) HTTP response code. Every request resulting in a 404 will be printed on the console.

<configuration>
  <statusListener class="ch.qos.logback.core.status.OnConsoleStatusListener" />

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <filter class="ch.qos.logback.core.filter.EvaluatorFilter">
      <evaluator>
        <expression>event.getStatusCode() == 404</expression>
      </evaluator>
      <onMismatch>DENY</onMismatch>
    </filter>
   <encoder><pattern>%h %l %u %t %r %s %b</pattern></encoder>
  </appender>

  <appender-ref ref="STDOUT" />
</configuration>

In the next example, we still log requests resulting in 404 errors, except those requests asking for CSS files.

<configuration>
  <statusListener class="ch.qos.logback.core.status.OnConsoleStatusListener" />
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <filter class="ch.qos.logback.core.filter.EvaluatorFilter">
      <evaluator name="Eval404">
        <expression>
         (event.getStatusCode() == 404)
           &amp;&amp;  <!-- ampersand characters need to be escaped -->
         !(event.getRequestURI().contains(".css"))
        </expression>
      </evaluator>
      <onMismatch>DENY</onMismatch>
    </filter>

   <encoder><pattern>%h %l %u %t %r %s %b</pattern></encoder>
  </appender>

  <appender-ref ref="STDOUT" />
</configuration>