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<h1>Chapter 7: Filters</h1>
<div class="quote">
<p><em>Have lots of ideas and throw away the bad ones. You aren't
going to have good ideas unless you have lots of ideas and some
sort of principle of selection.</em></p>
<p>—LINUS PAULING</p>
</div>
<script src="../templates/creative.js" type="text/javascript"></script>
<p>In the preceding chapters, the <a
href="architecture.html#basic_selection">basic selection rule</a>,
which lies at the heart of logback-classic, has been presented. In
this chapter, additional filtering methods will be introduced.
</p>
<p>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.
</p>
<script src="../templates/setup.js" type="text/javascript"></script>
<h2>In logback-classic</h2>
<p>Logback-classic offers two types of filters, regular filters
and turbo filters.
</p>
<h3 class="doAnchor" name="filter">Regular filters</h3>
<p>Regular logback-classic filters extend the <a
href="../xref/ch/qos/logback/core/filter/Filter.html"><code>Filter</code></a>
abstract class which essentially consists of a single
<code>decide()</code> method taking an <code>ILoggingEvent</code>
instance as its parameter.
</p>
<p>Filters are organized as an ordered list and are based on
ternary logic. The <code>decide(ILoggingEvent event)</code> method
of each filter is called in sequence. This method returns one of
the <a
href="../xref/ch/qos/logback/core/spi/FilterReply.html"><code>FilterReply</code></a>
enumeration values, i.e. one of <code>DENY</code>,
<code>NEUTRAL</code> or <code>ACCEPT</code>. If the value
returned by <code>decide</code>() is <code>DENY</code>, then the
log event is dropped immediately without consulting the remaining
filters. If the value returned is <code>NEUTRAL</code>, 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 <code>ACCEPT</code>, then the logging
event is processed immediately skipping the invocation of the
remaining filters.
</p>
<p>In logback-classic, filters can be added to
<code>Appender</code> 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.
</p>
<h3 class="doAnchor" name="yourOwnFilter">Implementing your own
Filter</h3>
<p>Creating your own filter is easy. All you have to do is extend
the <code>Filter</code> abstract class and implement the
<code>decide()</code> method.
</p>
<p>The SampleFilter class shown below provides an example. Its
<code>decide</code> method returns ACCEPT for logging events
containing the string "sample" in its message field. For other
events, the value NEUTRAL is returned.
</p>
<em>Example: Basic custom filter (<a
href="../xref/chapters/filters/SampleFilter.html">logback-examples/src/main/java/chapters/filters/SampleFilter.java</a>)</em>
<pre class="prettyprint source">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;
}
}
}</pre>
<p>The configuration files shown next attaches a
<code>SampleFilter</code> to a <code>ConsoleAppender</code>.
</p>
<em>Example: SampleFilter configuration
(logback-examples/src/main/java/chapters/filters/SampleFilterConfig.xml)</em>
<span class="asGroovy" onclick="return asGroovy('SampleFilterConfig');">View as .groovy</span>
<pre id="SampleFilterConfig" class="prettyprint source"><configuration>
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<b><filter class="chapters.filters.SampleFilter" /></b>
<encoder>
<pattern>
%-4relative [%thread] %-5level %logger - %msg%n
</pattern>
</encoder>
</appender>
<root>
<appender-ref ref="STDOUT" />
</root>
</configuration></pre>
<p>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 <code><filter></code>
element.
</p>
<p>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.
</p>
<p>Taking notice of this orthogonality, logback ships with the <a
href="../xref/ch/qos/logback/core/filter/AbstractMatcherFilter.html">
<code>AbstractMatcherFilter</code></a> class which provides a
useful skeleton for specifying the appropriate response on match
and on mismatch, with the help of two properties, named
<em>OnMatch</em> and <em>OnMismatch</em>. Most of the regular
filters included in logback are derived from
<code>AbstractMatcherFilter</code>.
</p>
<h3 class="doAnchor" name="levelFilter">LevelFilter</h3>
<p><a href="../xref/ch/qos/logback/classic/filter/LevelFilter.html">
<code>LevelFilter</code></a> 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 <span class="option">onMatch</span> and <span
class="option">onMismatch</span> properties. Here is a sample
configuration file.
</p>
<em>Example: Sample LevelFilter configuration
(logback-examples/src/main/java/chapters/filters/levelFilterConfig.xml)</em>
<span class="asGroovy" onclick="return asGroovy('levelFilterConfig');">View as .groovy</span>
<pre id="levelFilterConfig" class="prettyprint source"><configuration>
<appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
<b><filter class="ch.qos.logback.classic.filter.LevelFilter">
<level>INFO</level>
<onMatch>ACCEPT</onMatch>
<onMismatch>DENY</onMismatch>
</filter></b>
<encoder>
<pattern>
%-4relative [%thread] %-5level %logger{30} - %msg%n
</pattern>
</encoder>
</appender>
<root level="DEBUG">
<appender-ref ref="CONSOLE" />
</root>
</configuration></pre>
<h3 class="doAnchor" name="thresholdFilter">ThresholdFilter</h3>
<p>The <a
href="../xref/ch/qos/logback/classic/filter/ThresholdFilter.html">
<code>ThresholdFilter</code></a> filters events below the
specified threshold. For events of level equal or above the
threshold, <code>ThresholdFilter</code> will respond NEUTRAL when
its <code>decide</code>() method is invoked. However, events with
a level below the threshold will be denied. Here is a sample
configuration file.
</p>
<em>Example: Sample ThresholdFilter configuration
(logback-examples/src/main/java/chapters/filters/thresholdFilterConfig.xml)</em>
<span class="asGroovy" onclick="return asGroovy('thresholdFilterConfig');">View as .groovy</span>
<pre id="thresholdFilterConfig" class="prettyprint source"><configuration>
<appender name="CONSOLE"
class="ch.qos.logback.core.ConsoleAppender">
<!-- deny all events with a level below INFO, that is TRACE and DEBUG -->
<b><filter class="ch.qos.logback.classic.filter.ThresholdFilter">
<level>INFO</level>
</filter></b>
<encoder>
<pattern>
%-4relative [%thread] %-5level %logger{30} - %msg%n
</pattern>
</encoder>
</appender>
<root level="DEBUG">
<appender-ref ref="CONSOLE" />
</root>
</configuration></pre>
<h2 class="doAnchor" name="evalutatorFilter">EvaluatorFilter</h2>
<p><a
href="../xref/ch/qos/logback/core/filter/EvaluatorFilter.html"><code>EvaluatorFilter</code></a>
is a generic filter encapsulating an
<code>EventEvaluator</code>. As the name suggests, an
<a
href="../xref/ch/qos/logback/core/boolex/EventEvaluator.html">
<code>EventEvaluator</code></a> evaluates whether a given criteria
is met for a given event. On match and on mismatch,
the hosting <code>EvaluatorFilter</code> will return the value
specified by the <span class="option">onMatch</span>
or <span class="option">onMismatch</span> properties respectively.
</p>
<p>Note that <code>EventEvaluator</code> is an abstract class. You
can implement your own event evaluation logic by sub-classing
<code>EventEvaluator</code>.
</p>
<!-- ======================== GEventEvaluator ========================= -->
<h3 class="doAnchor" name="GEventEvaluator">GEventEvaluator</h3>
<p><a
href="../xref/ch/qos/logback/classic/boolex/GEventEvaluator.html">GEventEvaluator</a>
is a concrete <a
href="../xref/ch/qos/logback/core/boolex/EventEvaluator.html"><code>EventEvaluator</code></a>
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. <code>GEventEvaluator</code> requires the Groovy
runtime. Please see the <a
href="../setup.html#groovy">corresponding section</a> of
the setup document on adding the Groovy runtime to your class
path.
</p>
<p>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.
</p>
<p>The evaluation expression acts on the current logging event.
Logback automatically inserts the current logging event of type <a
href="../apidocs/ch/qos/logback/classic/spi/ILoggingEvent.html">ILoggingEvent</a>
as a variable referred to as '<em>event</em>' as well as its
shorthand referred to as '<em>e</em>'. 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
<code>true</code> 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
<code>toInt()</code> operator.
</p>
<p>Here is a more complete example.</p>
<span class="asGroovy" onclick="return asGroovy('GEventEvaluator');">View as .groovy</span>
<pre id="GEventEvaluator" class="prettyprint source"><configuration>
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<b><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></b>
<encoder>
<pattern>
%-4relative [%thread] %-5level %logger - %msg%n
</pattern>
</encoder>
</appender>
<root level="DEBUG">
<appender-ref ref="STDOUT" />
</root>
</configuration></pre>
<p>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
<code>/Googlebot|msnbot|Yahoo/</code> regular expression. Note
that since the MDC map can be null, we are also using Groovy's <a
href="http://groovy.codehaus.org/Null+Object+Pattern">safe
dereferencing operator</a>, that is the ?. operator. The equivalent
logic would have been much longer if expressed in Java.
</p>
<p>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 <a
href="mdc.html#mis"><code>MDCInsertingServletFilter</code></a>
designed for this purpose. It will be described in a later
chapter.
</p>
<!-- ==================== JaninoEventEvaluator ======================== -->
<h3 class="doAnchor"
name="JaninoEventEvaluator">JaninoEventEvaluator</h3>
<p>Logback-classic ships with another concrete
<code>EventEvaluator</code> implementation called <a
href="../xref/ch/qos/logback/classic/boolex/JaninoEventEvaluator.html">JaninoEventEvaluator</a>
taking an arbitrary Java language block returning a boolean value
as the evaluation criteria. We refer to such Java language boolean
expressions as "<em>evaluation expressions</em>". Evaluation
expressions enable great flexibility in event
filtering. <code>JaninoEventEvaluator</code> requires the <a
href="http://docs.codehaus.org/display/JANINO/Home">Janino
library</a>. Please see the <a
href="../setup.html#janino">corresponding section</a> of the setup
document. Compared to <code>JaninoEventEvaluator</code>,
<code>GEventEvaluator</code>, by virtue of the Groovy language, is
significantly more convenient to use, but
<code>JaninoEventEvaluator</code> will usually run (much) faster
for equivalent expressions.
</p>
<p>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. </p>
<p>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.
</p>
<table class="bodyTable">
<tr>
<th>Name</th>
<th>Type</th>
<th>Description</th>
</tr>
<tr>
<td>event</td>
<td><code>LoggingEvent</code></td>
<td>The raw logging event associated with the logging
request. All of the following variables are also available
from the event. For example, <code>event.getMessage()</code>
returns the same String value as the <em>message</em> variable
described next.
</td>
</tr>
<tr class="alt">
<td>message</td>
<td><code>String</code></td>
<td>The raw message of the logging request. For some logger
<em>l</em>, when you write l.info("Hello {}", name); where
name is assigned the value "Alice", then "Hello {}" is the
message.</td> </tr>
<tr>
<td>formattedMessage</td>
<td><code>String</code></td>
<td>The formatted message in the logging request. For some
logger <em>l</em>, when you write l.info("Hello {}", name);
where name is assigned the value "Alice", then "Hello Alice"
is the formatted message.</td>
</tr>
<tr class="alt">
<td>logger</td>
<td><code>String</code></td>
<td>The name of the logger.
</td>
</tr>
<tr>
<td>loggerContext</td>
<td><a
href="../xref/ch/qos/logback/classic/spi/LoggerContextVO.html"><code>LoggerContextVO</code></a></td>
<td>A restricted (value object) view of the logger context to which the logging event belongs to.
</td>
</tr>
<tr class="alt">
<td>level</td>
<td><code>int</code></td>
<td>The int value corresponding to the level. To help create
easily expressions involving levels, the default value
<em>DEBUG</em>, <em>INFO</em>, <em>WARN</em> and
<em>ERROR</em> are also available. Thus, using <em>level >
INFO</em> is a correct expression.
</td>
</tr>
<tr>
<td>timeStamp
</td>
<td><code>long</code></td>
<td>The timestamp corresponding to the logging event's
creation.
</td>
</tr>
<tr class="alt">
<td>marker</td>
<td><code>Marker</code></td>
<td>The <code>Marker</code> object associated with the logging
request. Note that marker can be null and it is your
responsibility to check for this condition in order to avoid
<code>NullPointerException</code>.
</td>
</tr>
<tr>
<td>mdc</td>
<td><code>Map</code></td>
<td>A map containing all the MDC values at the time of the
creation of the logging event. A value can be accessed by
using the following expression: <em>mdc.get("myKey")</em>. As
of logback-classic version 0.9.30, the 'mdc' variable will
never be null.
<p>The <code>java.util.Map</code> type is non-parameterized
because Janino does not support generics. It follows that the
type returned by <code>mdc.get()</code> is <code>Object</code>
and not <code>String</code>. To invoke <code>String</code>
methods on the returned value, it must be cast as
<code>String</code>. For example,
<code>((String) mdc.get("k")).contains("val")</code>.
</p>
</td>
</tr>
<tr class="alt">
<td>throwable</td>
<td>java.lang.Throwable</td>
<td>If no exception is associated with the event, then the
value of the "throwable" variable will be null. Unfortunately,
"throwable" does not survive serialization. Thus, on remote
systems, its value will always be null. For location
independent expressions, use the <code>throwableProxy</code>
variable described next.
</td>
</tr>
<tr>
<td>throwableProxy</td>
<td><a
href="../xref/ch/qos/logback/classic/spi/IThrowableProxy.html"><code>IThrowableProxy</code></a></td>
<td>A proxy for the exception associated with the logging
event. If no exception is associated with the event, then the
value of the "throwableProxy" variable will be null. In
contrast to "throwable", when an exception is associated with
an event, the value of "throwableProxy" will be non-null even
on remote systems, that is even after serialization.
</td>
</tr>
</table>
<p>Here is a concrete example.</p>
<em>Example: Basic event evaluator usage
(logback-examples/src/main/java/chapters/filters/basicEventEvaluator.xml)</em>
<span class="asGroovy" onclick="return asGroovy('basicEventEvaluator');">View as .groovy</span>
<pre id="basicEventEvaluator" class="prettyprint source longline"><configuration>
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<b><filter class="ch.qos.logback.core.filter.EvaluatorFilter">
<evaluator> <!-- defaults to type ch.qos.logback.classic.boolex.JaninoEventEvaluator -->
<expression><span class="green">return message.contains("billing");</span></expression>
</evaluator>
<OnMismatch>NEUTRAL</OnMismatch>
<OnMatch>DENY</OnMatch>
</filter></b>
<encoder>
<pattern>
%-4relative [%thread] %-5level %logger - %msg%n
</pattern>
</encoder>
</appender>
<root level="INFO">
<appender-ref ref="STDOUT" />
</root>
</configuration></pre>
<p>The bold part in the above configuration file adds an
<code>EvaluatorFilter</code> to a <code>ConsoleAppender</code>. An
evaluator of type <code>JaninoEventEvaluator</code> is then
injected into the <code>EvaluatorFilter</code>. In the absence of
<span class="attr">class</span> attribute in the
<code><evaluator></code> element specified by the user, Joran
will infer a default type of <code>JaninoEventEvaluator</code>
for the evaluator. This is one of the <a
href="onJoran.html#defaultClassMapping">few occurrences</a> where
Joran implicitly infers the type of a component.
</p>
<p>The <em>expression</em> element corresponds to the evaluation
expression just discussed. The expression
<code>return message.contains("billing");</code> returns a boolean
value. Notice that the <em>message</em> variable is exported
automatically by <code>JaninoEventEvaluator</code>.
</p>
<p>Given that the <span class="option">OnMismatch</span> property is
set to NEUTRAL and the <span class="option">OnMatch</span>
property set to DENY, this evaluator filter will drop all logging
events whose message contains the string "billing".
</p>
<p>The <a
href="../xref/chapters/filters/FilterEvents.html"><code>FilterEvents</code></a>
application issues ten logging requests, numbered 0 to 9. Let us
first run <code>FilterEvents</code> class without any filters:
</p>
<div class="source"><pre>
java chapters.filters.FilterEvents src/main/java/chapters/filters/basicConfiguration.xml
</pre></div>
<p>All requests will be displayed, as shown below:</p>
<div class="source"><pre>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 - <b>billing statement 6</b>
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</pre></div>
<p>Suppose that we want to get rid of the "billing statement".
The <em>basicEventEvaluator.xml</em> configuration file listed
above filters messages containing the string "billing" which is
precisely the desired outcome.</p>
<p>Running with <em>basicEventEvaluator.xml</em>:</p>
<p class="source">java chapters.filters.FilterEvents src/main/java/chapters/filters/basicEventEvaluator.xml</p>
<p>we obtain:
</p>
<p class="source">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</p>
<p>Evaluation expressions can be Java blocks. For example, the
following is a valid expression.</p>
<pre class="prettyprint source"><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></pre>
<h2 class="doAnchor" name="matcher">Matchers</h2>
<p>While it is possible to do pattern matching by invoking the <a
href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html#matches%28java.lang.String%29">matches()</a>
method in the <code>String</code> class, this incurs the cost of
compiling of a brand new <code>Pattern</code> object each time the
filter is invoked. To eliminate this overhead, you can predefine
one or more <a
href="../xref/ch/qos/logback/core/boolex/Matcher.html">Matcher</a>
objects. Once a matcher is defined, it can be repeatedly
referenced by name in the evaluator expression.</p>
<p>An example should clarify the point:</p>
<em>Example: Defining matchers in an event evaluator (logback-examples/src/main/java/chapters/filters/evaluatorWithMatcher.xml)</em>
<span class="asGroovy" onclick="return asGroovy('evaluatorWithMatcher');">View as .groovy</span>
<pre id="evaluatorWithMatcher" class="prettyprint source"><configuration debug="true">
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<filter class="ch.qos.logback.core.filter.EvaluatorFilter">
<evaluator>
<b><matcher>
<Name>odd</Name>
<!-- filter out odd numbered statements -->
<regex>statement [13579]</regex>
</matcher>
<expression>odd.matches(formattedMessage)</expression></b>
</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></pre>
<p>Running with <em>evaluatorWithMatcher.xml</em>:</p>
<p class="source">java chapters.filters.FilterEvents src/main/java/chapters/filters/evaluatorWithMatcher.xml</p>
<p>we obtain:
</p>
<p class="source">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</p>
<p>If you need to define additional matchers, you can do so by
adding further <code><matcher></code> elements.</p>
<!-- ================================================================ -->
<!-- ===================== TURBO FILTER ============================= -->
<!-- ================================================================ -->
<h2 class="doAnchor" name="TurboFilter">TurboFilters</h2>
<p><code>TurboFilter</code> objects all extend the
<a href="../xref/ch/qos/logback/classic/turbo/TurboFilter.html">
<code>TurboFilter</code></a> abstract class. Like the regular
filters, they use ternary logic to return their evaluation of
the logging event.
</p>
<p>Overall, they work much like the previously mentioned
filters. However, there are two main differences between
<code>Filter</code> and <code>TurboFilter</code> objects.
</p>
<p><code>TurboFilter</code> 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.
</p>
<p>More importantly, they are called before the
<code>LoggingEvent</code> object creation.
<code>TurboFilter</code> 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.
</p>
<h3 class="doAnchor" name="yourOwnTurboFilter">Implementing your
own TurboFilter</h3>
<p>To create your own <code>TurboFilter</code> component, just
extend the <code>TurboFilter</code> abstract class. As previously,
when implementing a customized filter object, developing a custom
<code>TurboFilter</code> only asks that one implement the
<code>decide()</code> method. In the next example, we create a
slightly more complex filter:
</p>
<em>Example: Basic custom <code>TurboFilter</code> (<a href="../xref/chapters/filters/SampleTurboFilter.html">logback-examples/src/main/java/chapters/filters/SampleTurboFilter.java</a>)</em>
<pre class="prettyprint source">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();
}
}
}
</pre>
<p>The <code>TurboFilter</code> 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.
</p>
<p>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 <code>start()</code>
method, to check that the option has been specified during the
configuration process.
</p>
<p>Here is a sample configuration that makes use of our newly
created <code>TurboFilter</code>.
</p>
<em>Example: Basic custom <code>TurboFilter</code> configuration
(logback-examples/src/main/java/chapters/filters/sampleTurboFilterConfig.xml)</em>
<span class="asGroovy" onclick="return asGroovy('sampleTurboFilterConfig');">View as .groovy</span>
<pre id="sampleTurboFilterConfig" class="prettyprint source"><configuration>
<b><turboFilter class="chapters.filters.SampleTurboFilter">
<Marker>sample</Marker>
</turboFilter></b>
<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></pre>
<p>Logback classic ships with several <code>TurboFilter</code>
classes ready for use. The <a
href="../xref/ch/qos/logback/classic/turbo/MDCFilter.html"><code>MDCFilter</code></a>
checks the presence of a given value in the MDC whereas <a
href="../apidocs/ch/qos/logback/classic/turbo/DynamicThresholdFilter.html"><code>DynamicThresholdFilter</code></a>
allows filtering based on MDC key/level threshold associations. On
the other hand, <a
href="../xref/ch/qos/logback/classic/turbo/MarkerFilter.html"><code>MarkerFilter</code></a>
checks for the presence of a specific marker associated with the
logging request.
</p>
<p>Here is a sample configuration, using both
<code>MDCFilter</code> and <code>MarkerFilter</code>.
</p>
<em>Example: <code>MDCFilter</code> and <code>MarkerFilter</code>
configuration
(logback-examples/src/main/java/chapters/filters/turboFilters.xml)</em>
<span class="asGroovy" onclick="return asGroovy('turboFilters');">View as .groovy</span>
<pre id="turboFilters" class="prettyprint source"><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></pre>
<p>You can see this configuration in action by issuing the
following command:
</p>
<p class="source">java chapters.filters.FilterEvents src/main/java/chapters/filters/turboFilters.xml</p>
<p>As we've seen previously, the <a
href="../xref/chapters/filters/FilterEvents.html"><code>FilterEvents</code></a>
application issues 10 logging requests, numbered 0 to 9. Except
for requests 3 and 6, all of the requests are of level
<em>INFO</em>, the same level as the one assigned to the root
logger. The 3rd request, is issued at the the <em>DEBUG</em>
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 <code>MDCFilter</code>
specifically accepts the request (and only that request). The 6th
request, issued at the <em>ERROR</em> level, is marked as
"billing". As such, it is denied by the MarkerFilter (the second
turbo filter in the configuration).
</p>
<p>Thus, the output of <code>FilterEvents</code> application
configured with <em>turboFilters.xml</em> file shown above is:
</p>
<p class="source">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</p>
<p>One can see that the 3rd request, which should not be displayed
if we only followed the overall <em>INFO</em> level, appears
anyway, because it matched the first <code>TurboFilter</code>
requirements and was accepted.
</p>
<p>On the other hand, the 6th request, that is an <em>ERROR</em>
level request should have been displayed. But it satisfied the
second <code>TurboFilter</code> whose <span
class="option">OnMatch</span> option is set to <em>DENY</em>.
Thus, the 6th request was not displayed.
</p>
<h3 class="doAnchor"
name="DuplicateMessageFilter">DuplicateMessageFilter</h3>
<p>The <code>DuplicateMessageFilter</code> merits a separate
presentation. This filter detects duplicate messages, and beyond
a certain number of repetitions, drops repeated messages.
</p>
<p>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:
</p>
<pre class="prettyprint source">logger.debug("Hello "+name0);
logger.debug("Hello "+name1);</pre>
<p>Assuming <code>name0</code> and <code>name1</code> 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.
</p>
<p>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.
</p>
<pre class="prettyprint source">logger.debug("Hello {}.", name0);
logger.debug("Hello {}.", name1);</pre>
<p>The number of allowed repetitions can be specified by the <span
class="option">AllowedRepetitions</span> 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 <span
class="option">AllowedRepetitions</span> property is set to 5.
</p>
<p>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 <span class="option">CacheSize</span>
property. By the default, this is set to 100.
</p>
<em>Example: <code>DuplicateMessageFilter</code>
configuration (logback-examples/src/main/java/chapters/filters/duplicateMessage.xml)</em>
<span class="asGroovy" onclick="return asGroovy('duplicateMessage');">View as .groovy</span>
<pre id="duplicateMessage" class="prettyprint source"><configuration>
<b><turboFilter class="ch.qos.logback.classic.turbo.DuplicateMessageFilter"/></b>
<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></pre>
<p>Thus, the output for <code>FilterEvents</code> application
configured with <em>duplicateMessage.xml</em> is:
</p>
<p class="source">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</p>
<p>"logging statement 0" is the first <em>occurrence</em> of the
message "logging statement {}". "logging statement 1" is the first
<em>repetition</em>, "logging statement 2" is the second
repetition. Interestingly enough, "logging statement 3" of level
DEBUG, is the <em>third</em> repetition, even though it is later
dropped by virtue of the <a
href="architecture.html#basic_selection">basic selection
rule</a>. This can be explained by the fact that turbo filters are
invoked before other types of filters, including the basic
selection rule. Thus, <code>DuplicateMessageFilter</code>
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.
</p>
<h1 class="doAnchor" name="logbac-access">In logback-access</h1>
<p>Logback-access offers most of the features available with
logback-classic. In particular, <code>Filter</code> objects are
available and work in the same way as their logback-classic
counterparts, with one notable difference. Instead of
<code>LoggingEvent</code> instances logback-access filters act
upon <a
href="../xref/ch/qos/logback/access/spi/AccessEvent.html"><code>AccessEvent</code></a>
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.
</p>
<h2 class="doAnchor"
name="countingFilter"><code>CountingFilter</code></h2>
<p>With the help of <a
href="xref/ch/qos/logback/access/filter/CountingFilter.html"><code>CountingFilter</code></a>
class, logback-access can provide statistical data about access to
the web-server. Upon initialization, <code>CountingFilter</code>
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.
</p>
<p>The following <em>logback-access.xml</em> configuration file
declares a <code>CountingFilter</code>. </p>
<pre class="prettyprint source"><configuration>
<statusListener class="ch.qos.logback.core.status.OnConsoleStatusListener" />
<b><filter class="ch.qos.logback.access.filter.CountingFilter">
<name>countingFilter</name>
</filter></b>
<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></pre>
<p>You can examine the various statistics maintained by
<code>CountingFilter</code> on your platform's JMX server via the
<code>jconsole</code> application. </p>
<img alt="CountingFilter via jconsole" src="images/chapters/filters/countingFilter.png" />
<h3 class="doAnchor" name="access_EvalutorFilter">EvaluatorFilter</h3>
<p><a
href="../xref/ch/qos/logback/core/filter/EvaluatorFilter.html"><code>EvaluatorFilter</code></a>
is a generic filter encapsulating an
<code>EventEvaluator</code>. As the name suggests, an
<a
href="../xref/ch/qos/logback/core/boolex/EventEvaluator.html">
<code>EventEvaluator</code></a> evaluates whether a given criteria
is met for a given event. On match and on mismatch,
the hosting <code>EvaluatorFilter</code> will return the value
specified by the <span class="option">onMatch</span> or
<span class="option">onMismatch</span> properties respectively. Note that
<code>EvaluatorFilter</code> has been previously discussed in the
context of logback-classic (<a href="#evalutatorFilter">see
above</a>). The present text is mostly a repetition of the
previous discussion.</p>
<p>Note that <code>EventEvaluator</code> is an abstract class. You
can implement your own event evaluation logic by sub-classing
<code>EventEvaluator</code>. Logback-access ships with a concrete
implementation named <a
href="../xref/ch/qos/logback/access/boolex/JaninoEventEvaluator.html">JaninoEventEvaluator</a>.
It takes arbitrary Java language boolean expressions as the
evaluation criteria. We refer to such Java language blocks as
"<em>evaluation expressions</em>". Evaluation expressions enable
great flexibility in event
filtering. <code>JaninoEventEvaluator</code> requires the <a
href="http://docs.codehaus.org/display/JANINO/Home">Janino
library</a>. Please see the <a
href="../setup.html#janino">corresponding section</a> of the setup
document.
</p>
<p>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. </p>
<p>The evaluation expression is evaluated on the current access
event. Logback-access automatically exports the current
<code>AccessEvent</code> instance under the variable name
<b><code>event</code></b>. You can read the various data
associated with the HTTP request as well as the HTTP response via
the <code>event</code> variable. Please refer to the <a
href="../xref/ch/qos/logback/access/spi/AccessEvent.html"><code>AccessEvent</code>
class source code</a> for the exact list.
</p>
<p>The next logback-access configuration file illustrates
filtering based on the <a
href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.5">404
(Not Found)</a> HTTP response code. Every request resulting in a
404 will be printed on the console.</p>
<em>Example: Access Evaluator (logback-examples/src/main/java/chapters/filters/accessEventEvaluator.xml)</em>
<pre class="prettyprint source"><configuration>
<statusListener class="ch.qos.logback.core.status.OnConsoleStatusListener" />
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<b><filter class="ch.qos.logback.core.filter.EvaluatorFilter">
<evaluator>
<expression>event.getStatusCode() == 404</expression>
</evaluator>
<onMismatch>DENY</onMismatch>
</filter></b>
<encoder><pattern>%h %l %u %t %r %s %b</pattern></encoder>
</appender>
<appender-ref ref="STDOUT" />
</configuration></pre>
<p>In the next example, we still log requests resulting in 404 errors,
except those requests asking for CSS files.
</p>
<em>Example 6.10: Access Evaluator (logback-examples/src/main/java/chapters/filters/accessEventEvaluator2.xml)</em>
<pre class="prettyprint source"><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>
<b>(event.getStatusCode() == 404)</b>
<b>&amp;&amp;</b> <!-- ampersand characters need to be escaped -->
<b>!(event.getRequestURI().contains(".css"))</b>
</expression>
</evaluator>
<onMismatch>DENY</onMismatch>
</filter>
<encoder><pattern>%h %l %u %t %r %s %b</pattern></encoder>
</appender>
<appender-ref ref="STDOUT" />
</configuration>
</pre>
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