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<a id="Overview"></a>
<h2>Overview</h2>
<p>
A Checkstyle listener monitors the progress of a <span class="code">Checker</span> during the audit of files. The <span class="code">Checker</span> notifies its attached listeners of
significant events such as the start of the audit of a file and
the logging of a Check error, and the listeners respond
appropriately. Any number of listeners can be attached to a
<span class="code"> Checker</span>. An audit always adds one of
the distribution listeners, <a href="api/com/puppycrawl/tools/checkstyle/DefaultLogger.html">DefaultLogger</a>
or <a href="api/com/puppycrawl/tools/checkstyle/XMLLogger.html">XMLLogger</a>,
to report events. A <span class="code">DefaultLogger</span>
produces simple text output for the events it receives, and a
<span class="code">XMLLogger</span> produces an XML document for
its events.
</p>
<p>
Listeners <span class="code">DefaultLogger</span> and <span class="code"> XMLLogger</span> are sufficient for most
Checkstyle users, but you may find a need for a custom
listener. For example, a user has requested verbose output of
progress information during a Checkstyle run. Another user would
like to filter error events. This document explains how to write
listeners for such tasks and how to integrate them in a Checker
module. It also describes two custom listeners that are inspired
by ANT listeners: a listener that is a wrapper for the Jakarta
Commons Logging API, and a listener that sends its results via
email.
</p>
<p>
A listener is an implementation of the <a href="api/com/puppycrawl/tools/checkstyle/api/AuditListener.html">AuditListener</a>
interface. During an audit, a <span class="code">Checker</span>
informs its attached <span class="code">AuditListeners</span> of
six kinds of events: audit started/ended, file started/ended,
and logging of an error/exception.
</p>
<p>
An audit passes an event to a listener as an <a href="api/com/puppycrawl/tools/checkstyle/api/AuditEvent.html">AuditEvent</a>.
A file-related <span class="code">AuditEvent</span> contains the
name of that file. An <span class="code">AuditEvent</span> for
error logging has a message, a severity level, a message source
such as the name of a <span class="code">Check</span>, and file
line and column numbers that may be relevant to the error. The
notification of an exception to a <span class="code">AuditListener</span> includes an error <span class="code">AuditEvent</span> and the details of the
exception. Here is a UML diagram for classes <span class="code">AuditListener</span> and <span class="code">AuditEvent</span>.
</p>
<image src="images/AuditListener.gif" width="381" height="488" alt="AuditListener UML diagram" />
<a id="Writing Listeners"></a>
<h2>Writing Listeners</h2>
<p>
A custom listener is an implementation of the <a href="api/com/puppycrawl/tools/checkstyle/api/AuditListener.html">AuditListener</a>
interface. If the listener has properties that can be set from a
configuration, the listener must extend <a href="api/com/puppycrawl/tools/checkstyle/api/AutomaticBean.html">AutomaticBean</a>.
An <span class="code">AutomaticBean</span> uses JavaBean
introspection to set JavaBean properties.
</p>
<p>
The custom listener that we demonstrate here is a verbose
listener that simply prints each event notification to an output
stream, and reports the number of errors per audited file and
the total number of errors. The default output stream is <span class="code">System.out</span>. In order to enable the
specification of output to a file through property <span class="code">file</span>, the class extends <span class="code">AutomaticBean</span> and defines method <span class="code">setFile(String)</span>.
</p>
<div align="left">
<table cellspacing="4" cellpadding="0" border="0">
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
<tr>
<td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#ffffff"><pre>package com.mycompany.listeners;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import com.puppycrawl.tools.checkstyle.api.AuditEvent;
import com.puppycrawl.tools.checkstyle.api.AuditListener;
import com.puppycrawl.tools.checkstyle.api.AutomaticBean;
import com.puppycrawl.tools.checkstyle.api.SeverityLevel;
public class VerboseListener
extends AutomaticBean
implements AuditListener
{
private PrintWriter mWriter = new PrintWriter(System.out);
private boolean mCloseOut = false;
private int mTotalErrors;
private int mErrors;
public void setFile(String aFileName)
throws FileNotFoundException
{
final OutputStream out = new FileOutputStream(aFileName);
mWriter = new PrintWriter(out);
mCloseOut = true;
}
public void auditStarted(AuditEvent aEvt)
{
mTotalErrors = 0;
mWriter.println("Audit started.");
}
public void auditFinished(AuditEvent aEvt)
{
mWriter.println("Audit finished. Total errors: " + mTotalErrors);
mWriter.flush();
if (mCloseOut) {
mWriter.close();
}
}
public void fileStarted(AuditEvent aEvt)
{
mErrors = 0;
mWriter.println(
"Started checking file '" + aEvt.getFileName() + "'.");
}
public void fileFinished(AuditEvent aEvt)
{
mWriter.println("Finished checking file '" + aEvt.getFileName()
+ "'. Errors: " + mErrors);
}
public void addError(AuditEvent aEvt)
{
printEvent(aEvt);
if (SeverityLevel.ERROR.equals(aEvt.getSeverityLevel())) {
mErrors++;
mTotalErrors++;
}
}
public void addException(AuditEvent aEvt, Throwable aThrowable)
{
printEvent(aEvt);
aThrowable.printStackTrace(System.out);
mErrors++;
mTotalErrors++;
}
private void printEvent(AuditEvent aEvt)
{
mWriter.println("Logging error -"
+ " file: '" + aEvt.getFileName() + "'"
+ " line: " + aEvt.getLine()
+ " column: " + aEvt.getColumn()
+ " severity: " + aEvt.getSeverityLevel()
+ " message: " + aEvt.getMessage()
+ " source: " + aEvt.getSourceName());
}
}</pre></td>
<td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
</table>
</div>
<p>
A listener that filters error events could perform the filtering
in methods <span class="code"> addError</span> and <span class="code">addException</span>. As further examples of
listeners, <a href="#CommonsLoggingListener">CommonsLoggingListener</a>
reports its events through the Commons Logging API, and <a href="#MailLogger">MailLogger</a> e-mails the audit report of a
<span class="code">DefaultLogger</span>.
</p>
<a id="Using Listeners"></a>
<h2>Using Listeners</h2>
<p>
To incorporate a custom listener in the set of listeners for a
<span class="code">Checker</span>, include a module element for
the listener in the <a href="config.html#auditlisteners">configuration file</a>. For
example, to configure a <span class="code">Checker</span> so
that it uses custom listener <span class="code">VerboseListener</span> to print audit messages to a
file named "audit.txt", include the following module
in the configuration file:
</p>
<div align="left">
<table cellspacing="4" cellpadding="0" border="0">
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
<tr>
<td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#ffffff"><pre><module name="com.mycompany.listeners.VerboseListener">
<property name="file" value="audit.txt"/>
</module></pre></td>
<td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
</table>
</div>
<p>
Here is a truncated example of audit output from a <span class="code"> VerboseListener</span>:
</p>
<div align="left">
<table cellspacing="4" cellpadding="0" border="0">
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
<tr>
<td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#ffffff"><pre>Audit started.
Started checking file 'CommonsLoggingListener.java'.
Finished checking file 'CommonsLoggingListener.java'. Errors: 0
Started checking file 'MailLogger.java'.
Finished checking file 'MailLogger.java'. Errors: 0
Started checking file 'VerboseListener.java'.
Logging error - file: 'VerboseListener.java' line: 23 ...
Finished checking file 'VerboseListener.java'. Errors: 1
Audit finished. Total errors: 1</pre></td>
<td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
</table>
</div>
<a id="Examples"></a>
<h2>Examples</h2>
<p>
This section describes two examples based on <a href="http://ant.apache.org/">ANT</a> listeners. The first
listener, <span class="code"> CommonsLoggingListener</span>,
hands off events to the <a href="http://jakarta.apache.org/commons/logging.html">Jakarata
Commons Logging</a> facade and the second, <span class="code">MailLogger</span>, sends a report of an audit via
e-mail. The discussion of these examples and how to use them is
derived from material in <a href="http://www.manning.com/antbook">"Java Development
with Ant"</a> by Eric Hatcher and Steve Loughran, an
excellent ANT book.
</p>
<h4>CommonsLoggingListener</h4>
<p>
<a href="http://jakarta.apache.org/commons/logging.html">Jakarata
Commons Logging</a> provides a facade for logging tools
<a href="http://jakarta.apache.org/log4j/docs/index.html">log4j</a>,
<a href="http://jakarta.apache.org/avalon/logkit/index.html">Avalon
LogKit</a>, J2SE 1.4, and others. Checkstyle listener <span class="code"> CommonsLoggingListener</span> responds to an
AuditEvent by handing it off to the current Commons Logging Log.
</p>
<p>
The source code for <span class="code">CommonsLoggingListener</span> is in distribution
directory <span class="default">contrib/examples/listeners</span>. Notice that
each <span class="code">AuditListener</span> method that
receives an <span class="code"> AuditEvent</span> calls a method
for the Commons Logging log level corresponding to the
Checkstyle <span class="code">SeverityLevel</span> of the <span class="code"> AuditEvent</span>.
</p>
<p>
The easiest way to use <span class="code">CommonsLoggingListener</span> is to include <span class="default">checkstyle-all-4.4.jar</span>
in the classpath because that jar file contains all the Commons
Logging classes. The default Log under J2SE 1.4 is wrapper
class <a href="http://jakarta.apache.org/commons/logging/api/org/apache/commons/logging/impl/Jdk14Logger.html">Jdk14Logger</a>.
Under earlier Java versions, the default Log is a simple wrapper
class, <a href="http://jakarta.apache.org/commons/logging/api/org/apache/commons/logging/impl/SimpleLog.html">SimpleLog</a>.
Both default logging tools can be used directly from Commons
Logging; if you need to use other tools such as log4j, then you
must include the appropriate jar file(s) in the classpath.
</p>
<p>
Logging configuration details for Jakarta Commons Logging are in
the <a href="http://jakarta.apache.org/commons/logging.html">documentation</a>.
As a simple example, assume that <span class="default">log4j.jar</span> is in the classpath and the
following <span class="default">log4j.properties</span> file is
in the current directory:
</p>
<div align="left">
<table cellspacing="4" cellpadding="0" border="0">
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
<tr>
<td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#ffffff"><pre># Set root logger level to INFO and its only appender to A1.
log4j.rootLogger=INFO, A1
# A1 is set to be a ConsoleAppender.
log4j.appender.A1=org.apache.log4j.ConsoleAppender
# A1 uses PatternLayout.
log4j.appender.A1.layout=org.apache.log4j.PatternLayout
log4j.appender.A1.layout.ConversionPattern=%-5p %c %x- %m%n</pre></td>
<td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
</table>
</div>
<p>
Running a Checkstyle audit with a <span class="code">CommonsLoggingListener</span> yields this
(abbreviated) output:
</p>
<div align="left">
<table cellspacing="4" cellpadding="0" border="0">
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
<tr>
<td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#ffffff"><pre>INFO com.puppycrawl...Checker - Audit started.
INFO com.puppycrawl...Checker - File "CommonsLoggingListener.java" started.
INFO com.puppycrawl...Checker - File "CommonsLoggingListener.java" finished.
INFO com.puppycrawl...Checker - File "MailLogger.java" started.
INFO com.puppycrawl...Checker - File "MailLogger.java" finished.
INFO com.puppycrawl...Checker - File "VerboseListener.java" started.
ERROR com.puppycrawl...ParenPadCheck - Line: 23 Column: 28 ...
INFO com.puppycrawl...Checker - File "VerboseListener.java" finished.
INFO com.puppycrawl...Checker - Audit finished.</pre></td>
<td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
</tr>
</table>
</div>
<h4>MailLogger</h4>
<p>
<span class="code">MailLogger</span> sends an audit report in an
email message. The listener uses a <span class="code">DefaultLogger</span> to prepare the text of the
message. The listener obtains other message parameters such as
<span class="default">to</span> and <span class="default">subject</span> from environment properties that
can be read from a properties file.
</p>
<p>
The source code for <span class="code">CommonsLoggingListener</span> is in distribution
directory <span class="default">contrib/examples/listeners</span>. This
implementation uses the <a href="http://java.sun.com/products/javamail">JavaMail API</a> as
the mail system, and you must include appropriate jar files in
the classpath.
</p>
<p>
As an example of using <span class="code">MailLogger</span>, set
system property <span class="default">-DMailLogger.properties.file=MailLogger.properties</span>,
so that <span class="code">MailLogger</span> reads message
parameters from file <span class="default">MailLogger.properties</span> of the current
directory:
</p>
<div align="left">
<table cellspacing="4" cellpadding="0" border="0">
<tr>
<td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
<td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td>
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<td bgcolor="#ffffff"><pre>MailLogger.from=user@example.org
MailLogger.failure.to=user@example.org
MailLogger.success.to=user@example.org
MailLogger.mailhost=localhost</pre></td>
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<a id="Huh? I can't figure it out!"></a>
<h2>Huh? I can't figure it out!</h2>
<p>
That's probably our fault, and it means that we have to
provide better documentation. Please do not hesitate to ask
questions on the user mailing list, this will help us to improve
this document. Please ask your questions as precisely as
possible. We will not be able to answer questions like "I
want to write a listener but I don't know how, can you help
me?". Tell us what you are trying to do (the purpose of the
listener), what you have understood so far, and what exactly you
are getting stuck on.
</p>
<a id="Contributing"></a>
<h2>Contributing</h2>
<p>
We need <em>your</em> help to keep improving Checkstyle.
Whenever you write a listener that you think is generally
useful, please consider <a href="contributing.html">contributing</a> it to the Checkstyle
community and submit it for inclusion in the next release of
Checkstyle.
</p>
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Copyright © 2001-2007, Oliver Burn
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