A Checkstyle listener monitors the progress of a Checker during the audit
of files. The Checker notifies its attached listeners of
significant events such as the start of the audit of a file and
the logging of a Check violation, and the listeners respond
appropriately. Any number of listeners can be attached to a
Checker. An audit always adds one of
the distribution listeners, DefaultLogger
or XMLLogger,
to report events. A DefaultLogger
produces simple text output for the events it receives, and a
XMLLogger produces an XML document for
its events.
Listeners DefaultLogger and XMLLogger 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 violation 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.
A listener is an implementation of the AuditListener
interface. During an audit, a Checker
informs its attached AuditListeners of
six kinds of events: audit started/ended, file started/ended,
and logging of an error(violation)/exception.
An audit passes an event to a listener as an AuditEvent.
A file-related AuditEvent contains the
name of that file. An AuditEvent for
violation logging has a message, a severity level, a message source
such as the name of a Check, and file
line and column numbers that may be relevant to the violation. The
notification of an exception to a AuditListener includes a violation
AuditEvent and the details of the exception. Here is a UML diagram for
classes AuditListener and AuditEvent.
A custom listener is an implementation of the AuditListener
interface. If the listener has properties that can be set from a
configuration, the listener must extend AutomaticBean.
An AutomaticBean uses JavaBean
introspection to set JavaBean properties.
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 violations per audited file and
the total number of violations. The default output stream is System.out.
In order to enable the specification of output to a file through property
file, the class extends AutomaticBean and defines method
setFile(String).
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.AbstractAutomaticBean;
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());
}
}
A listener that filters error events could perform the filtering
in methods addError and addException. As further examples of
listeners, CommonsLoggingListener
reports its events through the Commons Logging API, and MailLogger e-mails the audit report of a
DefaultLogger.
To incorporate a custom listener in the set of listeners for a
Checker, include a module element for
the listener in the configuration file. For
example, to configure a Checker so
that it uses custom listener
VerboseListener
to print audit messages to a
file named "audit.txt", include the following module
in the configuration file:
<module name="com.mycompany.listeners.VerboseListener">
<property name="file" value="audit.txt"/>
</module>
Here is a truncated example of audit output from a VerboseListener:
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
This section describes two examples based on ANT listeners. The first
listener, CommonsLoggingListener,
hands off events to the Apache
Commons Logging facade and the second, MailLogger,
sends a report of an audit via
e-mail. The discussion of these examples and how to use them is
derived from material in "Java Development
with Ant" by Eric Hatcher and Steve Loughran, an
excellent ANT book.
Apache
Commons Logging provides a facade for logging tools
log4j,
J2SE 1.4, and others. Checkstyle listener CommonsLoggingListener
responds to an AuditEvent by handing it off to the current Commons Logging Log.
The source code for
CommonsLoggingListener. Notice that
each AuditListener method that
receives an AuditEvent calls a method
for the Commons Logging log level corresponding to the
Checkstyle SeverityLevel of the AuditEvent.
The easiest way to use CommonsLoggingListener is to include
checkstyle-10.15.0-all.jar
in the classpath because that jar file contains all the Commons
Logging classes. The default Log under J2SE 1.4 is wrapper
class
Jdk14Logger.
Under earlier Java versions, the default Log is a simple wrapper
class,
SimpleLog.
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.
Logging configuration details for Jakarta Commons Logging are in
the documentation.
As a simple example, assume that log4j.jar is in the classpath and the
following log4j.properties file is
in the current directory:
# 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
Running a Checkstyle audit with a CommonsLoggingListener yields this
(abbreviated) output:
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.
MailLogger
sends an audit report in an
email message. The listener uses a DefaultLogger to prepare the text of the
message. The listener obtains other message parameters such as
to and subject from environment properties that
can be read from a properties file.
This implementation uses the JavaMail API as the mail system, and you must include appropriate jar files in the classpath.
As an example of using MailLogger, set
system property -DMailLogger.properties.file=MailLogger.properties,
so that MailLogger reads message
parameters from file MailLogger.properties of the current
directory:
MailLogger.from=user@example.org
MailLogger.failure.to=user@example.org
MailLogger.success.to=user@example.org
MailLogger.mailhost=localhost
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.
We need your help to keep improving Checkstyle. Whenever you write a listener that you think is generally useful, please consider contributing it to the Checkstyle community and submit it for inclusion in the next release of Checkstyle.