# Writing your own Filters

## Overview

A Checker has a set of Filters that decide which audit events the Checker reports through its listeners. Interface Filter and class FilterSet are intended to support general AuditEvent filtering using a set of Filters.

A Filter has boolean method accept(AuditEvent) that returns true if the Filter accepts the AuditEvent parameter and returns false if the Filter rejects it.

A FilterSet is a particular Filter that contains a set of Filters. A FilterSet accepts an AuditEvent if and only if all Filters in the set accept the AuditEvent.

Here is a UML diagram for interface Filter and class FilterSet.

## Writing Filters

The Filter that we demonstrate here rejects audit events for files whose name matches a regular expression. In order to enable the specification of the file name pattern as a property in a configuration file, the Filter is an AutomaticBean with mutator method setFiles(String) that receives the file name pattern. An AutomaticBean uses JavaBean introspection to set JavaBean properties such as files.

    package com.mycompany.filters;

    import org.apache.regexp.RE;
    import org.apache.regexp.RESyntaxException;

    import com.puppycrawl.tools.checkstyle.api.AuditEvent;
    import com.puppycrawl.tools.checkstyle.api.AutomaticBean;
    import com.puppycrawl.tools.checkstyle.api.Filter;
    import com.puppycrawl.tools.checkstyle.api.Utils;

    public class FilesFilter
        extends AutomaticBean
        implements Filter
    {
        private RE mFileRegexp;

        public FilesFilter()
            throws RESyntaxException
        {
            setFiles("^$");
        }

        public boolean accept(AuditEvent aEvent)
        {
            final String fileName = aEvent.getFileName();
            return ((fileName == null) || !mFileRegexp.match(fileName));
        }

        public void setFiles(String aFilesPattern)
            throws RESyntaxException
        {
            mFileRegexp = Utils.getRE(aFilesPattern);
        }
    }

## Using Filters

To incorporate a Filter in the filter set for a Checker, include a module element for the Filter in the configuration file. For example, to configure a Checker so that it uses custom filter FilesFilter to prevent reporting of audit events for files whose name contains "Generated", include the following module in the configuration file:

        <module name="com.mycompany.filters.FilesFilter">
            <property name="files" value="Generated"/>
        </module>

## Huh? I can't figure it out!

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 filter but I don't know how, can you help me?". Tell us what you are trying to do (the purpose of the filter), what you have understood so far, and what exactly you are getting stuck on.

## Contributing

We need your help to keep improving Checkstyle. Whenever you write a filter 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.

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Copyright © 2002-2003 Oliver Burn. All rights Reserved.