Checkstyle 5.5

##### About

- Checkstyle
- Release Notes

##### Documentation

- Configuration
  - Property Types
- Running
  - Ant Task
  - Command Line
- Available Checks
- Standard Checks
  - Annotations
  - Block Checks
  - Class Design
  - Coding
  - Duplicate Code
  - Headers
  - Imports
  - Javadoc Comments
  - Metrics
  - Miscellaneous
  - Modifiers
  - Naming Conventions
  - Regexp
  - Size Violations
  - Whitespace
- Extending Checkstyle
  - Writing checks
  - Writing filters
  - Writing listeners

##### Developers

- Javadoc
- Project Page
- Contributing

##### Project Documentation

- Project Information
- Project Reports

## Overview

A `Checker` has a set of `Filter`s 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 `Filter`s.

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 `Filter`s. A `FilterSet` accepts an `AuditEvent` if and only if all `Filter`s 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 java.util.regex.Matcher;
    import java.util.regex.Pattern;
    import java.util.regex.PatternSyntaxException;

    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 Pattern mFileRegexp;

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

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

        public void setFiles(String aFilesPattern)
            throws PatternSyntaxException
        {
            mFileRegexp = Utils.getPattern(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.