Author: Oliver Burn
Checkstyle is a development tool to help programmers write Java code that adheres to a coding standard. Its purpose is to automate the process of checking Java code, and to spare humans of this boring (but important) task.
My experience from using a similar proprietary tool at a previous employer was that it increased the productivity of code reviews. This is because it allowed the reviewers to get on with reviewing the code and not get into "style" debates. It also helped that all the code had the same style.
The things that checkstyle checks for are:
The following list outlines some things checkstyle is not:
Binary and source distributions are from http://checkstyle.sourceforge.net/.
Source code is stored under CVS at SourceForge. The project page is http://sourceforge.net/projects/checkstyle.
To run checkstyle you will need:
To build checkstyle you will need:
The following ways are supported for running checkstyle:
I have tested checkstyle using JRE1.2.2 and JRE1.3 on Windows 2000.
Checks for import statements that are not used. It will warn about wild-card imports like import java.io.*;. It will also warn about duplicate import statements.
Removing unused import statements reduces unnecessary dependencies in a code base. This feature can be turned off.
Checks that the following constructs have a Javadoc comment:
Javadoc comments for class and interface declarations are checked to ensure that the @author tag exists. This can be turned off.
You can make checkstyle relax the checking for Javadoc comments. When relaxed, checkstyle requires a Javadoc comment for public and protected variables and methods, and for interface and class definitions.
Javadoc comments for methods are checked to ensure that the following tags exist (if required):
For example the following is valid:
/**
* Checks for a return tag.
* @return the index of the next unchecked tag
* @param aTagIndex the index to start in the tags
* @param aTags the tags to check
* @param aLineNo the line number of the expected tag
**/
public int checkReturnTag(final int aTagIndex,
JavadocTag[] aTags,
int aLineNo)
It can be extremely painful writing or duplicating Javadoc for a method required for an interface. Hence checkstyle supports using the convention of using a single @see tag instead of all the other tags. For example, if the previous method was implementing a method required by the com.puppycrawl.tools.checkstyle.Verifier interface, then the Javadoc could be done as:
/** @see com.puppycrawl.tools.checkstyle.Verifier **/
public int checkReturnTag(final int aTagIndex,
JavadocTag[] aTags,
int aLineNo)
Checks for lines that are longer that a specified length. The default is "80".
Checks for lines that contain tab ('\t') characters. This can be turned off.
Verifies that the format of the variable names conform to a specified regular expression. The formats can be overridden. The following table outlines the defaults:
| Scope | Default Format | Example |
| static and final | ^[A-Z]([A-Z0-9_]*[A-Z0-9])?$ The exception to the rule is for serialVersionUID. |
public static final int MAX_ROWS = 2; |
| static only | ^s[A-Z][a-zA-Z0-9]*$ | private static int sNumCreated = 0; |
| non static | ^m[A-Z][a-zA-Z0-9]*$ | private int mNumCreated = 0; |
| non static public | ^f[A-Z][a-zA-Z0-9]*$ | public int fCMPField; |
Verifies that the format of parameter names conform to a specified regular expression. The default is ^a[A-Z][a-zA-Z0-9]*$.
Verifies that the format of class and interface names conform to a specified regular expression. The default is ^[A-Z][a-zA-Z0-9]*$.
Checks for data members that are not declared private. Also finds static non-final data members that are not declared as private.
Note: you can turn on allowing protected data members.
Container Managed Persistence EJBs require that managed fields are declared public. This will cause checkstyle to complain that the fields should be declared private. Hence checkstyle supports ignoring public that match a specified regular expression. The default is ^f[A-Z][a-zA-Z0-9]*$.
Checks for the following use of white space:
This feature can be turned off.
Checks for missing braces {}'s for the following constructs:
This feature can be turned off.
Ensure that the file starts with a specified header. The header contents are specified in a file. If no file is specified, checkstyle does not check for a header.
Most copyright headers will contain date information (for example the year) which will change over time. To support this, checkstyle supports specifying a line in the header to ignore when comparing the start of the file with the header. For example, consider the following header:
line 1: /////////////////////////////////////////////////////////////////////// line 2: // checkstyle: Checks Java source code for adherence to a set of rules. line 3: // Copyright (C) 2001 Oliver Burn line 4: ///////////////////////////////////////////////////////////////////////
Since the year information will change over time, you can tell checkstyle to ignore line 3.
checkstyle: Checks Java source code for adherence to a set of rules. Copyright (C) 2001 Oliver Burn This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
If you have any feedback on this tool, please email me at checkstyle@puppycrawl.com. I am interested in any improvements you would like to see (or hopefully made!).
Also, send me email if you would like to be notified when a newer version of checkstyle is released.
Copyright © 2001 Oliver Burn. All rights Reserved.