Tools for Java Coding Standards
... now we can all get along!

R. Mark Volkmann, Partner
Object Computing, Inc. (OCI)

Ever tried to get a group of software developers to agree on a coding standard? We're all reasonable people, right? Certainly we're mature enough to come to an agreement on decisions that are based on personal preference. Well after 20 years of working in the software business I've never seen a team of developers successfully do this. Sure, some dissenting team members can be beaten down until they conform, but they're not happy about it.

How do we get past personal biases about coding standards and move on to getting work done? The answer, in my opinion, is to let everyone do what they want! I can hear the issues being raised as I write this. We'll have too much trouble reading code if we all do it differently. The end product will be inconsistent ... and so on.

We need to make a clear distinction between standards related to style and standards related to best practices. Examples of style standards are where to place braces and how to use whitespace. Examples of best practices are when to use interfaces and how to implement an equals method. When I say "let everyone do what they want" I'm only referring to style standards.

So how can we let everyone use whatever style they prefer and not suffer for it? Use a tool like Jalopy, an open source tool available at SourceForge, to reformat code to a single style. When developers get code from the source repository they can use Jalopy to reformat it to their preferred style. Before they commit changes to the source repository they run Jalopy again to reformat the code to a style selected for the project. It's a good idea to find a way to hook Jalopy into the check-in/commit functionality of the repository so that developers can't bypass this step. Putting the code back in a common format is essential so that reasonable diffs can be performed to determine the changes made between revisions of the code.

What about those non-style, best practice issues? That's were CheckStyle comes in. It's another open-source tool available at SourceForge.

Both tools can be run from the command-line, an Ant build file or be integrated with an IDE. This article will only discuss the first two options since there are too many variations in IDE setup to cover in a single article. See the documentation of each tool for instructions on how to integrate them with a particular IDE.


Jalopy is an open-source tool for reformatting Java source code. It is released under a BSD licence. It can control

Jalopy can also insert Javadoc and header/footer templates.

Jalopy has plug-ins for running in many different environments. A version containing the source code and every plug-in, or versions containing a single, specific plug-in can be downloaded. It can be run from the command line (using the console plug-in), an Ant build file or an IDE. Jalopy has been successfully integrated with these IDES.

Some IDEs, such as IDEA, have such good support for code formatting that there would be little interest in creating a Jalopy plug-in.

Downloading and Installing Jalopy

Jalopy can be downloaded from Installation only requires unzipping and adding its bin directory to the PATH environment variable.

Configuring Jalopy

Running the preferences script (either .bat or .sh) in the bin directory opens a Swing GUI where coding style preferences can be specified.

Figure 1

Jalopy Preferences GUI

In addition, a preview window is displayed to show the effect of your choices.

Figure 2

Jalopy Preview Window

Selecting a category in the left navigation area changes the content of the main area of the GUI to allow preferences for that category to be specified. It also changes what is displayed in the preview window.

Figure 3

Jalopy Project Selection

Multiple sets of preferences, called "projects", can be created. Preferences for each project are saved in separate, binary files. This makes using the GUI the only practical way to view and modify preferences. Under Windows XP, the preference files are stored under C:\Documents and Settings\user-name\.jalopy. A default project is created the first time the GUI is used. Click on the "Projects" category to view the current list of projects (see figure 3). The active project will appear in bold. This is the project that will be affected by changes made under the "Printer" category (see figure 1) and is also the project whose preferences will be used when Jalopy is run. To change the active project, select a non-bold project name and click the "Activate" button. To create a new project, click the "Add..." button. Remember to activate it if you want to specify preferences for it and use it!

Running Jalopy from the Command Line

To run Jalopy from the command-line, add bin\jalopy-console-version.jar to your CLASSPATH and enter commands with the following format. When running under Linux, it may be necessary to change the .sh script files to UNIX format and add getopt-version.jar to your CLASSPATH.

      jalopy [options] args

Common usages are described below.

To run on a single source file and overwrite it,


To run on a single source file and avoid overwriting it, redirect output to a different file.

      jalopy >

To run on all source files below the current directory and overwrite them,

      jalopy -r .

To run on all source files below the current directory and avoid overwriting them,

      jalopy -r . -d output-directory

To get help on all the available options,

      jalopy -h

Running Jalopy from Ant

To enable Ant to find the Jalopy JAR files, either copy them to the Ant lib directory or insert the following taskdef into your Ant build file.

    <taskdef name="jalopy"
        <fileset dir="jalopy-dir/bin">
          <include name="*.jar"/>

Here's an example Ant target for running Jalopy on all Java source files in a project.

    <target name="format" depends="compile"
      description="reformats all Java source files in the project">
      <taskdef name="jalopy" classname="de.hunsicker.jalopy.plugin.ant.AntPlugin"/> 
      <jalopy classpathref="classpath">
        <fileset dir="${src.dir}">
          <include name="**/*.java"/>

The taskdef in the example above is only needed if the previous one is not used. As written, it expects the Jalopy JAR files to be in the Ant lib directory.

This target depends on the compile target because formatting code that contains syntax errors may produce undesirable results. classpath is the id of a path element that is also used to compile the source code. Depending on compile and specifying the classpath are only necessary to take advantange of import optimization.

Import optimization has three possible settings: disabled, expand and collapse. When set to expand, all wildcard imports are replaced by explicit imports. When set to collapse, multiple explicit imports for the same package are replaced by a single wildcard import for the package.

For details on other attributes that can be specified on the jalopy task see docs\plugin-ant-usage.html.


CheckStyle is an open-source tool for checking conformance of Java source code against a set of coding standards that are highly configurable. It is released under a GNU Lesser General Public Licence (LGPL). Here are highlights of what it can check.

Downloading and Installing CheckStyle

CheckStyle can be downloaded from Installation only requires unzipping and adding checkstyle-all-version.jar to the CLASSPATH environment variable.

Configuring CheckStyle

To override the coding standards checked by CheckStyle, create a Java property file describing alternate settings. An example of a CheckStyle property is "checkstyle.maxlinelen" which defaults to 80. For details on specific properties that can be set, see docs\config.html. The property file to be used can be specified in a command-line option or in the properties attribute of the checkstyle Ant task.

Running CheckStyle from the Command Line

To run CheckStyle from the command line, enter commands with one of the following formats. The difference between them is that the first requires the CLASSPATH environment variable to be set and the second doesn't.

      java [options] [source-files]


      java -jar %CHECKSTYLE_HOME%\checkstyle-all-2.4.jar [options] [source-files]

Options include:

Running CheckStyle from Ant

To enable Ant to find the CheckStyle JAR file, either copy it to the Ant lib directory or insert the following taskdef into your Ant build file.

    <taskdef name="checkstyle"

Here's an example Ant target that runs CheckStyle on all Java source files in a project and produces an HTML report.

    <target name="check"
      description="checks all Java source files for coding standard violations">
      <taskdef name="checkstyle" classname=""/>
      <checkstyle properties=""
        failOnViolation="false" failureProperty="check.failed">
        <fileset dir="${src.dir}" includes="**/*.java"/>
        <formatter type="xml" toFile="checkstyle.xml"/>
      <xslt in="checkstyle.xml" out="checkstyle.html"

The taskdef in the example above is only needed if the previous one is not used. As written, it expects the Checkstyle JAR file to be in the Ant lib directory.

Setting failOnViolation to false causes it to continue checking source files after one has failed to pass all the checks. failureProperty specifies an Ant property to set if one or more checks fail. This can be used in subsequent Ant targets to prevent them from running.