Running Checkstyle as an automatic JUnit test

Update June 12, 2011

This solution is now released as an own open source project. More information is here:

Running Checkstyle as an automatic JUnit test

If you are looking for a great code analysis tool to help you review your Java code, Checkstyle is a good choice. It basically checks your code towards a set of given checks of your choice, everything from checking that the code is following naming conventions, to spotting duplicated code.

There are several ways to use Checkstyle, either by using it with a build automation tool such as Apache Maven or as an Ant task, but you can also simply execute it through a command line.

However if you are interested you can also wrap it into a JUnit test. Even though it is really not a unit test, it could be really convenient as it allows you run it along with the rest of your tests.

Here is a basic example code:

public class CheckStyleTest extends TestCase {

    public void testCheckStyle() throws Exception {

         * Files

        File ROOT = new File("src/");
        System.out.println("Root is set to \"" + ROOT.getAbsolutePath() + "\".");

        List files = new ArrayList();
        listFiles(files, ROOT, "java");
        System.out.println("Found " + files.size() + " Java source files.");

         * Listener

        ByteArrayOutputStream sos = new ByteArrayOutputStream();
        AuditListener listener = new DefaultLogger(sos, false);

         * Configuration

        InputSource inputSource = new InputSource(CheckStyleTest.class.getResourceAsStream("configuration.xml"));
        Configuration configuration = ConfigurationLoader.loadConfiguration(inputSource, new PropertiesExpander(System.getProperties()), false);

         * Create checker

        Checker checker = new Checker();

         * Process

        int errors = checker.process(files);
        System.out.println("Found " + errors + " check style errors.");
        assertTrue(errors + " check style errors found. " + sos.toString(), errors == 0);

         * Clean up



    private static void listFiles(List files, File folder, String extension) {
        if (folder.canRead()) {
            if (folder.isDirectory()) {
                for (File f : folder.listFiles()) {
                    listFiles(files, f, extension);
            } else if (folder.toString().endsWith("." + extension)) {


As you can notice the test will try and read from a file called "configuration.xml". File this has to be in the same folder as where you put this test. "configuration.xml" is simply the checks that Checkstyle should perform. How to write this document is described on the their configuration page.

A sample of the "configuration.xml" file would be:

<module name="Checker">

    <module name="TreeWalker">

         <module name="CovariantEquals"/>
         <module name="EmptyStatement"/>
        <module name="FallThrough"/>
        <module name="SuperClone"/>
        <module name="AvoidStarImport"/>
        <module name="UnusedImports"/>
        <module name="AvoidStaticImport"/>
        <module name="EqualsHashCode"/>