Maven Code Style And Code Conventions

This document describes how developers and contributors should write code. The reasoning of these styles and conventions is mainly for consistency, readability and maintainability reasons.

Generic Code Style And Convention

All working files (java, xml, others) should respect the following conventions:

  • License Header: Always add the current ASF license header in all versionned files.
  • Trailing Whitespaces: Remove all trailing whitespaces. If your are an Eclipse user, you could use the Anyedit Eclipse Plugin.

and the following style:

  • Indentation: Never use tabs!
  • Line wrapping: Always use a 120-column line width.

Note: The specific styles and conventions, listed in the next sections, could override these generic rules.


Java Code Style

The Maven style for Java is mainly:

  • White space: One space after control statements and between arguments (i.e. if ( foo ) instead of if(foo)), myFunc( foo, bar, baz ) instead of myFunc(foo,bar,baz)). No spaces after methods names (i.e. void myMethod(), myMethod( "foo" ))
  • Indentation: Always use 4 space indents and never use tabs!
  • Blocks: Always enclose with a new line brace.
  • Line wrapping: Always use a 120-column line width for Java code and Javadoc.
  • Readingness: Specify code grouping members, if needed. For instance in a Mojo class, you could have:
    public class MyMojo
        // ----------------------------------------------------------------------
        // Mojo components
        // ----------------------------------------------------------------------
         * Artifact factory.
         * @component
        private ArtifactFactory artifactFactory;
        // ----------------------------------------------------------------------
        // Mojo parameters
        // ----------------------------------------------------------------------
         * The POM.
         * @parameter expression="${project}"
         * @required
        private MavenProject project;
        // ----------------------------------------------------------------------
        // Mojo options
        // ----------------------------------------------------------------------
        // ----------------------------------------------------------------------
        // Public methods
        // ----------------------------------------------------------------------
         * {@inheritDoc}
        public void execute()
            throws MojoExecutionException
        // ----------------------------------------------------------------------
        // Protected methods
        // ----------------------------------------------------------------------
        // ----------------------------------------------------------------------
        // Private methods
        // ----------------------------------------------------------------------
        // ----------------------------------------------------------------------
        // Static methods
        // ----------------------------------------------------------------------

The following sections show how to set up the code style for Maven in IDEA and Eclipse. It is strongly preferred that patches use this style before they are applied.

IntelliJ IDEA 4.5+

Download maven-idea-codestyle.xml and copy it to ~/.IntelliJIDEA/config/codestyles then restart IDEA. On Windows, try C:\Documents and Settings<username>\.IntelliJIDEA\config\codestyles

After this, restart IDEA and open the settings to select the new code style.

Eclipse 3.2+

Download maven-eclipse-codestyle.xml.

After this, select Window > Preferences, and open up the configuration for Java > Code Style > Code Formatter. Click on the button labeled Import... and select the file you downloaded. Give the style a name, and click OK.

Java Code Convention

For consistency reasons, our Java code convention is mainly:

  • Naming: Constants (i.e. static final members) values should always be in upper case. Using short, descriptive names for classes and methods.
  • Organization: Avoid using a lot of public inner classes. Prefer interfaces instead of default implementation.
  • Modifier: Avoid using final modifier on all member variables and arguments. Prefer using private or protected member instead of public member.
  • Exceptions: Throw meaningful exceptions to makes debugging and testing more easy.
  • Documentation: Document public interfaces well, i.e. all non-trivial public and protected functions should include Javadoc that indicates what it does. Note: it is an ongoing convention for the Maven Team.
  • Testing: All non-trivial public classes should include corresponding unit or IT tests.

JavaDoc Convention



XML Code Style

The Maven style for XML files is mainly:

  • Indentation: Always use 2 space indents, unless you're wrapping a new XML tags line in which case you should indent 4 spaces.
  • Line Breaks: Always use a new line with indentation for complex XML types and no line break for simple XML types. Always use a new line to separate XML sections or blocks, for instance:
      <simpleType>This is a simple type</simpleType>
        <simpleType>This is a complex type</simpleType>

    In some cases, adding comments could improve the readability of blocks, for instance:

        <!-- Simple XML documentation                                               -->


        <!-- ====================================================================== -->
        <!-- Block documentation                                                    -->
        <!-- ====================================================================== -->

Generic XML Code Convention

No generic code convention exists yet for XML files.

POM Code Convention

The team has voted during the end of June 2008 to follow a specific POM convention to ordering POM elements. The consequence of this vote is that the Maven project descriptor is no more considered as the reference for the ordering.

The following is the recommended ordering for all Maven POM files:

<project xmlns="" xmlns:xsi="" xsi:schemaLocation="">
















  1. The <project/> element is always on one line.
  2. The blocks are voluntary separated by a new line to improve the readingness.
  3. The dependencies in <dependencies/> and <dependencyManagement/> tags have no specific ordering. Developers are free to choose the ordering, but grouping dependencies by topics (like groupId i.e. org.apache.maven) is a good practice.

Note: There existing two alternativs to change order of a pom file Tidy Maven Plugin or the maven-sortpom-plugin.

XDOC Code Convention

For consistency and readability reasons, XDOC files should respect:

  • Metadata: Always specify metadata in the <properties/> tag.
  • Sections: Always use a new line with indentation for <section/> tags.

FML Code Convention

For readability reasons, FML files should respect:

  • FAQ: Always use a new line with indentation for <faq/> tags.