Chapter 4. Maven

4.1. Learn about Maven

4.1.1. About Maven

Apache Maven is a distributed build automation tool used in Java application development to build and manage software projects. Maven uses configuration XML files called POM (Project Object Model) to define project properties and manage the build process. POM files describe the project's module and component dependencies, build order, and targets for the resulting project packaging and output. This ensures that projects are built in a correct and uniform manner.
Maven uses repositories to store Java libraries, plug-ins, and other build artifacts. Repositories can be either local or remote. A local repository is a download of artifacts from a remote repository cached on a local machine. A remote repository is any other repository accessed using common protocols, such as http:// when located on an HTTP server, or file:// when located on a file server. The default repository is the public remote Maven 2 Central Repository.
Configuration of Maven is performed by modifying the settings.xml file. You can either configure global Maven settings in the M2_HOME/conf/settings.xml file, or user-level settings in the USER_HOME/.m2/settings.xml file.
For more information about Maven, see Welcome to Apache Maven.
For more information about Maven repositories, see Apache Maven Project - Introduction to Repositories.
For more information about Maven POM files, see the Apache Maven Project POM Reference.

4.1.2. About the Maven POM File

The Project Object Model, or POM, file is a configuration file used by Maven to build projects. It is an XML file that contains information about the project and how to build it, including the location of the source, test, and target directories, the project dependencies, plug-in repositories, and goals it can execute. It can also include additional details about the project including the version, description, developers, mailing list, license, and more. A pom.xml file requires some configuration options and will default all others. See Section 4.1.3, “Minimum Requirements of a Maven POM File” for details.
The schema for the pom.xml file can be found at http://maven.apache.org/maven-v4_0_0.xsd.
For more information about POM files, see the Apache Maven Project POM Reference.

4.1.3. Minimum Requirements of a Maven POM File

Minimum requirements

The minimum requirements of a pom.xml file are as follows:

  • project root
  • modelVersion
  • groupId - the id of the project's group
  • artifactId - the id of the artifact (project)
  • version - the version of the artifact under the specified group
Sample pom.xml file

A basic pom.xml file might look like this:

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.jboss.app</groupId>
  <artifactId>my-app</artifactId>
  <version>1</version>
</project> 

4.1.4. About the Maven Settings File

The Maven settings.xml file contains user-specific configuration information for Maven. It contains information that should not be distributed with the pom.xml file, such as developer identity, proxy information, local repository location, and other settings specific to a user.
There are two locations where the settings.xml can be found.
In the Maven install
The settings file can be found in the M2_HOME/conf/ directory. These settings are referred to as global settings. The default Maven settings file is a template that can be copied and used as a starting point for the user settings file.
In the user's install
The settings file can be found in the USER_HOME/.m2/ directory. If both the Maven and user settings.xml files exist, the contents are merged. Where there are overlaps, the user's settings.xml file takes precedence.
The following is an example of a Maven settings.xml file:
<settings>
    <profiles>
        <profile>
            <id>my-profile</id>
            <activation>
                <activeByDefault>true</activeByDefault>
            </activation>
            <repositories>
                <repository>
                    <id>fusesource</id>
                    <url>http://repo.fusesource.com/nexus/content/groups/public/</url>
                    <snapshots>
                        <enabled>false</enabled>
                    </snapshots>
                    <releases>
                        <enabled>true</enabled>
                    </releases>
                </repository>
                ...
            </repositories>
        </profile>
    </profiles>
    ...
</settings>
The schema for the settings.xml file can be found at http://maven.apache.org/xsd/settings-1.0.0.xsd.

4.1.5. KIE Plugin

The KIE plugin for Maven ensures that artifact resources are validated and pre-compiled, it is recommended that this is used at all times. To use the plugin simply add it to the build section of the Maven pom.xml

Example 4.1. Adding the KIE plugin to a Maven pom.xml

  <build>
    <plugins>
      <plugin>
        <groupId>org.kie</groupId>
        <artifactId>kie-maven-plugin</artifactId>
        <version>${project.version}</version>
        <extensions>true</extensions>
      </plugin>
    </plugins>
  </build>
Building a KIE module without the Maven plugin will copy all the resources, as is, into the resulting JAR. When that JAR is loaded by the runtime, it will attempt to build all the resources then. If there are compilation issues it will return a null KieContainer. It also pushes the compilation overhead to the runtime. In general this is not recommended, and the Maven plugin should always be used.

Note

For compiling decision tables and processes, appropriate dependencies must be added either to project dependencies (as compile scope) or as plugin dependencies. For decision tables the dependency is org.drools:drools-decisiontables and for processes org.jbpm:jbpm-bpmn2.

4.1.6. Maven Versions and Dependencies

Maven supports a number of mechanisms to manage versioning and dependencies within applications. Modules can be published with specific version numbers, or they can use the SNAPSHOT suffix. Dependencies can specify version ranges to consume, or take avantage of SNAPSHOT mechanism.
If you always want to use the newest version, Maven has two keywords you can use as an alternative to version ranges. You should use these options with care as you are no longer in control of the plugins/dependencies you are using.
When you depend on a plugin or a dependency, you can use the a version value of LATEST or RELEASE. LATEST refers to the latest released or snapshot version of a particular artifact, the most recently deployed artifact in a particular repository. RELEASE refers to the last non-snapshot release in the repository. In general, it is not a best practice to design software which depends on a non-specific version of an artifact. If you are developing software, you might want to use RELEASE or LATEST as a convenience so that you don't have to update version numbers when a new release of a third-party library is released. When you release software, you should always make sure that your project depends on specific versions to reduce the chances of your build or your project being affected by a software release not under your control. Use LATEST and RELEASE with caution, if at all.
Here's an example illustrating the various options. In the Maven repository, com.foo:my-foo has the following metadata:
<metadata>
  <groupId>com.foo</groupId>
  <artifactId>my-foo</artifactId>
  <version>2.0.0</version>
  <versioning>
    <release>1.1.1</release>
    <versions>
      <version>1.0</version>
      <version>1.0.1</version>
      <version>1.1</version>
      <version>1.1.1</version>
      <version>2.0.0</version>
    </versions>
    <lastUpdated>20090722140000</lastUpdated>
  </versioning>
</metadata>
If a dependency on that artifact is required, you have the following options (other version ranges can be specified of course, just showing the relevant ones here): Declare an exact version (will always resolve to 1.0.1):
<version>[1.0.1]</version>
Declare an explicit version (will always resolve to 1.0.1 unless a collision occurs, when Maven will select a matching version):
<version>1.0.1</version>
Declare a version range for all 1.x (will currently resolve to 1.1.1):
<version>[1.0.0,2.0.0)</version>
Declare an open-ended version range (will resolve to 2.0.0):
<version>[1.0.0,)</version>
Declare the version as LATEST (will resolve to 2.0.0):
<version>LATEST</version>
Declare the version as RELEASE (will resolve to 1.1.1):
<version>RELEASE</version>
Note that by default your own deployments will update the "latest" entry in the Maven metadata, but to update the "release" entry, you need to activate the "release-profile" from the Maven super POM. You can do this with either "-Prelease-profile" or "-DperformRelease=true"

4.1.7. Remote Repository Setup

The maven settings.xml is used to configure Maven execution.
The settings.xml file can be located in 3 locations, the actual settings used is a merge of those 3 locations.
  • The Maven install: $M2_HOME/conf/settings.xml
  • A user's install: ${user.home}/.m2/settings.xml
  • Folder location specified by the system propert kie.maven.settings.custom
The settings.xml is used to specify the location of remote repositories. It is important that you activate the profile that specifies the remote repository, typically this can be done using "activeByDefault":
<profiles>
  <profile>
    <id>profile-1</id>
    <activation>
      <activeByDefault>true</activeByDefault>
    </activation>
    ...
  </profile>
</profiles>