Understanding Pom.xml


<project xmlns="http://maven.apache.org/POM/4.0.0"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                      http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.mycompany.app</groupId>
  <artifactId>my-app</artifactId>
  <packaging>jar</packaging>
  <version>1.0-SNAPSHOT</version>
  <name>Maven Quick Start Archetype</name>
  <url>http://maven.apache.org</url>
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
</project>
  • project This is the top-level element in all Maven pom.xml files.
  • modelVersion This element indicates what version of the object model this POM is using. The version of the model itself changes very infrequently but it is mandatory in order to ensure stability of use if and when the Maven developers deem it necessary to change the model.
  • groupId This element indicates the unique identifier of the organization or group that created the project. The groupId is one of the key identifiers of a project and is typically based on the fully qualified domain name of your organization. For example org.apache.maven.plugins is the designated groupId for all Maven plug-ins.
  • artifactId This element indicates the unique base name of the primary artifact being generated by this project. The primary artifact for a project is typically a JAR file. Secondary artifacts like source bundles also use the artifactId as part of their final name. A typical artifact produced by Maven would have the form <artifactId>-<version>.<extension> (for example, myapp-1.0.jar).
  • packaging This element indicates the package type to be used by this artifact (e.g. JAR, WAR, EAR, etc.). This not only means if the artifact produced is JAR, WAR, or EAR but can also indicate a specific lifecycle to use as part of the build process. (The lifecycle is a topic we will deal with further on in the guide. For now, just keep in mind that the indicated packaging of a project can play a part in customizing the build lifecycle.) The default value for the packaging element is JAR so you do not have to specify this for most projects.
  • version This element indicates the version of the artifact generated by the project. Maven goes a long way to help you with version management and you will often see the SNAPSHOT designator in a version, which indicates that a project is in a state of development. We will discuss the use of snapshots and how they work further on in this guide.
  • name This element indicates the display name used for the project. This is often used in Maven’s generated documentation.
  • url This element indicates where the project’s site can be found. This is often used in Maven’s generated documentation.
  • description This element provides a basic description of your project. This is often used in Maven’s generated documentation.

All possible elements in the pom are listed here

Pom Dependency

Dependency scope is used to limit the transitivity of a dependency, and also to affect the classpath used for various build tasks.

There are 6 scopes available:

  • compile
    This is the default scope, used if none is specified. Compile dependencies are available in all classpaths of a project. Furthermore, those dependencies are propagated to dependent projects.
  • provided
    This is much like compile, but indicates you expect the JDK or a container to provide the dependency at runtime. For example, when building a web application for the Java Enterprise Edition, you would set the dependency on the Servlet API and related Java EE APIs to scope provided because the web container provides those classes. This scope is only available on the compilation and test classpath, and is not transitive.
  • runtime
    This scope indicates that the dependency is not required for compilation, but is for execution. It is in the runtime and test classpaths, but not the compile classpath.
  • test
    This scope indicates that the dependency is not required for normal use of the application, and is only available for the test compilation and execution phases.
  • system
    This scope is similar to provided except that you have to provide the JAR which contains it explicitly. The artifact is always available and is not looked up in a repository.
  • import (only available in Maven 2.0.9 or later)
    This scope is only used on a dependency of type pom in the <dependencyManagement> section. It indicates that the specified POM should be replaced with the dependencies in that POM’s <dependencyManagement> section. Since they are replaced, dependencies with a scope of import do not actually participate in limiting the transitivity of a dependency.

Maven Dependencies

Whenever a project references a dependency that isn’t available in the local repository, Maven will download the dependency from a remote repository into the local repository. You probably noticed Maven downloading a lot of things when you built your very first project (these downloads were dependencies for the various plugins used to build the project). By default, the remote repository Maven uses can be found (and browsed) at http://repo.maven.apache.org/maven2/. You can also set up your own remote repository (maybe a central repository for your company) to use instead of or in addition to the default remote repository. For more information on repositories you can refer to the Introduction to Repositories.

Maven Repositories

A repository in Maven is used to hold build artifacts and dependencies of varying types.

There are strictly only two types of repositories: local and remote. The local repository refers to a copy on your own installation that is a cache of the remote downloads, and also contains the temporary build artifacts that you have not yet released.

Remote repositories refer to any other type of repository, accessed by a variety of protocols such as file:// and http://. These repositories might be a truly remote repository set up by a third party to provide their artifacts for downloading (for example,repo.maven.apache.org and uk.maven.org house Maven’s central repository). Other “remote” repositories may be internal repositories set up on a file or HTTP server within your company, used to share private artifacts between development teams and for releases.

The local and remote repositories are structured the same way so that scripts can easily be run on either side, or they can be synced for offline used. In general use, the layout of the repositories is completely transparent to the Maven user, however.

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