Maven Concepts: Difference between revisions
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Revision as of 06:22, 9 January 2018
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Internal
Maven Coordinates
groupId:artifactId:version is the minimum amount of information to fully and unequivocally identify an artifact, and constitute an address and a timestamp. This information is referred to as maven coordinates. More details about each component below.
Group ID
The group ID is an unique identifier (ideally, globally) for a project. It is specified as <groupId> at the top of the POM file.
Conventionally uses a dot notation, but it does not have to. When it does use the dot notation, the groupId does NOT have to correspond to the package structure of the project it designates; it is, though, a good practice to follow. When stored within a repository, the dots are replaced by OS specific directory separators, which becomes a relative directory structure from the base repository.
Group ID and Parent-Children Module Relationship
The group ID namespace and the parent-chidren module namespace are completely independent: the children modules can belong to totally different groups than their parents - the parent's groupId and the children's groupIds may not overlap at all, yet Maven enforces the coordinated parent/children build cycles and places the artifacts on completely un-related locations in the repository. For more details on module and module naming, see:
Artifact ID
The artifact ID is generally the name that the project is known by. It is specified as <artifactId> at the top of the POM file. The artifact ID, along with the group ID, create a key that ideally separates this project from every other project in the world.
Under particular circumstances, the artifact ID may contain version information. For example, if the project is an extension for JBoss DataGrid 7 (and only works with JBoss DataGrid version 7), it is fine to use the following artifact ID: "jboss-datgrid-7.0". The project will have its own versioning scheme, independent of "7.0", and a project artifact will be named similarly to "jboss-datagrid-7.0-1.0.0-SNAPSHOT-1.jar"
Artifact Name
The artifactId is used to generate the project's artifact name, according to the pattern artifactId-version.packaging.
The name of the artifact file generated in the target directory of the project can be overridden using the finalName element. However, the name of the artifact published to repository ignores finalName.
Version
The version represents a timestamp for a specific incarnation of the project's artifact, in the history of the project. The information is used within an artifact's repository to separate versions from each other. It is specified as <version> at the top of the POM file.
For more details on a project's module version management, see "Modules and Versions" section.
Stable Versions and Snapshots
Stable Version
A stable version is guaranteed not to change in the repository.
Snapshots
A snapshot is a version of a Maven artifact that has not been officially released. For example 1.0.0-SNAPSHOT refers to a version that may become 1.0.0 or that may be overwritten by one or more updates until it becomes 1.0.0, so using
<version>1.0.0-SNAPSHOT</version>
in the definition of a dependency means that you should be prepared to deal with a different version of the artifact every time a build is run and in consequence, the latest version is pulled from the artifact server. A remote repository may accumulate several actual snapshot versions that correspond to a *-SNAPSHOT, and the resolution is done by Maven by pulling the remote maven-metadata.xml that corresponds to the snapshot and looking up the latest actual version of the file, which is written in that file. Once downloaded, maven-metadata.xml is written as maven-metadata-local.xml in the local repository.
Handling snapshots is different from handling stable versions, in that Maven may connect to the remote repository in an attempt to pull the latest version of a SNAPSHOT, even if there's a version of that SNAPSHOT in the local repository. The actual behavior is given by the configuration of the corresponding repository:
<repository>
<id>...</id>
<snapshots>
<enabled>true</enabled>
<updatePolicy>always|daily|interval:<interval-in-mins>|never</updatePolicy>
</snapshots>
</repository>
With a <updatePolicy> of "never", SNAPSHOT will be handled as a stable version, in that it will pulled only if it does not exist in the local repository, and then, once cached, it will never be updated, even if newer snapshots appear in the remote repository.
For more details on the <repository> element syntax, see:
Packaging
The packaging represents the project's artifact type. It is specified as <packaging> at the top of the POM file.
When no packaging is declared, Maven assumes the artifact is, by default, a "jar". The valid types are Plexus role-hints of the component role org.apache.maven.lifecycle.mapping.LifecycleMapping. The current core packaging values are: "pom" (for an example see multi-module projects), "jar", "maven-plugin", "ejb", "war", "ear", "rar" and "par". The packaging value defines the default list of goals to be bound to phases associated with that particular package structure.
Some packaging types are available only if a particular plugin was declared in <build> and the <extensions> was turned on to true for that plugin.
If you need to specify packaging in the Maven coordinates of a project, it is specified after artifact ID: groupId:artifactId:packaging:version.
Classifier
The classifier allows to distinguish artifacts that were built from the same POM but differ in their content. It is some optional and arbitrary string that - if present - is appended to the artifact name just after the version number.
A common use case for classifiers is the need to attach secondary artifacts to the project's main artifact (for example, javadocs or sources).
It is specified as <classifier> at the top of the POM file. If you need to specify the classifier in the Maven coordinates of a project, it is specified after packaging: groupId:artifactId:packaging:classifier:version.
Classifiers are also used by the assembly plugin.
Name
It is specified as <name> at the top of the POM file.
Lifecycle
For details on the Maven lifecycles and phases, see:
Type
POM
Profile
Plugin
A Maven plugin are pieces of functionality that provide goals. A plugin has one or more goals, and each goal represents a capability of that plugin. Available Plugins:
Plugins can be used independently from command line, or their goals can be executed as part of a project release sequence.
In order to insure that a plugin is used in a project release sequence, the plugin must be declared in the <plugins> container element. If POM inheritance is used, the plugins to be used by a hierarchy of projects can be declared in a <pluginManagement> element at the top of the hierarchy. For more details on how <plugins> compares with <pluginManagement>, see "<pluginManagement>" section in the POM reference.
Plugin Execution
Declaring the plugins in POM is a pre-requisite for execution as part of the project lifecycle. In order to be actually executed, their goals must be linked to a specific phase of the project. For some plugins and some phases, this association is done automatically.
If the association is needed to be declared explicitly, this is how it's done:
<plugin>
...
<executions>
<execution>
<id>attach-sources</id>
<phase>package</phase>
<goals>
<goal>jar</goal>
</goals>
</execution>
</executions>
</plugin>
Command Line Execution
Plugins' goals can be executed from command line, as shown below. The plugin configuration parameters can be passed with -D<param-name>=<param-value> convention.
mvn dependency:build-classpath -Dsilent=true
Plugin Registry
$HOME/.m2/plugin-registry.xml contains the plugin versions.
Configured in settings.xml
Variables
Maven configuration files support variables, which are of five different types:
Environment Variables
Project Properties
A dot (.) notated path in the POM prefixed with project. will contain the corresponding POM element value. For example:
<project> <version>...</version> </project>
is accessible as ${project.version}.
More details about project properties are available here:
Settings Properties
A dot (.) notated path in the POM prefixed with settings. will contain the corresponding settings.xml value. For example:
<settings> <offline>...</offline> </settings >
is accessible as ${settings.offline}.
Also see:
System Properties
These are the Java system properties. These are the properties accessible with java.lang.system.getProperties(), and can be used as pom.xml variables (example: ${java.home}).
Set in the <properties> Elements
Properties can also be declared and exposed to the build in the <properties> section of a pom.xml or of a <profile>:
TODO: Maven Standard System Properties
Dependencies
BOM
Repositories
Servers
A server is a configuration element allowing to specify sensitive information such as the username and the password for the URLs used by repositories or mirrors, in a location different than the publicly shared pom.xml. A server is identified by its "id" element and it is declared in settings.xml. The server's "id" matches the "id" element of the repository or mirror Maven tries to connect to.
Mirror
Configured in settings.xml
Proxy
Configured in settings.xml
Multi-Module Maven Projects
Fileset
Resources
The concept of "resource" is used by the "resources" plugin. See Maven Resources Plugin - The "Resources" section.
Interactive Mode
Maven attempts to interact with the user for input in interactive mode. Configured in settings.xml.
Offline Mode
Configured in settings.xml.