Plugins in Maven and POM.xml

Question

I just started using Maven and I read that plugins are additional components that can be used.
A typical structure of pom.xml file is

Answer 1

A late clarification on two important points

Where to place plugin
A plugin should indeed be added in most of the cases within the build/plugins section, however there is an important difference between placing it within plugins against placing it within pluginManagement/plugins.

This misunderstanding is often the cause of a non invoked plugin in Maven or an harder troubleshooting:

• Plugins under build/plugins are directly part of the default Maven build, if they specify an execution or if they configure something for the default build (see below)

• Plugins under build/pluginManagement/plugins are not necessarely part of the default Maven build, that is, is a management, it's an hint to maven: it you happen to use this plugin, then please use the version, the configuration, the executions I specify here, in this management.

  But what happen to use means? Means: if the same plugin is also present in the build/plugins section, then apply this management (and only then it will be effective); or if the plugin is invoked by default by Maven, then also apply it.

  But how is a plugin invoked by default? That's part of the main philosophy behind maven: convention over configuration. By convention, when you specify a certain packaging (default jar, but it can be war for example), you want certain plugins to be invoked. To build a jar, by default invoke the maven-jar-plugin; to build a war, by default invoke the maven-war-plugin and so on. So, if you specify a plugin configuration in the build/pluginManagement/plugin for a plugin which has a default binding to the Maven build, then it will be also be used.

Ordering
Concerning the ordering of sections within the pom.xml file, a further clarification is required: it's indeed irrelevant in most of the cases, however the order of plugin element wihtin the build/plugins section may be important. Since Maven 3.0.3 (MNG-2258), different plugin executions attached to the same Maven phase will be invoked in their order of declaration in the pom.xml file. That is, ordering is important in this case, since it may affect the behavior of the build.

Additionally, also order of dependency declarations may affect your build towards Dependency Mediation, that is, the first declared dependency wins in case of conflict against a transitive dependency. So, once again, ordering is important in certain cases.

Last but not least, although ordering is not important for other sections of the pom.xml file, good habit is to follow official Maven recommendations and, as a simplified version, follow this order of declaration:

The sortpom-maven-plugin can also be used to automatically apply this standard ordering, simply invoking the following on the concerned pom.xml file:

mvn com.github.ekryd.sortpom:sortpom-maven-plugin:2.5.0:sort \
     -Dsort.keepBlankLines -Dsort.predefinedSortOrder=recommended_2008_06 

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For further reading:

• Stack Overflow: Maven: what is pluginManagement?
• Official Maven doc: Maven POM Reference, PluginManagement
• Official Maven default bindings
• Official Maven doc: Dependency Mediation
• Official Maven doc: Maven Code Style And Code Conventions