How to build a JAR file

When you want to create a JAR-file with Maven, you first have to create a pom.xml-file with at least the following content:

<project>
  <modelVersion>4.0.0</modelVersion>
  
  <groupId>com.mycompany.project</groupId>
  <artifactId>core</artifactId>
  <version>1.0-SNAPSHOT</version>
  <!-- <packaging>jar</packaging>  -->
</project>

Since 'jar' is the default packaging type it is not required to set it in this case. Apart from the above you will normally want some real java source files which should be located within src/main/java. If you need extra resources on your classpath (for example property files) they should be located in src/main/resources. Now we can create a JAR-file by using the command below:

mvn package

The 'package' phase is always responsible for bundling all the files in the artifact, in this case a JAR-file.

In your project's target directory you'll see the generated jar file which is named like: 'core-1.0-SNAPSHOT.jar'. The resulting 'jar' file contains the compiled java class files as well as the files from src/main/resources.

Usually there is no need to mentioned the 'maven-jar-plugin' explicit cause it's bound to the Maven Build Life Cycle.

For full documentation, click here.

Modular JAR files

The Java Platform Module System (JPMS) introduced changes in the JAR file specifications as well - Modular JAR files. Modular JAR files are JAR files with module-info.class file in the root directory (or in the versioned area for multi-release JAR files). If the project contains module-info.class the resulting JAR will be a Modular JAR without a need for any configuration regardless of the plugin version used.

Starting with version 3.1.2, if the JAR file contains module-info.class, this plugin will update the modular descriptor (module-info.class) with additional attributes such as the list of packages included and will validate the resulting modular descriptor (for example if all services provided by the module are actually included in the JAR file). The most notable additional attribute added is the module main class. If the JAR manifest contains Main-Class attribute this plugin will set the module main class to the same value (for example how to set it read Make The Jar Executable). Internally the plugin uses the JDK jar tool so in order to add the additional attributes and to verify the modular descriptor Maven should be run with JDK version 9 or newer. If version 8 or earlier is used the resulting JAR still will be Modular JAR (as it contains module-info.class) but no additional attributes will be added and no validation will be performed.