You can add it into project's pom file, inside <project>
tag:
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-jar-plugin</artifactId>
<version>2.4</version>
<configuration>
<archive>
<manifest>
<mainClass>your.main.class</mainClass>
</manifest>
</archive>
</configuration>
</plugin>
</plugins>
</build>
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