<project>
  [...]
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-dependency-plugin</artifactId>
        <version>3.1.2</version>
        <executions>
          <execution>
            <id>src-dependencies</id>
            <phase>package</phase>
            <goals>
              <!-- use copy-dependencies instead if you don't want to explode the sources -->
              <goal>unpack-dependencies</goal>
            </goals>
            <configuration>
              <classifier>sources</classifier>
              <failOnMissingClassifierArtifact>false</failOnMissingClassifierArtifact>
              <outputDirectory>${project.build.directory}/sources</outputDirectory>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
  [...]
</project>