You can add a new source directory with build-helper:
<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>build-helper-maven-plugin</artifactId>
<version>3.2.0</version>
<executions>
<execution>
<phase>generate-sources</phase>
<goals>
<goal>add-source</goal>
</goals>
<configuration>
<sources>
<source>src/main/generated</source>
</sources>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
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