This problem is related to the local environment, and normally happens when

• the user don't have permissions to write in this folder
• another process is accessing the folder
• an IDE is open, and when maven cleans the target folder, the IDE detects that classes disappeared and recompiles the project.

Could you please provide more info about you local setup?

Yes, it is a problem related with the the user permissions.
So the documentation need to be re-write:
( https://forge.fi-ware.org/plugins/mediawiki/wiki/fiware/index.php/StreamOriented_-_Installation_and_Administration_Guide )
–
End to End testing
Kurento Media Server must be installed and started before running the following example, which is called magic-mirror and it is developed with the Kurento Java Client. You should run this example in a machine with camera and microphone since live media is needed. To launch the application first you need to clone the GitHub project where it is hosted and then run the main class, as follows:
git clone https://github.com/Kurento/kurento-tutorial-java.git
cd kurento-tutorial-java/kurento-magic-mirror
mvn compile exec:java
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Should be add the command "sudo" before the "mvn compile exec:java" command.

Regards

This command should not be run as root. "sudo" will for sure work, but it just hides the fact that there is something wrong in the local setup, or that you are trying to compile something where you shouldn't. If the tutorial is followed, a user will not run into this. The only way you could find yourself in this situation, is if you run one of the previous commands (especially git clone) as super user.

Could you check who is the owner of the folder, and post here the output of ls -l run in the root of the cloned repo?

Yes, indeed the file owner was root.
For some reason at the cloning of the GIT project, the command was done with the sudo.

Just check all the examples and they work fine.
I think that you can close the Bug.

Regards,