Resolved. But you are not going to believe it; coping code from samples in the Internet, I had to use @Valid and searching I discover that I needed "javax.validation.Valid", searching how to get it with
Maven I ended using
<dependency>
<groupId>javax.validation</groupId>
<artifactId>validation-api</artifactId>
<version>2.0.1.Final</version>
</dependency>
and I had no compilation errors. Keeping looking at samples in the Internet I found and application that wasn't using the above and so far, it is not clear to me how is it doing it. However, I found a page clarifying that earlier, "spring-boot-starter-web" supported "javax.validation.Valid", but that had been removed and the dependency to be used is
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-validation</artifactId>
</dependency> <!--
https://mvnrepository.com/artifact/org.hamcrest/java-hamcrest -->
Well, my application works now and I thank you all and bid you well.