Javax validation on nested objects - not working

Use @ConvertGroup from Bean Validation 1.1 (JSR-349).

Introduce a new validation group say Pk.class. Add it to groups of BuildingDto:

public class BuildingDto {

    @NotNull(groups = {Pk.class, Existing.class, LocationGroup.class})
    // Other constraints
    private Integer id;

    //
}

And then in LocationDto cascade like following:

@Valid
@ConvertGroup.List( {
    @ConvertGroup(from=New.class, to=Pk.class),
    @ConvertGroup(from=LocationGroup.class, to=Pk.class)
} )
// Other constraints
private BuildingDto building;

Further Reading:

5.5. Group conversion from Hibernate Validator reference.

Just try adding @valid to collection. it would be working as per reference hibernate

  @Getter
  @Setter
  @Valid
  @NotNull(groups = { Existing.class })
  private List<LocationDto> locations;

@Valid annotation must be added to cascade class attributes.

LocationDTO.class

public class LocationDto {

  @Valid
  private BuildingDto building;
   
  .........

}