Using the DTOs in the Transaction Handlers
Now it's time to learn how to code methods in the transaction handler that return either a DVD with only the key of the director or a complete director. In the following example, you only need to call the DVD DAO to get the result:
public DVD_DTO findDVD(String dvdId) throws DAOException {
DVD_DTO dto = null;
DAOFactoryIF factory = new JdbcDAOFactory();
DVDDAOIF dvdDao = factory.createDVDDAO();
dvdDao.setTransaction(trans);
DVD dvd = null;
try {
dvd = dvdDao.findDVD(dvdId);
} catch (DAOException e) {
String msg = "findDVD failed for id=" + dvdId;
logger.error(msg);
throw new DAOException(msg, e);
}
if (dvd != null) {
dto = new DVD_DTO();
dto.setKey(ConvertKey.toDTO(dvd.getKey()));
dto.setTitle(dvd.getTitle());
dto.setDirectorKey(ConvertKey.toDTO(dvd.getDirector()));
}
return dto;
}
The DAO beans and the DTOs are often the same, even so, they don't have to be. Hence the utility ConvertKey:
package dk.hansen.dto;
import dk.hansen.dao.bean.Key;
public class ConvertKey {
public static Key_DTO toDTO(Key key) {
return new Key_DTO(key.getId());
}
public static Key toBean(Key_DTO key) {
return new Key(key.getId());
}
}
Conversion from a DAO bean to a DTO is often trivial, since they may have the same
attributes. It's a good idea to place the conversion in two utility methods. This allows you to convert DAO beans into DTOs and vice versa.
Now, the transaction handler code for the complete DVD:
public DVDWithDirector_DTO findDVDWhDirector(String dvdId)
throws DAOException {
DVDWithDirector_DTO dvdDTO = null;
DAOFactoryIF factory = new JdbcDAOFactory();
DVDDAOIF dvdDao = factory.createDVDDAO();
DirectorDAOIF directorDao = factory.createDirectorDAO();
dvdDao.setTransaction(trans);
DVD dvd;
try {
dvd = dvdDao.findDVD(dvdId);
} catch (DAOException e) {
String msg = "getDVD failed for id=" + dvdId;
logger.error(msg);
throw new DAOException(msg, e);
}
if (dvd != null) {
directorDao.setTransaction(trans);
Director director;
try {
director = directorDao.getDirector(dvd.getDirector().getId());
} catch (DAOException e) {
String msg = "getDVD failed for directorid=" +
dvd.getDirector().getId();
logger.error(msg);
throw new DAOException(msg, e);
}
Director_DTO directorDTO = new Director_DTO();
directorDTO.setKey(ConvertKey.toDTO(director.getKey()));
directorDTO.setName(director.getName());
dvdDTO = new DVDWithDirector_DTO();
dvdDTO.setKey(ConvertKey.toDTO(dvd.getKey()));
dvdDTO.setTitle(dvd.getTitle());
dvdDTO.setDirector(directorDTO);
}
return dvdDTO;
}
Validation of DAO beans and DTOs
Should the DAOs or the transaction managers validate the data they receive in these classes? As always, the answer is dependent on the application at hand. Look at the transaction managers first—you should know who the clients are. If the clients are part of the same project as the handlers, then your design criterion might be to let the clients have the responsibility for producing valid input to the handlers. If the application is "mission-critical," then you might want to add validation in the handlers as well. Another possibility is to let the handlers offer two interfaces—one with validation, one without.
The same considerations could be made for the DAOs. If the clients of the DAOs are the handlers, you might make them responsible for producing valid input to the DAOs. Since DAOs write data to the backend, it's important to be sure that the data is valid, or you might end up with a corrupt database.
The Importance of Making the Right Design
Making the correct data classes in a DAO application—or in any application—is by no means trivial. It's worthwhile to take the extra time for this design, since
development and maintenance of the application will be made much easier with a well considered and properly documented design.
Happy coding!
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