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POBest practice propagating Unique Violation Exceptions to UI
 

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    <p>We are working on Java web project based on JPA 2, Hibernate, Spring 3 and JSF 2 running in Tomcat 7. We are using Oracle 11g as database. </p> <p>We are currently holding a debate on approaches to populate database constraint violations as user-friendly messages to the UI. More or less we see two ways, both are not really satisfying. Could somebody give some advise?</p> <p><strong>Approach 1 - Validate programmatically and throw specific exception</strong></p> <p>In CountryService.java each Unique constraint will be validated and a corresponding exception is thrown. The exceptions are handled individually in a backing bean.</p> <p><em>Advantage</em>: Easy to understand and maintain. Specific User messages possible.</p> <p><em>Disadvantage</em>: A lot of code just for having nice messages. Basically all DB Constraints are written again in the application. A lot of queries - unnecessary db load.</p> <pre class="lang-java prettyprint-override"><code>@Service("countryService") public class CountryServiceImpl implements CountryService { @Inject private CountryRepository countryRepository; @Override public Country saveCountry(Country country) throws NameUniqueViolationException, IsoCodeUniqueViolationException, UrlUniqueViolationException { if (!isUniqueNameInDatabase(country)) { throw new NameUniqueViolationException(); } if (!isUniqueUrl(country)) { throw new UrlUniqueViolationException(); } if (!isUniqueIsoCodeInDatabase(country)) { throw new IsoCodeUniqueViolationException(); } return countryRepository.save(country); } } </code></pre> <p>In the View's Backing Bean you handle the exceptions:</p> <pre class="lang-java prettyprint-override"><code>@Component @Scope(value = "view") public class CountryBean { private Country country; @Inject private CountryService countryService; public void saveCountryAction() { try { countryService.saveCountry(country); } catch (NameUniqueViolationException e) { FacesContext.getCurrentInstance().addMessage("name", new FacesMessage("A country with the same name already exists.")); } catch (IsoCodeUniqueViolationException e) { FacesContext.getCurrentInstance().addMessage("isocode", new FacesMessage("A country with the same isocode already exists.")); } catch (UrlUniqueViolationException e) { FacesContext.getCurrentInstance().addMessage("url", new FacesMessage("A country with the same url already exists.")); } catch (DataIntegrityViolationException e) { // update: in case of concurrent modfications. should not happen often FacesContext.getCurrentInstance().addMessage(null, new FacesMessage("The country could not be saved.")); } } } </code></pre> <p><strong>Approach 2 - Let the database detect constraint violations</strong></p> <p><em>Advantage</em>: No boiler plate code. No unnecessary queries to db. No duplication of data constraint logic.</p> <p><em>Disadvantage</em>: Dependencies to constraint names in DB, so generation of Schema through hibernate not possible. Mechanism needed to bind messages to input components (e.g. for highlighting).</p> <pre class="lang-java prettyprint-override"><code>public class DataIntegrityViolationExceptionsAdvice { public void afterThrowing(DataIntegrityViolationException ex) throws DataIntegrityViolationException { // extract the affected database constraint name: String constraintName = null; if ((ex.getCause() != null) &amp;&amp; (ex.getCause() instanceof ConstraintViolationException)) { constraintName = ((ConstraintViolationException) ex.getCause()).getConstraintName(); } // create a detailed message from the constraint name if possible String message = ConstraintMsgKeyMappingResolver.map(constraintName); if (message != null) { throw new DetailedConstraintViolationException(message, ex); } throw ex; } } </code></pre>
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    1. COYou are still relying on the database to detect constraint violations in Approach 1, in the event that a user saves a duplicate country after the unique check but before the save of a second user.
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    2. COwe are aware of the concurrent issue here. it is not mandatory for our use cases. it is enough, if in 90% of the cases the message is specific. if in exceptional cases, a more generic message is triggered by db doesn't matter.
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    3. COi updated approach 1
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