Hibernate:constraintName é nulo no MySQL

Cheguei à seguinte solução:

Estenda o Hibernate MySQL5Dialect existente:

public class MySQL5Dialect extends org.hibernate.dialect.MySQL5Dialect {

    /**
    * Pattern to extract violated constraint name from {@link SQLException}.
    */
    private static final Pattern PATTERN = Pattern.compile(".*constraint\\W+(\\w+).*", Pattern.CASE_INSENSITIVE);

    private ViolatedConstraintNameExtracter constraintNameExtracter;

    public MySQL5Dialect() {
        constraintNameExtracter = new ConstraintNameExtractor();
    }

    @Override
    public ViolatedConstraintNameExtracter getViolatedConstraintNameExtracter() {
        return constraintNameExtracter;
    }

    private class ConstraintNameExtractor implements ViolatedConstraintNameExtracter {

        @Override
        public String extractConstraintName(SQLException sqle) {
            final String msg = sqle.getMessage();
            final Matcher matcher = PATTERN.matcher(msg);
            String constraintName = null;
            if (matcher.matches()) {
                constraintName = matcher.group(1);
            }

            return constraintName;
        }

    }

}

Especifique o dialeto recém-criado no arquivo de configuração do Hibernate (hibernate.cfg.xml):
```
<property name="dialect">your.package.MySQL5Dialect</property>
```

Agora getConstraintName() retornará o nome real da restrição violada:

try {
    ...
} catch (ConstraintViolationException e) {
    LOG.error(String.format("Constraint=%s, code=%d", e.getConstraintName(), e.getErrorCode()));
}