In Spring MVC, where to catch Database exceptions

Question

I am following the structure suggested above at ( http://viralpatel.net/blogs/spring3-mvc-hibernate-maven-tutorial-eclipse-example/ ). I tried adding a duplicate entry, which resulted in the following exception:

SEVERE: Servlet.service() for servlet [appServlet] in context with path [/cct] threw exception [Request processing failed; nested exception is org.springframework.dao.DataIntegrityViolationException: Duplicate entry '[email protected]' for key 'PRIMARY'; SQL [n/a]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationException: Duplicate entry '[email protected]' for key 'PRIMARY'] with root cause
com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry '[email protected]' for key 'PRIMARY'
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
    at << removed for readability>> org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:202)
    at com.sun.proxy.$Proxy26.addUser(Unknown Source)
    at com.bilitutor.cct.control.HomeController.signup(HomeController.java:56)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    << removed for readability>> 

I have the following questions:

1. Why is the exception being caught by the Controller ( userService.addUser(user) in com.bilitutor.cct.control.HomeController ) and not by the DAO ( sessionFactory.getCurrentSession().save(user); ) and then bubbled up to the Controller?

2. I understand that I am getting a org.springframework.dao.DataIntegrityViolationException because I am using the @Repository annotation, which perhaps does exception translation (correct me if I am wrong). In that case, when I catch the exception, how do I find the error code for it?

3. As a best practice, on which layer (DAO, Service or Controller) is the best spot to catch the exception?

Relevant classes :

COntroller :

package com.bilitutor.cct.control;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.dao.DataIntegrityViolationException;
import org.springframework.web.bind.annotation.ModelAttribute;

import com.bilitutor.cct.bean.*;
import com.bilitutor.cct.service.*;

/**
 * Handles requests for the application home page.
 */
@Controller
public class HomeController {

    @Autowired
    UserService userService;

    @ModelAttribute("user")
    public User getUserObect() {
      return new User();
    }

    private static final Logger logger = LoggerFactory.getLogger(HomeController.class);

    /**
     * Landing page. Just return the login.jsp
     */
    @RequestMapping(value = "/", method = RequestMethod.GET)
    public String home(Model model) {
        logger.info("home() called");
        return "login";
    }

    /**
     * Login. Either forward to the user's homepage or return back the error
     */
    @RequestMapping(value = "/login", method = RequestMethod.GET)
    public String login(Model model) {
        logger.info("login() called");
        return "login";
    }

    /**
     * New User signup. If user already exists, send back the error, or send an email and forward to the email validation page
     */
    @RequestMapping(value = "/signup", method = RequestMethod.POST)
    public String signup(@ModelAttribute("user")User user, BindingResult result) {
        logger.info("signup() : email="+user.getEmail()+" pass="+user.getPassword()+" accessCode="+user.getAccessCode());
        userService.addUser(user);
        return "login";
    }

}

Service:

package com.bilitutor.cct.service;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

import com.bilitutor.cct.dao.UserDAO;
import com.bilitutor.cct.bean.User;

@Service
public class UserServiceImpl implements UserService {

    @Autowired
    private UserDAO userDAO;

    @Transactional
    public void addUser(User user) {
        userDAO.addUser(user);
    }

    @Transactional
    public void removeUser(String email) {
        userDAO.removeUser(email);
    }

}

DAO:

package com.bilitutor.cct.dao;

import com.bilitutor.cct.bean.User;

import org.hibernate.SessionFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Repository;

@Repository
public class UserDAOImpl implements UserDAO {

    @Autowired
    private SessionFactory sessionFactory;

    public void setSessionFactory(SessionFactory sessionFactory) {
        this.sessionFactory = sessionFactory;
    }

    public void addUser(User user) {
        sessionFactory.getCurrentSession().save(user);
    }

    public void removeUser(String email) {
        User user = (User) sessionFactory.getCurrentSession().load(User.class, email);
        if (user!=null) {
            sessionFactory.getCurrentSession().delete(user);
        }

    }
}

Answer 1

Exception is occuring at service layer. You can see this in trace

at com.sun.proxy.$Proxy26.addUser(Unknown Source)

@Transactional
public void addUser(User user) {
    userDAO.addUser(user);
}

As previous answer says your transaction boundary is at service layer, so exception occurs there.

I would recommend to catch/throw proper business exceptions(checked exceptions) from service methods. Service methods incorporate your business logic so , if anything goes wrong it should be properly communicated to outer world through the exceptions that service methods throw. For eg : WeakPasswordException, UserNameExistsException etc

Regarding org.springframework.dao.DataIntegrityViolationException try calling getCause() to see the wrapped exception