DAO layer.

Hi,
i m using jdk1.5 and data base is mysql.
i executed the query & got the results from database, my java class have jdbc connection seperately.
To avoid this redundancy , i ll will go to create common java class only for database connection. whenever i want to connect datadase, at that time only i ll call that class..
give some samples & idea about it.
Regards,
kumar

I agree with Duffy in regards to the connection pool definitely better to use something that's already been built and tested rather than trying to work through it yourself (unless you just want to or want to learn about such things).
I would add to it that in terms of getting the connection from the pool, I've had success with a Singleton object that knows how to fetch a connection from the pool and that's it. I have used this concept along with classes to represent the objects persisted in the db and it's worked very well.
Another option would be to use a layer such as Hibernate, but that can easily become overkill and in some ways adds complexity to the whole thing.
It entirely depends on what you're after with it as to how you'd want to proceed.
Cheers,
PS.

Similar Messages

How to deal with validation errors from DAO layer.

I have been pondering on how to deal with validation errors from DAO layer.
Lets say you have a DAO that can save a car object. A car has a year, make, model, vin and so on. During the save operation of this DAO, it validates the car attributes to see if they pass some business rules. If it does not it throws some validation exception that contains all the validation errors. These validation errors know nothing about jsf or my components it just knows what attributes on the object are invalid and why.
If I just want to show those errors at the top of the page that would be no problem I could just create some FacesMessage objects and add them to the FacesContext messages. But if the DAO layer is telling me that the make attribute is invalid it would be nice to map it to the make field on the screen. I am wondering if any of you have tackled this problem or have some ideas on how to tackle it?
Brian

Let it throw an exception with a self explaining message, then catch it and embed that message in a FacesMessage.
Or let it throw more specific exception types (InvalidCarMakeException extends CarDAOException and so on) and let JSF handle it with own FacesMessage message.

JSF calls DAO layer based on JPA/Hibernate throws NPE

I'm new in JSF and Hibernate;
I'm using Hibernate as JPA provider, the dataaccess is tied to DAO layer; when my JSF controller(UserController) calls the getUsers() of my DAO layer
(Glassfish is the app Server I use)
NullpointerException is thrown, I assume it's because instance of BasicDAO is initialized, where do I have to instantiate BasicDAO
here is my code :
public class BasicDAO {
@PersistenceUnit(unitName = "test5PU")
private EntityManager entityManager;
public void setEntityManager(EntityManager entityManager)
{        this.entityManager = entityManager;    }
public List getUsers() {
Query query = entityManager.createNamedQuery("User.findAllUsers");
return query.getResultList();
public User findUser(String id) {
try{
Query q = entityManager.createNamedQuery("User.findByIdUser");
User o = (User)q.getSingleResult();
return o;
} finally {
entityManager.close();
public class UserController {
/** Creates a new instance of UserController */
public UserController() {
private DataModel model;
private BasicDAO basicDAO;
public DataModel getUsers() {
return new ListDataModel(basicDAO.getUsers());
public void setBasicDAO(BasicDAO basicDAO) {
this.basicDAO = basicDAO;
public BasicDAO getBasicDAO() {
return this.basicDAO;
public User findUser(String id) {
return basicDAO.findUser(id);
List.jsp :
<h:dataTable value='#{user.users}' var='item' border="1" cellpadding="2" cellspacing="0">
<h:column>
<f:facet name="header">
<h:outputText value="IdUser"/>
</f:facet>
<h:outputText value="#{item.idUser}"/>
</h:column>
<h:column>
<f:facet name="header">
<h:outputText value="Password"/>
</f:facet>
<h:outputText value="#{item.password}"/>
</h:column>
</h:dataTable>

in fact, i did some tests, and the reason why the NPE happens revealed to be caused by the call to
this.emf = Persistence.createEntityManagerFactory(persistenceUnitName, emfProperties);
which returns NULL value, and later when emf is accessed to create entitymanager NullPointerException is thrown.
I think there is a configuration probem some where, except in persistence.xml file. my persistence.xml content is :
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
<persistence-unit name="test9PU" transaction-type="JTA">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<jta-data-source>JeeCertDB</jta-data-source>
<class>persistence.user.User</class>
<properties>
<property name="hibernate.jdbc.driver"
value="com.mysql.jdbc.Driver" />
<property name="hibernate.jdbc.url"
value="jdbc:mysql://localhost:3306/JeeCertDB" />
<property name="hibernate.jdbc.user" value="root" />
<property name="hibernate.jdbc.password" value="belly" />
<property name="hibernate.logging.level" value="INFO" />
</properties>
</persistence-unit>
</persistence>
Message was edited by:
S_screen

Creating DAO layer from java beans(setter and getter methods)

Can anybody explain me how to create an adapter layer (DAO layer) from java beans with no connection and transaction code inside DAO layer?

Sure, have another layer do the transaction work.
Your DAO would make us of this layer.

How to delegate from controller to busines/database layer in web MVC?

Hello all,
I'm building a web application using MVC , DAO patterns by following the FrontController and DAO J2EE patterns listed here: http://java.sun.com/blueprints/corej2eepatterns/Patterns/index.html
I have two FrontControllers (http://java.sun.com/blueprints/corej2eepatterns/Patterns/FrontController.html),
1) Displays item details
2) Edit item details
The FrontController is currently making calls to the DAO layer to get the item details. This code is duplicated in both FrontControllers.
I want to extract this duplicated code and put it in a class, so that both of the FrontControllers will get the data from this class.
In both DisplayItemController and EditItemController (FrontControllers) I have
    private void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //Code to validate item id
        if(itemId> 0){
            DAOFactory daoFactory = DAOFactory.getDAOFactory(DAOFactory.MYSQL);
            ItemDAO itemDAO = daoFactory.getItemDAO();
            ItemTO item = itemDAO.getItemById(ItemId);
            request.setAttribute("item",item);
        String requestForwardPage = ApplicationResource.ITEM_DISPLAY_PAGE; //ITEM_EDIT_PAGE
        dispatch(request,response, requestForwardPage);
    }Is there a design pattern that allows moving the above common logic to a different class?
I appreciate any help.

yawmark wrote:
appy77 wrote:
Thanks for your reply, I know that there should be a model layer in MVC , but what type of classes are in the Model layer?
[http://java.sun.com/developer/technicalArticles/javase/mvc/]
[http://static.springframework.org/docs/Spring-MVC-step-by-step/]
~Ok thanks, those examples are better.

Best practices for DAO with relationships

Suppose I have have a relationship structure similar to this:
School -> 1:M -> Classrooms -> 1:M -> Students -> 1:M TextbooksShould I have 4 DAOs? (SchoolDAO, ClassroomDAO, StudentDAO, TextBookDAO)...
How would be the best way to insert a Student?
Would I insert the Student from the ClassroomDAO (because I need to know the ClassroomID in order to insert a Student)? Or would it be better design-wise to insert a Student in the StudentDAO, and assume the correct ClassroomID has been retrieved before the insert?
I guess this sounds a lot like an ORM tool -- we've tried to use Hibernate in the past but we're working against a large legacy database and we've had lots of trouble hooking hibernate up to it. We're trying to construct a nice DAO layer because the app is lacking a good one -- but I'd like to make sure we're following the best practices.
Thanks, Kevin

kp5150 wrote:
Suppose I have have a relationship structure similar to this:
>
>
Should I have 4 DAOs? (SchoolDAO, ClassroomDAO, StudentDAO, TextBookDAO)...Just noting that that doesn't seem realistic. Either the students own the books or the school does.> School -> 1:M -> Classrooms -> 1:M -> Students -> 1:M Textbooks
If the first then a school system wouldn't track them. If the second then you need ownership in the database that reflects that.
And students are not part of a class room but rather part of a class. Classes are held/scheduled in class rooms.
Additionally best practices generally dictate that table names should not be plural unless they contain sets (plural of set) data.
>
How would be the best way to insert a Student?
Would I insert the Student from the ClassroomDAO (because I need to know the ClassroomID in order to insert a Student)? Or would it be better design-wise to insert a Student in the StudentDAO, and assume the correct ClassroomID has been retrieved before the insert?
Probably irrelevant. You could do one or the other or even both. In one case you have a class to which a student is added. In the other you have a student and add them to a class.
I guess this sounds a lot like an ORM tool -- we've tried to use Hibernate in the past but we're working against a large legacy database and we've had lots of trouble hooking hibernate up to it. We're trying to construct a nice DAO layer because the app is lacking a good one -- but I'd like to make sure we're following the best practices.Huh?
If you have a unrealistic datamodel (like the above) then that is the cause of problems, not a tool.
But in a generic sense if you have the following
A -> 1:M -> B -> 1:M -> C -> 1:M D
Then that is easy for tools to handle.

A design question related to generic dao pattern

I follow this link https://www.hibernate.org/328.html to design my DAO layer using jpa/ hibernate. And basically it works ok. However, I encounter a dilemma regarding to persistence relation between domain objects.
My classes include
User <--(many to many)--> Group
Tables are USERS, USERS_GROUPS, GROUPS
In User class, I have properties
     @JoinTable(name="USERS_GROUPS",
          joinColumns=
          @JoinColumn(name="USER_ID"),
          inverseJoinColumns=
          @JoinColumn(name="GROUP_ID")
     private List<Group> groups = new ArrayList<Group>();
     public User(String id, String account, String name, String password){
          this.id = id;
          this.account = account;
          this.name = name;
          this.password = password;
     public List<Group> getGroups(){
          return this.groups;
     public void setGroups(List<Group> groups){
          this.groups = groups;
     }In Group class,
     @ManyToMany(mappedBy="groups",fetch=FetchType.EAGER)
     private List<User> users;
     public Group(String id, GroupName name){
          this.id = id;
          this.name = name;
     public List<User> getUsers(){
          return this.users;
     public void setUsers(List<User> users){
          this.users = users;
...Now in the database (mysql), I have already had two groups existing. They are
mysql> select * from GROUPS;
+----+------+
| ID | NAME |
+----+------+
| 1 | Root |
| 2 | User |
+----+------+and in the table USERS_GROUPS
mysql> select * from USERS_GROUPS;
+--------------------------------------+----------------+
| USER_ID                               | GROUP_ID |
+--------------------------------------+----------------+
| 1                                               | 1                 |
| 1                                               | 2                 |
+--------------------------------------+----------------+When I want to create a new user object and assig user object with an existing GROUP (e.g. GroupName.User)
At the moment the way how I implement it is done in the DAO layer (e.g. UserDaoImpl). (But what I want is to get this procedure done in the domain object layer i.e. User class, not DAO layer. )
steps:
1.) search the GROUP table (entityManager.cerateQuery(" from Group").getReulstList())
So it will return a list of all groups in GROUPS table.
2.) then check if the group (e.g GroupName.User) I want to assign to the User object existing in the GROUP table or not.
3.) if not existing, create a new Group and add it to a list; then User.setGroups(newList);
or
if the group (to be assigned to User class) already exists, use the group retrieved from the GROUP table and assign this group to the User class (e.g. groupList.add(groupFoundInGroupTable) User.setGroups(groupList))
However, what I want to do is to encapsulate this procedure into User calss. So that in DAO class whilst performing function createUser(), it would only need to do
User user = new User(....);
user.addGroup(GroupName.User); // GroupName is an enum in which it contains two values - Root, User
entityManager.merge(user);Unfortunately, because I need to obtaining Group collections from GROUP table first by using entityManager, which does not exist in the domain object (e.g. User class). So this road looks like blocked.
I am thinking about this issue, but at the moment there is no better solution popped upon my head.
Is there any chance that this procedure can be done more elegantly?
I appreciate any suggestion.
Edited by: shogun1234 on Oct 10, 2009 7:25 PM

You should be able to link your objects using methods in your entities to create an object graph.
When you save your user the associated objects will also be persisted based on the cascade types that you have specified on the relationships between the entities.

DAO pattern Value Objects and Data Streams

Suppose we have a PersonVO (java bean) with simple attributes i.e. first and last name then the DAO is quite simple and clients of the DAO layer pass VO back and forth (really cleanly).
But it is a different story if a person has a picture. If the picture is small then we
could define a field that is simple an array of bytes i.e. byte pic[], and would work ,
but will not scale if our pic becomes too large or there are lots of persons.
How can streams be used in this case ? (without dao code leaking into the client layer) and does this break the DAO pattern ?

I can picture not saving the image bytes themselves
but a link to a JPG or GIF on the file system.And how would that be guarenteed on a cluster? Or even per OS?
Putting streams into a database like that can choke
performance. Can't do WHERE clauses on byte
streams.For example in PostgreSQL:
CREATE TABLE images (
name VARCHAR(32) NOT NULL,
image bytea NOT NULL,
imagetype VARCHAR(4),
CONSTRAINT image_pk PRIMARY KEY (name)
);After retrieving the image into a byte[] you can either
- directly serve it to the client (browser) over HTTP through a servlet
(don't forget to set your content type through HttpServletResponse.setContentType())
- convert the byte[] to a java.awt.image.BufferedImage using javax.imageio.ImageIO.read(byte[]), and ofcourse do everything you want (including transformations) from there.
- What ever you can think off.
Now if you're passing that VO to a JSP, the link to
the image on the server file system fits nicely into
the <img> tag and Bob's your uncle.But this will still not be guarenteed to work on a cluster, across platforms, etc.
10 % 0
java.lang.ArithmeticException: / by zero;-)

Hibernate vs DAO's

Hi Guys,
I've been using hibernate as the persistence mechanism in an enterprise project for a while now. Originally the datamodel was architected to use a DAO to store (through hibernate) and the BusinessObjects (BO's) would use/update the DO's for persistence.
My question is, given that Hibernate is (almost) database independent, is there any actual need for a DAO layer? It's almost redundant, consistiong of member fields that are to be persisted and no actual persistence logic. AFAIK there will always be a database layer in our system, so needing to abstract the persistence to be anything other than a database is a bit of overkill (although a good Software Engineer always leaves things open for change).
Should the DAO layer exist at all? I can see an argument architectually that you wouldn't want to always rely on hibernate as the persistence mechanism (I don't like to rely exclusively on any 3rd party code, I'd prefer to depend on my own abstraction), and the DAO's provide a relatively easy abstraction for changing the persistence mechanism, but in a hibernate based architecture they really are just "dumb" objects.
Cheers,
Aidos

Hi Guys,
I've been using hibernate as the persistence
mechanism in an enterprise project for a while now.
Originally the datamodel was architected to use a DAO
to store (through hibernate) and the BusinessObjects
(BO's) would use/update the DO's for persistence.
My question is, given that Hibernate is (almost)
database independent, is there any actual need for a
DAO layer? Most emphatically - yes.
It's almost redundant, consistiong of
member fields that are to be persisted and no actual
persistence logic.
AFAIK there will always be a
database layer in our system, so needing to abstract
the persistence to be anything other than a database
is a bit of overkill (although a good Software
Engineer always leaves things open for change).
Should the DAO layer exist at all? I can see an
argument architectually that you wouldn't want to
always rely on hibernate as the persistence mechanism
(I don't like to rely exclusively on any 3rd party
code, I'd prefer to depend on my own abstraction),
and the DAO's provide a relatively easy abstraction
for changing the persistence mechanism, but in a
hibernate based architecture they really are just
"dumb" objects.
Cheers,
AidosHibernate is one way to implement the DAO, but it will remain if you ever switch from Hibernate.
%

Design Issue: How to implement as DAO for limited no of tables.

Hi Friends,
I am currently working on one small app.
I need to design the app the challenge is the interaction of the app will be with 5 db table only.
In this scenarios i can't use hibernate which will be an overhead.
Pls suggest me some approach wht should i use to interact with DB or dao implementation.
Here is the application domain:
One java main program will read some information using dao layer from oracle db(consisting of 5 table max ) and based on the read rows i want to generate some alerts.
I am planing to use spring as containner to load dao and other beans directly from main java program as xmlwebappctx.
I am confussed wht shall i use Spring DAO implementation or direct jdbc call from dao layer or any other thing?
Hope you understand my qn.
Thanks
Novin

Novin-Jaiswal wrote:
Hi Friends,
I am currently working on one small app.
I need to design the app the challenge is the interaction of the app will be with 5 db table only.since you don't talk about objects, i'll assume you don't have any. in that case, i won't recommend hibernate.
just write table gateway classes using JDBC to perform CRUD operations for each table.
>
In this scenarios i can't use hibernate which will be an overhead.don't see that. hibernate uses JDBC. If it generates the same SQL you don't, it's not an overhead.
%

DAO with JDeveloper

Hello,
I am totally new with DAO, I only know it a little bit theorically.
I want to use it from JDeveloper. And my question is if it is possible to use it from JDeveloper without using ADF, Spring or Hibernate? Is possible to use directly?
Thanks and regards.

There will always be a need to change your database accessing layer if the table definition is changed - no matter what approach you take.
What you can do though is abstract the other layers of your application from these changes by providing another layer that provides a more generic set of methods.
And yes you can do this with JDeveloper, you'll just need to program that interface on your own - just like in any other Java IDE.
JDeveloper however will make the development of the layer that does communicate with the database simpler for you by using either JPA or ADF BC.
Your DAO layer can be a layer above those components.

Java servlets

I am using Java Servlet and SQL.. i want to open two tables with two resultset objects...
i closed the first resultset and statement object. now open the second resultset and statement..
here the objects are created but the while doling "while(rs.next())" is not executed...
please solve this problem ...
another one is i am using the requestdispatcher
in the first servlet i havae the dopost method in this i put the requestdispatcher with forward or include from one servlet to another servlet .. here gives the expection is
do not match with post url
why this is happened....

ramkrishna.goli wrote:
I am using Java Servlet and SQL.. make sure you aren't putting database calling directly in your Servlet
create a DAO layer to decouple the business logic from the data access methods
i want to open two tables with two resultset objects...i think you mean you want to perform 2 queries and get back 2 ResultSets
i closed the first resultset and statement object. now open the second resultset and statement..
here the objects are created but the while doling "while(rs.next())" is not executed...was an error thrown?
if not, just sounds to me like no database records were found/returned
i.e. empty ResultSet
please solve this problem ...that's your job!

My jsp page does not get refreshed

I have a jsp page which looks like -
the top portion of the page has 3 textboxes and a submit button to add an entity.
the rest of the page displays a list of all the entities in the system.
when i load the jsp and add a new entity, the new entity shows up on the list. when i try to add a new entity second time, the entity does get added into the database, but does not show up in the list.
In the action class, i make a request to the dao and print all the values from the list, returned by the dao. Since the newly added values are present in the list, returned by the dao, i dont think there is anything wrong with my business layer or the DAO layer.i dont understand, why it does not show up in the jsp page.
any ideas. i am wondering if the data is being cached, so, in the jsp page i have also set the following parameters:
<head><meta http-equiv="Expires" CONTENT="0"><meta http-equiv="Cache-Control" CONTENT="no-cache"><meta http-equiv="Pragma" CONTENT="no-cache">
--------------------------------------------------------------------------------Has this to do anything with Struts and Hibernate which i use in my application? The same thing happens, when i edit. when i click on edit button, it takes the user to a different page, i edit the details and click on save button. the control returns back to the list page, but the changes are not shown, although the changes are saved to the db. how do i prevent cache. i am putting my entire list in a request object, before doing an action forward to the jsp page

Hi,
instead of using the cache in the meta tag try with the following lines,
response.setHeader("Cache-Control", "no-cache");
response.setHeader("Pragma", "no-cache");
response.setDateHeader("Expires", 0); Regards,
Bala A

I am facing a caching problem in the Web-Application that I've developed us

Dear Friends,
I am facing a caching problem in the Web-Application that I've developed using Java/JSP/Servlet.
Problem Description: In this application when a hyperlink is clicked it is supposed to go the Handling Servlet and then servlet will fetch the data (using DAO layer) and will store in the session. After this the servlet will forward the request to the view JSP to present the data. The JSP access the object stored in the session and displays the data.
However, when the link is clicked second time then the request is not received by our servlet and the cached(prev data) page is shown. If we refresh the page then request come to the servlet and we get correct data. But as you will also agree that we don't want the users to refresh the page again and again to get the updated data.
We've included these lines in JSPs also but it does no good:
<%
response.setHeader("Expires", "0");
response.setHeader("Cache-Control" ,"no-cache, must-revalidate");
response.setHeader("Pragma", "no-cache");
response.setHeader("Cache-Control","no-store");
%>
Request you to please give a solution for the same.
Thanks & Regards,
Mohan

However, when the link is clicked second time then the request is not received by our servlet Impossible mate.. can you show your code. You sure there are no javascript errors ?
Why dont you just remove your object from the session after displaying the data from it and see if your page "automatically" hits the servlet when the link is clicked.
cheers..
S

Adding object to collection outside of unit of work?

Hi,
Tried to find the answer in this forum but difficult to know what keywords to use...anyway
What I am trying to do is so simple I can only believe I am missing the point somewhat ;-)
I have an object Licence that has a Set of LicenceHolders. I have a licence already saved in the db and the application now has cause to add a licenceHolder. I create the licenceHolder as a new object then get the licence out of the db, add the licenceHolder to the Set, then I make a call to my data persistance business class to make the save to the database. I.e. I have a separate layer to do the persistance from my business logic. The latter being the specific method to say 'add a licence holder to the licence', the former being 'save the licenceHolder, and update the link table between the licence and licenceHolder' (m - m relationship).
The problem I have is that in my business method licence.addLicencee(licenceHolder) doesn't save my licenceHolder and link table if I don't include the bold line below. This does work but see below for more comments
code snippet for business method....
// lm is the persistance method that gets licence from db
PremisesLicence licence = (PremisesLicence) lm.getLicence(premisesLicenceId);
// licenceePerson - new object not yet persisted
licence.addLicenceHolder(licenceePerson);
//lhm is another persistance method to save licenceePerson and update licence_licenceHolder link table
lhm.addLicenceHolder(licenceePerson, licence);
code for lhm...
public void addLicenceHolder(ILicenceHolder licenceHolder, Licence licence) throws DataAccessException {
UnitOfWork uow = aSession.acquireUnitOfWork();
try {
     Licence licenceClone = (Licence) uow.readObject(licence);
licenceClone.addLicenceHolder(licenceHolder);
     uow.registerNewObject(licenceHolder);
     uow.commit();
} catch (Exception e) {
     e.printStackTrace();
     throw new DataAccessException(e);
I don't believe I should have to do the bold line as it is business logic in the persistance logic. I have already (in business method) said 'this licence has this licenceHolder' why should I have to repeat it in the persistance layer too. This can only lead to bugs in my software - I need all the help I can not to introduce any more ;-)
Comments please?
Thanks
Conrad

We've been working with TopLink for a while now, and had some issues with what you are doing. The following way is how we interact with TopLink to enable modifications outside the UoW:
- When returning objects from you DAO layer always return a copy of the retrieved objects (try Session.copyObject()), not the object returned by TopLink.
- Do modifications to the copied objects
- Pass the modified object graph to the DAO layer and use uow.mergeCloneWithReferences() to merge in differences
- Save to database with TopLink
From our experience, the reason you have to copy things is because what TopLink returns is the cached objects. So because you then will modify the cached objects, TopLink will not discover changes (comparing an object to itself will not give any differences). At least that was the only explanation we could find to our strange behaviour.
This is not a very efficient way to do things, and I think that if you are able to have a UoW open during this whole process it would be much more efficient. For an easy way to do this with web applications check out the TopLink integration with Springframework.
Please let me know if anyone has any feedback to this way of doing it. I know it's not ideal, but it is the best solution we found for detaching the objects.

DAO layer.

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