Object Run time typecasting

[Schandha Ravi]

Hello All,

I'm working on a Sample DAO pattern, where in my pattern should be able to insert any record into any table. My basic interface is as below

    
package com.utility;
import java.sql.*;
public interface DAO {
public boolean create(Object o,Class clazz) throws SQLException;
public Object retrieve(Object key, Class clazz) throws SQLException;
public boolean update(Object o,Class clazz,Object key) throws SQLException;
public boolean delete(Object o, Class clazz, Object key) throws SQLException;
}

I have a very vague idea about the implementation. My job would have been easier if I want to insert a specific data into a specific table. But I want a generalized solution so that I can use this utility across different applications. My idea is when a caller calls the methods of this interface say for example, he wants to insert a record in to PERSON table with fields PID,FNAME,LNAME,ADDR. Caller would create a Data Transfer Object called "Product" and could call the method like

(Assuming that we have an implementation of our DAO already existing)

    
Person p=new Person("A","ADAM","SMITH","SAVANNAH CIRCLE");
dao.create(p,Person.class);

This call should be able to insert the record Person object in PERSON table.

similarly another call to insert a Customer object in CUSTOMER table should also be successful.

But the question is, how do determine that incoming object is of typer Person or Customer. I tried to work on implementation like below
(One comprimise here assuming the class name is same as table name)

    
public boolean create(Object o, Class clazz) throws SQLException{
   if(clazz.isInstance(o)){
System.out.println("Valid");
        [B]/* At this point, I should be able to determine my object O is of type Person 
          or Customer or Animal any random class and also should be able to access its member fields.*/
       /* We can find out the available methods, fields and its types using reflection 
        from clazz object. But how do we extract the data from Object o */[/B]   }
   else
System.out.println("Invalid");

[Ernest Friedman-Hill]

Are you aware of the existence of, e.g., Hibernate, and many other object-relational mapping frameworks for data persistence? Basically what you're asking is how to reimplement something like Hibernate.

[Schandha Ravi]

Hi Ernest,

Right, what I'm trying to do is a prototype version of ORM. Basically, I'm trying to learn how ORM would have been implemented. You are right, I could use already available ORM frameworks, but wanted to implement using core JDBC. I mean I would get the confidence, If I see how things are implemented at the minute level,rather than using high level API.

[Arvind Choudhary]

Hi Ravi,

I am also having a simillar requirement. I am restricted to use any ORM tool. Please share if you have developed this.


Thanks.
Arvind

[Winston Gutkowski]

Schandha Ravi wrote:I'm working on a Sample DAO pattern, where in my pattern should be able to insert any record into any table.

As soon as I hear words like "anything into anything" I always suspect that someone hasn't sufficiently defined their requirements.
And if, indeed, you do mean "insert any record into any table" (including, presumably, ones you haven't dreamt up yet), then I'd suggest that Java is probably not the language you want to use.

However, if there is some "syntax" (ie, restrictions or rules) on what you want to be able to do, there are a few patterns that you might be able to use, just a couple of which might be Abstract Factory or Bridge.

However, if this really is an "anything to anything" scenario, then I fear you're into the realms of reflection; and the solution is likely to be complex, slow, difficult to understand, and even more difficult to test properly.

Winston

[Arvind Choudhary]

Dear Winston ,

Thanks for your reply.
Two things : a. This indeed is a "anything into anything" requirement meaning the caller will pass the query for CRUD operation and the rest will be taken care of by respective CRUD implemented method in the utility class.

b. The requirement itself starts with a pattern i.e. DAO. What I want is to build a generic DAO so that once it is developed developer will save time in coding and will readability of the code will be improved.


Thanks.
Arvind

[Winston Gutkowski]

Arvind Choudhary wrote:The requirement itself starts with a pattern i.e. DAO. What I want is to build a generic DAO so that once it is developed developer will save time in coding and will readability of the code will be improved.

OK, well the only other thing I can suggest of the top of my head is that you type your DAO rather than passing classes around, viz:

public interface DAO<T, K> {
   public boolean create(T o) throws SQLException;
   public T retrieve(K key) throws SQLException;
   public boolean update(T o, K key) throws SQLException;
   public boolean delete(K key) throws SQLException;
}

however, I'm not quite sure what all this gets you beyond templating basic CRUD operations. As Ernest said, Hibernate provides a lot more than just that; and I certainly wouldn't consider trying to emulate it (at least not on my own; or without a year or so of solid free time).

I'm also not sure what all these boolean returns are for (except possibly for create()). Either the task works or it throws an SQLException (ugh), surely?

Winston