AW: [OJB-developers] Select count(x)

[Mahler T. <tho...@it...>]

Hi Charles,


> 
> 1. When my RowReader returns null, 
> RSIterator.getObjectFromResultSet still
> tries to create an Identity, look it up in the cache etc. Creating an
> Indentity from a null reference results a NullPointer 
> exception. This is
> easily fixable, I believe, just by changeing the appropriate 
> part of the
> method to  
> 	Object result = 
> m_mif.getRowReader().readObjectFrom(m_rs, m_mif);
> 	if(result!=null) {
> 		...
> 	}

I will at this to the todo-list!
 
> 
> 2. I'm a little concerned about the ClassDescriptor.setRowReader in a
> multi-threaded environment. What would happen if one thread 
> was using a
> RowReaderDefaultImpl, and another was using my 
> PagingRowReader - on the same
> class ? I think that would cause problems.
> 

There will be no problems if different threads are using different
RowReaders for the same class.
But: there my be problems if to threads are manipulating a ClassDescriptor
simultaneously.
thread 1 says: use RowReaderDefaultImpl,
thread 2 says: use PagingRowReader 
thread 1 acquires a ResultSetIterator an will be using a PagingRowreader
although it asked for a default RowReader.

You'll have to implement some additional access management to avoid such
errors.

A general remark:
I'm not quite sure what you want to to with your paging component.
My concept of a paging display is as follows.
You a ResultSetIterator which may produce any number of Objects.
You don't want to display all those objects immediately, but in pages of say
20 objects.
The Paging component should support forward and backward navigation ("show
last 20 items", "show next 20 items").

I would implement this as follows:

public class Pager
{
	private ResultSetIterator iter;

	private Vector items = new Vector();

	private int offset = 0;

	private int pagesize;

	public Pager(ResultSetIterator i, int size)
	{
		iter = i;
		pagesize = size;
	}


	public Collection getNextPage()
	{
		Collection page = new Vector()
		for (int i = 0; i<pagesize; i++)
		{
			page.add(iter.next());	
			offset++;
		}
		items.addAll(page);
		return page;
	}
	
	public Collection getPreviousPage()
	{
		Collection page = new Vector();
		int oldoffset = offset;            
		offset = offset - pagesize;
		if (offset < 0) offset = 0;
		for (int i = offset; i < oldoffset; i++)
		{
			page.add(items.get(i));	
		}
		return page;		
	}


}

cheers, 

Thomas

> Thanks, and sorry to keep bothering you,
> 
> 
> Charles.
> 
> 
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