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As part of the same effort, I have done some work on making the Context less data-layer dependent (by having it hold a org.dspace.storage.dao.GlobalDAO rather than a java.sql.Connection, etc). I've also introduced a proxy for the Item that is a bit smarter about when it retrieves content from the data layer, and an ArchiveManager class that takes care of some core "archive operations" (so that other core classes don't need to).

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The Item class will be broken up into the following classes:

• org.dspace.content.Item: core class that doesn't go near the database (it doesn't even know about the DAOs); behaves much like the current implementation.
• org.dspace.content.dao.ItemDAO: interface defining DAO API
• org.dspace.content.dao.ItemDAOFactory: factory for dishing out implementations of the above interface
• org.dspace.content.dao.postgres.ItemDAOPostgres: default implementation of the above interface for use with PostgreSQL
• org.dspace.content.proxy.ItemProxy: subclass of Item that needs to know about the DAO. It will be used for (eg) only loading metadata on demand, to reduce the memory footprint of Items etc.

The following classes have also been introduced:

• org.dspace.core.ArchiveManager
• org.dspace.storage.dao.GlobalDAO
• org.dspace.storage.dao.GlobalDAOFactory
• org.dspace.storage.dao.GlobalDAOPostgres

Note that it might be preferable to have a more generic implementation of the ItemDAO interface that supports both PostgreSQL and Oracle, but given that one motivation for adopting DAOs is to remove db-specificities from the code making it easier to port, I thought it was sensible to start with just PostgreSQL. Eventually, it ought to be possible to drop in ItemDAOHibernate (etc) implementations that make db portability far easier.

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Basic implementation of the Item object. This class has been stripped down to remove all contact with the database, including (but not limited to) contstructors, factory methods, update(), delete(), find(), etc. I haven't decided exactly how the Item API will look, but it will probably be much the same as before, only with any of the aforementioned methods. Another key difference is that it will have actual Java objects as member variables instead of pulling everything out of a TableRow.

org.dspace.content.proxy.ItemProxy

This will be a fairly simple proxy implementation. Specifically, it will be closest to being a virtual proxy, in that it will appear to be a regular Item object, but will have a slightly smarter implementation (not loading metadata until requested, keeping track of what has changed to make updates more efficient etc). 

public class ItemProxy extends Item
{ // Overrides relevant methods of Item. }

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This isn't final, but it's a good start. 

public interface ItemDAO extends ContentDAO
    implements CRUD<Item>, Link<Item, Bundle>
{
    public Item create() throws AuthorizeException;
    public Item retrieve(int id);
    public Item retrieve(UUID uuid);
    public void update(Item item) throws AuthorizeException;
    public void delete(int id) throws AuthorizeException;
    public List<Item> getItems();
    public List<Item> getItemsBySubmitter(EPerson eperson);
    public List<Item> getItemsByCollection(Collection collection);
    public List<Item> getParentItems(Bundle bundle);
}

org.dspace.content.dao.ItemDAOFactory

public class ItemDAOFactory
{
    public static ItemDAO getInstance(Context context)
    {   // Eventually, the implementation that is returned will be
        // defined in the configuration.
        return new ItemDAOPostgres(context);
    }
}

org.dspace.content.dao.postgres.ItemDAOPostgres

This is a fairly straightforward implementation of the above interface. As much as possible, code from the original Item class will be used. For instance, this is how getItems() is implemented:

public List<Item> getItems()
{
    try
    {
        TableRowIterator tri = DatabaseManager.queryTable(context, "item",
            "SELECT item_id FROM item WHERE in_archive = '1'");

        List<Item> items = new ArrayList<Item>();
        for (TableRow row : tri.toList())
        {
            int id = row.getIntColumn("item_id");
            items.add(retrieve(id));
        }
        return items;
    }
  catch (SQLException sqle){
        // Need to think more carefully about how we deal with SQLExceptions
        throw new RuntimeException(sqle);
    }
}

Some changes have been made to eliminate {{ItemIterator}}s, and to generally make things a little more consistent with the rest of the code (this looks almost identical to, eg, CollectionDAO.getCollections().

org.dspace.core

org.dspace.core.ArchiveManager

The idea behind this class came from the realisation that Item.withdraw() and Item.reinstate() don't really make sense. What I'd much rather do is call (eg) ArchiveManager.withdrawItem(Item item).
I've been thinking that the ArchiveManager could be used for certain maintenance operations as well, such as moving Items between Collections, and maybe acting as a wrapper for the CommunityFiliator.

public class ArchiveManager
{
    public static void withdrawItem(Context context, Item item)
    {        // ...      }

    public static void reinstateItem(Context context, Item item)
    {        // ...      }
    public static void moveItem(Context context, Item item, Collection source, Collection dest)
    {        // ...      }
}

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As suggested by Richard Jones, there probably ought to be a top-level general-purpose DAO interface that has implementations for the various storage mechanisms (GlobalDAOPostgres etc). The idea is to have this top-level object capture any implementation-specific details in a single top-level object, rather than in every Postgres DAO implementation. For example, with the current database "abstraction layer", the top-level implementation of GlobalDAO understands the Context object, whereas a Hibernate implementation would know what a SessionFactory is.

public interface GlobalDAO
{
    // The following methods actually currently throw SQLExceptions to
    // keep things simple, but in future SQLExceptions should be
    // eliminated from any code that doesn't directly touch a database.
    public void startTransaction() throws GlobalDAOException;
    public void endTransaction() throws GlobalDAOException;
    public void saveTransaction() throws GlobalDAOException;
    public void abortTransaction();
    public boolean transactionOpen();
    @Deprecated Connection getConnection();
}

org.dspace.storage.dao.GlobalDAOFactory

Super-simple GlobalDAO factory.

org.dspace.storage.dao.GlobalDAOPostgres

Implementation of the GlobalDAO interface for PostgreSQL.

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