/*
 This file is part of BORG.
 
 BORG is free software; you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
 the Free Software Foundation; either version 2 of the License, or
 (at your option) any later version.
 
 BORG is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU General Public License for more details.
 
 You should have received a copy of the GNU General Public License
 along with BORG; if not, write to the Free Software
 Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 
 Copyright 2003 by Mike Berger
 */
/*
 * JdbcDB.java
 *
 * Created on February 2, 2004, 12:57 PM
 */

package net.sf.borg.model.db.jdbc;

import net.sf.borg.model.entity.KeyedEntity;

import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;


/**
 * abstract base class for all classes that manage reading/writing of KeyedEntities
 * to and from a JDBC database. also provides an object cache.
 */
abstract class JdbcBeanDB<T extends KeyedEntity<T>> {

	// BORG needs its own caching. BORG rebuilds the map of DB data often
	// and going to the DB is too expensive. If BORG is changed to support
	// multi-user access, then the cache algorithm will have to be smarter.
	// currently, it is assumed that a single BORG client is the only DB
	// updater.
	// in the future, if there are multiple writers, the DB will have to
	// contain
	// some info to indicate when another process has written the DB to
	// force
	// a flush of the cache
	private final boolean objectCacheOn_; // is caching on?

	private final HashMap<Integer,T> objectCache_; // the cache

	/**
	 * Creates a new instance of JdbcDB.
	 */
	JdbcBeanDB()  {
		objectCacheOn_ = true;
		objectCache_ = new HashMap<Integer,T>();
	}

	/**
	 * flush the cache
	 */
	public void sync() {
		emptyCache();
	}

	protected void writeCache(T bean) {
		// put a copy of the bean in the cache
		if (objectCacheOn_) {
			objectCache_.put(Integer.valueOf(bean.getKey()), bean.copy());
		}
	}

	protected void emptyCache() {
		if (objectCacheOn_)
			objectCache_.clear();
	}

	protected void delCache(int key) {
		// remove the bean from the cache
		if (objectCacheOn_) {
			objectCache_.remove(Integer.valueOf(key));
		}
	}

	protected T readCache(int key) {
		// need to remove cache here if DB has been updated
		// by any other process besides this one
		// TBD

		// if the bean is in the cache - return it
		if (objectCacheOn_) {
			T o = objectCache_.get(Integer.valueOf(key));

			if (o != null) {
				return o.copy();
			}
		}

		return (null);
	}

	/**
	 * Read all Entities.
	 * 
	 * @return A collection of all Entities in the DB table
	 * 
	 * @throws Exception 
	 */
	public Collection<T> readAll() throws Exception {
		PreparedStatement stmt = null;
		ResultSet r = null;
		try {
			stmt = getPSAll();
			r = stmt.executeQuery();
			Collection<T> lst = new ArrayList<T>();
			while (r.next()) {
				T bean = createFrom(r);
				lst.add(bean);
				writeCache(bean);
			}
			return lst;
		} finally {
			if (r != null)
				r.close();
			if (stmt != null)
				stmt.close();
		}
	}

	/**
	 * Read a single KeyedEntity from the database by key
	 * 
	 * @param key the key
	 * 
	 * @return the entity
	 * 
	 * @throws Exception the exception
	 */
	public T readObj(int key) throws Exception {
		T bean = readCache(key);

		if (bean != null)
			return bean;

		PreparedStatement stmt = null;
		ResultSet r = null;
		try {
			stmt = getPSOne(key);
			r = stmt.executeQuery();
			if (r.next()) {
				bean = createFrom(r);
				writeCache(bean);
			}
			return bean;
		} finally {
			if (r != null)
				r.close();
			if (stmt != null)
				stmt.close();
		}
	}

	/**
	 * @return a PreparedStatement that reads one entity by key
	 * @throws SQLException
	 */
	abstract PreparedStatement getPSOne(int key) throws SQLException;
	
	/**
	 * 
	 * @return a PreparedStatement that reads all entities from a table
	 * @throws SQLException
	 */
	abstract PreparedStatement getPSAll() throws SQLException;

	/**
	 * Fills in an entity from a result set
	 * @param rs the result set
	 * @return the entity
	 * @throws SQLException
	 */
	abstract T createFrom(ResultSet rs) throws SQLException;
}
