import java.util.HashMap;
import java.util.Map;

/**
 * Template for read cache class.
 * @author Ronald Koster
 */
public class ReadCache {

	private static ReadCache cache;
	private Map map; //Requires thread-safe access.
	private long timestamp; //Idem.

	private ReadCache() {}

	public static synchronized ReadCache getInstance() {
		if (cache == null) {
			cache = new ReadCache();
		}
		return cache;
	}

	/**
	 * @return The cached data. Usually a database table (key: primary key, value: row object). Or a map
	 * of such maps, in which case the keys in the main map are the table names.
	 */
	public synchronized Map getMap() {
		if (this.map == null || System.currentTimeMillis() - this.timestamp > getRefreshPeriod()) {
			init();
		}
		return this.map;
	}

	public synchronized void refresh() {
		init();
	}

	private void init() {
		Map newMap = new HashMap();
		//TODO: load newMap from database.
		this.timestamp = System.currentTimeMillis();
		this.map = newMap;
	}

	private long getRefreshPeriod() {
		return 10 * 60 * 1000; //TODO: fetch this from a property file.
	}
}