de.julielab.xmlData.dataBase.DBCThreadedIterator Maven / Gradle / Ivy

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A utility for managing documents stored in a PostgreSQL database. The documents are imported into a PostgreSQL DB as full texts with the goal to be able to retrieve the documents by their PubMedID efficiently. For more sophisticated tasks, a user configuration file can be delivered which can take control of the table schema to use, the PostgreSQL schema to use and the actual database server to connect to as well as the concrete database.

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package de.julielab.xmlData.dataBase;

import de.julielab.xmlData.dataBase.util.CoStoSysSQLRuntimeException;

import java.util.Iterator;
import java.util.List;
import java.util.concurrent.Exchanger;

/**
 * 
 * This iterator class employs multiple Threads for database queries. While the
 * iterator offers access to retrieved values, additional values are
 * concurrently retrieved from the database by another thread.
 * 
 * 
 * The iterator uses the {@link Exchanger} class to communicate between threads.
 * 
 * 
 * @author hellrich/faessler
 * 
 * @param 
 */
public abstract class DBCThreadedIterator extends DBCIterator {
	// Exchangers switch results between 2 threads as needed
	protected Exchanger> listExchanger = new Exchanger<>();
	protected ConnectionClosable backgroundThread;
	private Iterator currentListIter;
	private boolean hasNext = true;



	public boolean hasNext() {
		return hasNext;
	}

	@Override
	public E next() {
		E next = currentListIter.next();
		if (!currentListIter.hasNext())
			update();
		return next;
	}

	/**
	 * unsupported
	 */
	public void remove() {
		throw new UnsupportedOperationException();
	}

	protected void update() {
		try {
			List list = listExchanger.exchange(null);
			while (list != null && list.isEmpty()) {
				list = listExchanger.exchange(null);
			}
			// list full or null
			if (list == null) {
				hasNext = false;
			} else
				currentListIter = list.iterator();
		} catch (InterruptedException e) {
			throw new CoStoSysSQLRuntimeException(e);
		}
	}

	@Override
	public abstract void close();


    public abstract void join() throws InterruptedException;
}