ResultSetIterator.java

package org.buddycloud.channelserver.db.jdbc;

import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Iterator;
import java.util.NoSuchElementException;

import org.apache.log4j.Logger;
import org.buddycloud.channelserver.db.CloseableIterator;

/**
 * Wraps a {@link ResultSet} to provide an {@link Iterator}, converting each row using a given
 * converter object.
 * <p>
 * The {@link ResultSet} and its {@link Statement} will be automatically closed when the iterator
 * returns the last item.
 * <p>
 * <b>NOTE: If the calling code does not reach the end of the iterator then {@link #close()} MUST be
 * called explicitly in order to free up database resources</b>
 * 
 * @param <T> the return type for each row.
 */
class ResultSetIterator<T> implements CloseableIterator<T> {

    private static final Logger logger = Logger.getLogger(ResultSetIterator.class);

    /**
     * An object to get the data from the current row of a {@link ResultSet} and return an object of
     * type T.
     * 
     * @param <T>
     */
    public interface RowConverter<T> {
        T convertRow(ResultSet rs) throws SQLException;
    }

    private ResultSet resultSet;
    private RowConverter<T> converter;

    private Boolean hasNextItem = null;

    public ResultSetIterator(final ResultSet resultSet, final RowConverter<T> converter) {
        this.resultSet = resultSet;
        this.converter = converter;
    }

    @Override
    public boolean hasNext() {
        if (hasNextItem == null) {
            try {
                hasNextItem = resultSet.next();

                if (!hasNextItem) {
                    close();
                }
            } catch (SQLException e) {
                logger.error("SQLException thrown while determining if next item available", e);
                hasNextItem = false;
            }
        }

        return hasNextItem;
    }

    @Override
    public T next() {
        if (!hasNext()) {
            throw new NoSuchElementException("No more results");
        }

        try {
            T item = converter.convertRow(resultSet);

            hasNextItem = null;

            return item;
        } catch (SQLException e) {
            throw new NoSuchElementException("No more results");
        }
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException();
    }

    public void close() {
        try {
            if (false == resultSet.isClosed()) {
                resultSet.getStatement().close();
            }
        } catch (SQLException e) {
            logger.error("SQLException thrown while closing statement", e);
        }
    }
}