ExporterToCSV.java

/*
 * Copyright 2017 Global Crop Diversity Trust
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.genesys.server.service.impl;

import java.io.BufferedWriter;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.util.stream.Stream;
import javax.servlet.http.HttpServletResponse;

import lombok.extern.slf4j.Slf4j;

import com.opencsv.CSVWriter;

/**
 * Writes data in CSV format
 * 
 * @author Andrey Lugovskoy.
 */
@Slf4j
public abstract class ExporterToCSV<T> {

	private static final int FLUSH_INTERVAL = 100;

	public void downloadMetadata(Stream<T> entities, HttpServletResponse response, char separator, char quoteChar, char escapeChar, String lineEnd, String encoding) throws IOException {
		int rowCounter = 0;
		try (Stream<T> stream = entities;
			 CSVWriter writer = new CSVWriter(new BufferedWriter(new OutputStreamWriter(response.getOutputStream(), encoding)), separator, quoteChar, escapeChar, lineEnd)) {

			writer.writeNext(writeHeaders());

			var iterator = stream.iterator();
			while (iterator.hasNext()) {
				writer.writeNext(prepareLine(iterator.next()), true);
				if (++rowCounter % FLUSH_INTERVAL == 0) {
					log.trace("Wrote {} rows...", rowCounter);
					writer.flush();
					response.flushBuffer();
				}
			}
			log.trace("Flushing the CSV writer!");
			writer.flush();
			response.flushBuffer();
		}
		log.debug("Done with metadata download!");
	}

	protected abstract String[] writeHeaders();

	protected abstract String[] prepareLine(T element);
}