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JRobin is a 100% pure Java alternative to RRDTool, with about exactly the same specifications. If you provide the same data to RRDTool and JRobin, you will get exactly the same results and graphs. All standard RRDTool operations are supported.
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/*******************************************************************************
 * Copyright (c) 2001-2005 Sasa Markovic and Ciaran Treanor.
 * Copyright (c) 2011 The OpenNMS Group, Inc.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 *******************************************************************************/

package org.jrobin.core.timespec;

import org.jrobin.core.RrdException;
import org.jrobin.core.Util;

import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

/**
 * Simple class to represent time obtained by parsing at-style date specification (described
 * in detail on the rrdfetch man page. See javadoc for {@link org.jrobin.core.timespec.TimeParser}
 * for more information.
 */
public class TimeSpec {
	static final int TYPE_ABSOLUTE = 0;
	static final int TYPE_START = 1;
	static final int TYPE_END = 2;

	int type = TYPE_ABSOLUTE;
	int year, month, day, hour, min, sec;
	int wday;
	int dyear, dmonth, dday, dhour, dmin, dsec;

	String dateString;

	TimeSpec context;

	TimeSpec(String dateString) {
		this.dateString = dateString;
	}

	void localtime(long timestamp) {
		GregorianCalendar date = new GregorianCalendar();
		date.setTime(new Date(timestamp * 1000L));
		year = date.get(Calendar.YEAR) - 1900;
		month = date.get(Calendar.MONTH);
		day = date.get(Calendar.DAY_OF_MONTH);
		hour = date.get(Calendar.HOUR_OF_DAY);
		min = date.get(Calendar.MINUTE);
		sec = date.get(Calendar.SECOND);
		wday = date.get(Calendar.DAY_OF_WEEK) - Calendar.SUNDAY;
	}

	GregorianCalendar getTime() throws RrdException {
		GregorianCalendar gc;
		// absoulte time, this is easy
		if (type == TYPE_ABSOLUTE) {
			gc = new GregorianCalendar(year + 1900, month, day, hour, min, sec);
		}
		// relative time, we need a context to evaluate it
		else if (context != null && context.type == TYPE_ABSOLUTE) {
			gc = context.getTime();
		}
		// how would I guess what time it was?
		else {
			throw new RrdException("Relative times like '" +
					dateString + "' require proper absolute context to be evaluated");
		}
		gc.add(Calendar.YEAR, dyear);
		gc.add(Calendar.MONTH, dmonth);
		gc.add(Calendar.DAY_OF_MONTH, dday);
		gc.add(Calendar.HOUR_OF_DAY, dhour);
		gc.add(Calendar.MINUTE, dmin);
		gc.add(Calendar.SECOND, dsec);
		return gc;
	}

	/**
	 * Returns the corresponding timestamp (seconds since Epoch). Example:
	 * 
	 * TimeParser p = new TimeParser("now-1day");
	 * TimeSpec ts = p.parse();
	 * System.out.println("Timestamp was: " + ts.getTimestamp();
	 * 
	 *
	 * @return Timestamp (in seconds, no milliseconds)
	 * @throws RrdException Thrown if this TimeSpec object does not represent absolute time.
	 */
	public long getTimestamp() throws RrdException {
		return Util.getTimestamp(getTime());
	}

	String dump() {
		return (type == TYPE_ABSOLUTE ? "ABSTIME" : type == TYPE_START ? "START" : "END") +
				": " + year + "/" + month + "/" + day +
				"/" + hour + "/" + min + "/" + sec + " (" +
				dyear + "/" + dmonth + "/" + dday +
				"/" + dhour + "/" + dmin + "/" + dsec + ")";
	}

	/**
	 * Use this static method to resolve relative time references and obtain the corresponding
	 * Calendar objects. Example:
	 * 
	 * TimeParser pStart = new TimeParser("now-1month"); // starting time
	 * TimeParser pEnd = new TimeParser("start+1week");  // ending time
	 * TimeSpec specStart = pStart.parse();
	 * TimeSpec specEnd = pEnd.parse();
	 * GregorianCalendar[] gc = TimeSpec.getTimes(specStart, specEnd);
	 * 
	 *
	 * @param spec1 Starting time specification
	 * @param spec2 Ending time specification
	 * @return Two element array containing Calendar objects
	 * @throws RrdException Thrown if relative time references cannot be resolved
	 */
	public static Calendar[] getTimes(TimeSpec spec1, TimeSpec spec2) throws RrdException {
		if (spec1.type == TYPE_START || spec2.type == TYPE_END) {
			throw new RrdException("Recursive time specifications not allowed");
		}
		spec1.context = spec2;
		spec2.context = spec1;
		return new Calendar[] {
				spec1.getTime(),
				spec2.getTime()
		};
	}

	/**
	 * Use this static method to resolve relative time references and obtain the corresponding
	 * timestamps (seconds since epoch). Example:
	 * 
	 * TimeParser pStart = new TimeParser("now-1month"); // starting time
	 * TimeParser pEnd = new TimeParser("start+1week");  // ending time
	 * TimeSpec specStart = pStart.parse();
	 * TimeSpec specEnd = pEnd.parse();
	 * long[] ts = TimeSpec.getTimestamps(specStart, specEnd);
	 * 
	 *
	 * @param spec1 Starting time specification
	 * @param spec2 Ending time specification
	 * @return array containing two timestamps (in seconds since epoch)
	 * @throws RrdException Thrown if relative time references cannot be resolved
	 */
	public static long[] getTimestamps(TimeSpec spec1, TimeSpec spec2) throws RrdException {
		Calendar[] gcs = getTimes(spec1, spec2);
		return new long[] {
				Util.getTimestamp(gcs[0]), Util.getTimestamp(gcs[1])
		};
	}
}