Parsing and formatting HTTP dates as used in cookies and other headers. : HttpURLConnection « Network « Java Tutorial

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import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Arrays;
import java.util.Calendar;
import java.util.Collection;
import java.util.Date;
import java.util.Locale;
import java.util.TimeZone;

/**
 * A utility class for parsing and formatting HTTP dates as used in cookies and
 * other headers.  This class handles dates as defined by RFC 2616 section
 * 3.3.1 as well as some other common non-standard formats.
 *
 * @author Christopher Brown
 * @author Michael Becke
 */
public final class DateUtil {

    /**
     * Date format pattern used to parse HTTP date headers in RFC 1123 format.
     */
    public static final String PATTERN_RFC1123 = "EEE, dd MMM yyyy HH:mm:ss zzz";

    /**
     * Date format pattern used to parse HTTP date headers in RFC 1036 format.
     */
    public static final String PATTERN_RFC1036 = "EEEE, dd-MMM-yy HH:mm:ss zzz";

    /**
     * Date format pattern used to parse HTTP date headers in ANSI C
     * <code>asctime()</code> format.
     */
    public static final String PATTERN_ASCTIME = "EEE MMM d HH:mm:ss yyyy";

    private static final Collection<String> DEFAULT_PATTERNS = Arrays.asList(
        new String[] {PATTERN_ASCTIME, PATTERN_RFC1036, PATTERN_RFC1123});

    private static final Date DEFAULT_TWO_DIGIT_YEAR_START;

    static {
        Calendar calendar = Calendar.getInstance();
        calendar.set(2000, Calendar.JANUARY, 1, 0, 0);
        DEFAULT_TWO_DIGIT_YEAR_START = calendar.getTime();
    }

    private static final TimeZone GMT = TimeZone.getTimeZone("GMT");

    /**
     * This class should not be instantiated.
     */
    private DateUtil() {
    }


    /**
     * Parses a date value.  The formats used for parsing the date value are retrieved from
     * the default http params.
     *
     * @param dateValue the date value to parse
     * @return the parsed date
     * @throws DateParseException if the value could not be parsed using any of the
     *                            supported date formats
     */
    public static Date parseDate(String dateValue) {
        return parseDate(dateValue, null, null);
    }

    /**
     * Parses the date value using the given date formats.
     *
     * @param dateValue   the date value to parse
     * @param dateFormats the date formats to use
     * @return the parsed date
     * @throws DateParseException if none of the dataFormats could parse the dateValue
     */
    public static Date parseDate(String dateValue, Collection<String> dateFormats)
        {
        return parseDate(dateValue, dateFormats, null);
    }

    /**
     * Parses the date value using the given date formats.
     *
     * @param dateValue   the date value to parse
     * @param dateFormats the date formats to use
     * @param startDate   During parsing, two digit years will be placed in the range
     *                    <code>startDate</code> to <code>startDate + 100 years</code>. This value may
     *                    be <code>null</code>. When <code>null</code> is given as a parameter, year
     *                    <code>2000</code> will be used.
     * @return the parsed date
     * @throws DateParseException if none of the dataFormats could parse the dateValue
     */
    public static Date parseDate(
        String dateValue,
        Collection<String> dateFormats,
        Date startDate
    )  {

        if (dateValue == null) {
            throw new IllegalArgumentException("dateValue is null");
        }
        if (dateFormats == null) {
            dateFormats = DEFAULT_PATTERNS;
        }
        if (startDate == null) {
            startDate = DEFAULT_TWO_DIGIT_YEAR_START;
        }
        // trim single quotes around date if present
        // see issue #5279
        if (dateValue.length() > 1
            && dateValue.startsWith("'")
            && dateValue.endsWith("'")
            ) {
            dateValue = dateValue.substring(1, dateValue.length() - 1);
        }

        SimpleDateFormat dateParser = null;
        for (String format : dateFormats) {
            if (dateParser == null) {
                dateParser = new SimpleDateFormat(format, Locale.US);
                dateParser.setTimeZone(TimeZone.getTimeZone("GMT"));
                dateParser.set2DigitYearStart(startDate);
            } else {
                dateParser.applyPattern(format);
            }
            try {
                return dateParser.parse(dateValue);
            } catch (ParseException pe) {
                // ignore this exception, we will try the next format
            }
        }

        // we were unable to parse the date
        throw new RuntimeException("Unable to parse the date " + dateValue);
    }

    /**
     * Formats the given date according to the RFC 1123 pattern.
     *
     * @param date The date to format.
     * @return An RFC 1123 formatted date string.
     * @see #PATTERN_RFC1123
     */
    public static String formatDate(Date date) {
        return formatDate(date, PATTERN_RFC1123);
    }

    /**
     * Formats the given date according to the specified pattern.  The pattern
     * must conform to that used by the {@link SimpleDateFormat simple date
     * format} class.
     *
     * @param date    The date to format.
     * @param pattern The pattern to use for formatting the date.
     * @return A formatted date string.
     * @throws IllegalArgumentException If the given date pattern is invalid.
     * @see SimpleDateFormat
     */
    public static String formatDate(Date date, String pattern) {
        if (date == null) {
            throw new IllegalArgumentException("date is null");
        }
        if (pattern == null) {
            throw new IllegalArgumentException("pattern is null");
        }

        SimpleDateFormat formatter = new SimpleDateFormat(pattern, Locale.US);
        formatter.setTimeZone(GMT);
        return formatter.format(date);
    }

}