org.xbib.helianthus.common.http.HeaderDateTimeFormat Maven / Gradle / Ivy

Go to download
package org.xbib.helianthus.common.http;

import static java.util.Objects.requireNonNull;

import java.time.Instant;
import java.time.Year;
import java.time.ZoneId;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeFormatterBuilder;
import java.time.temporal.ChronoField;
import java.util.Locale;

/**
 * Parses and formats 3 date-and-time representations.
 * 
 * 

 * Sun, 06 Nov 1994 08:49:37 GMT: standard specification, the only one with valid generation
 * Sun, 06 Nov 1994 08:49:37 GMT: obsolete specification
 * Sun Nov 6 08:49:37 1994: obsolete specification
 * 
 */
final class HeaderDateTimeFormat {

    private static final ZoneId GMT = ZoneId.of("GMT");

    /**
     * Standard date format:
     * 
     * 
     * Sun, 06 Nov 1994 08:49:37 GMT -> E, d MMM yyyy HH:mm:ss z
     * 
     */
    private static final DateTimeFormatter format1 = newFormat("E, dd MMM yyyy HH:mm:ss z");

    /**
     * First obsolete format:
     * 
     * 
     * Sunday, 06-Nov-94 08:49:37 GMT -> E, d-MMM-y HH:mm:ss z
     * 
     */
    private static final DateTimeFormatter format2 = newFormat("E, dd-MMM-yy HH:mm:ss z");

    /**
     * A variant of the first obsolete format, which handles 20th century years such as 1994.
     *
     * @see http://stackoverflow.com/a/29496149
     */
    private static final DateTimeFormatter format2a =
            new DateTimeFormatterBuilder().parseLenient()
                    .parseCaseInsensitive()
                    .appendPattern("E, dd-MMM-")
                    .appendValueReduced(
                            ChronoField.YEAR, 2, 2, Year.now().getValue() - 80)
                    .appendPattern(" HH:mm:ss z")
                    .toFormatter(Locale.ENGLISH).withZone(GMT);

    /**
     * Second obsolete format
     * 
     * 
     * Sun Nov 6 08:49:37 1994 -> EEE, MMM d HH:mm:ss yyyy
     * 
     */
    private static final DateTimeFormatter format3 = newFormat("E MMM d HH:mm:ss yyyy");

    private HeaderDateTimeFormat() {
    }

    private static DateTimeFormatter newFormat(String pattern) {
        return new DateTimeFormatterBuilder().parseLenient()
                .parseCaseInsensitive()
                .appendPattern(pattern)
                .toFormatter(Locale.ENGLISH).withZone(GMT);
    }

    static Instant parse(String text) {
        requireNonNull(text);
        try {
            return parse(format1, text);
        } catch (Exception e1) {
            // Try the second preferred format.
            try {
                return parse(format2, text);
            } catch (Exception e2) {
                // Try the variant of the second preferred format.
                try {
                    return parse(format2a, text);
                } catch (Exception e2a) {
                    // Try the third preferred format.
                    try {
                        return parse(format3, text);
                    } catch (Exception e3) {
                        // None worked.
                        throw new IllegalArgumentException("not a date: " + text);
                    }
                }
            }
        }
    }

    private static Instant parse(DateTimeFormatter formatter, String text) {
        return formatter.parse(text, Instant::from);
    }

    static String format(long timeMillis) {
        return format1.format(Instant.ofEpochMilli(timeMillis));
    }
}