Fastdateformat why thread safe

Chapter one FastDateFormat Why thread safe

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FastDateFormat Why thread safe

SimpleDateFormat The thread of is not safe

Everybody knows SimpleDateFormat It's not thread safe

protected Calendar calendar;

SimpleDateFormat Medium calendar Is a member variable , Multiple threads of the same instance will share this calendar object

When formatting, it may be because the first thread has not finished formatting , The second thread has modified the time

private StringBuffer format(Date date, StringBuffer toAppendTo,
                                FieldDelegate delegate) {
    
        //  If the first thread has not been formatted as a string after setting the time , At this time, the second thread overwrites the time , There will be thread safety issues 
        calendar.setTime(date);

        boolean useDateFormatSymbols = useDateFormatSymbols();

        for (int i = 0; i < compiledPattern.length; ) {
    
            int tag = compiledPattern[i] >>> 8;
            int count = compiledPattern[i++] & 0xff;
            if (count == 255) {
    
                count = compiledPattern[i++] << 16;
                count |= compiledPattern[i++];
            }

            switch (tag) {
    
            case TAG_QUOTE_ASCII_CHAR:
                toAppendTo.append((char)count);
                break;

            case TAG_QUOTE_CHARS:
                toAppendTo.append(compiledPattern, i, count);
                i += count;
                break;

            default:
                subFormat(tag, count, delegate, toAppendTo, useDateFormatSymbols);
                break;
            }
        }
        return toAppendTo;
    }

FastDateFormat How to deal with it

that FastDateFormat Why is thread safe ？ First FastDateFormat There is a cache , When instantiating, it is through cache Cache to get the instance

private static final FormatCache<FastDateFormat> cache= new FormatCache<FastDateFormat>() {
    
        @Override
        protected FastDateFormat createInstance(final String pattern, final TimeZone timeZone, final Locale locale) {
    
            return new FastDateFormat(pattern, timeZone, locale);
        }
    };

public static FastDateFormat getInstance(final String pattern) {
    
    return cache.getInstance(pattern, null, null);
}

Format will be formatted 、 Time zone and internationalization as a key There is cInstanceCache in ,cInstanceCache It's a ConcurrentHashMap, Equivalent to the same format 、 Time zone and internationalization will use the same FastDateFormat example

final MultipartKey key = new MultipartKey(pattern, timeZone, locale);
F format = cInstanceCache.get(key);
if (format == null) {
               
    format = createInstance(pattern, timeZone, locale);
    final F previousValue= cInstanceCache.putIfAbsent(key, format);
    if (previousValue != null) {
    
        // another thread snuck in and did the same work
        // we should return the instance that is in ConcurrentMap
        format= previousValue;              
    }
}

When formatting Calendar It uses local variables inside the method , There will be no thread safety problems

public String format(final Date date) {
    
    final Calendar c = newCalendar();
    c.setTime(date);
    return applyRulesToString(c);
}