Java中的SimpleDateFormat的线程安全问题详解

// Called from Format after creating a FieldDelegate
    private StringBuffer format(Date date, StringBuffer toAppendTo,
                                FieldDelegate delegate) {
        // Convert input date to time field list
        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;
    }

String inputValue = game.getId() + new SimpleDateFormat( "yyyyMMdd" ).format( new Date() );

private static SimpleDateFormat df = new SimpleDateFormat( "yyyyMMdd" );// 设置日期格式
	private static final Lock LOCK = new ReentrantLock();
	@Override
	public void addGame( Game game ) {
		......
		LOCK.lock();
		try {
			String inputValue = game.getId() + df.format( new Date() );
		} finally {
			LOCK.unlock();
		}
		......
	}

private static final String DATE_FORMAT = "yyyy-MM-dd";
private static final ThreadLocal<DateFormat> SDF  = new ThreadLocal<DateFormat>() {
    protected synchronized DateFormat initialValue() {
        return new SimpleDateFormat(
            DATE_FORMAT);
                }
        };
            ......
album.setReleaseDate(SDF.get().parse(show.getReleasedate().substring(0, 10))); // 发布日期