浅析Java中SimpleDateFormat为什么是线程不安全的

public static Date getData(String date) throws ParseException {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    return sdf.parse(date);
}

public static Date getDataByFormat(String date, String format) throws ParseException {
    SimpleDateFormat sdf = new SimpleDateFormat(format);
    return sdf.parse(date);
}

public static void getDataByZone(String param, String format) throws ParseException {
    SimpleDateFormat sdf = new SimpleDateFormat(format);

    // 默认时区解析时间表示
    Date date = sdf.parse(param);
    System.out.println(date + ":" + date.getTime());

    // 东京时区解析时间表示
    sdf.setTimeZone(TimeZone.getTimeZone("Asia/Tokyo"));
    Date newYorkDate = sdf.parse(param);
    System.out.println(newYorkDate + ":" + newYorkDate.getTime());
}

public static void main(String[] args) throws ParseException {
   getDataByZone("2023-11-10 10:00:00","yyyy-MM-dd HH:mm:ss");
}

public static void getDataByZoneFormat(String param, String format) throws ParseException {
   SimpleDateFormat sdf = new SimpleDateFormat(format);
    Date date = sdf.parse(param);
    // 默认时区格式化输出
    System.out.println(new SimpleDateFormat("[yyyy-MM-dd HH:mm:ss Z]").format(date));
    // 东京时区格式化输出
    TimeZone.setDefault(TimeZone.getTimeZone("Asia/Tokyo"));
    System.out.println(new SimpleDateFormat("[yyyy-MM-dd HH:mm:ss Z]").format(date));
}

public static void main(String[] args) throws ParseException {
   getDataByZoneFormat("2023-11-10 10:00:00","yyyy-MM-dd HH:mm:ss");
}

public static void getDataByZoneFormat8(String param, String format) throws ParseException {
    ZoneId zone = ZoneId.of("Asia/Shanghai");
    ZoneId tokyoZone = ZoneId.of("Asia/Tokyo");
    ZoneId timeZone = ZoneOffset.ofHours(2);

    // 格式化器
    DateTimeFormatter dtf = DateTimeFormatter.ofPattern(format);
    ZonedDateTime date = ZonedDateTime.of(LocalDateTime.parse(param, dtf), zone);

    // withZone设置时区
    DateTimeFormatter dtfz = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss Z");
    System.out.println(dtfz.withZone(zone).format(date));
    System.out.println(dtfz.withZone(tokyoZone).format(date));
    System.out.println(dtfz.withZone(timeZone).format(date));
}

public static void main(String[] args) throws ParseException {
    getDataByZoneFormat8("2023-11-10 10:00:00","yyyy-MM-dd HH:mm:ss");
}

public static void getDataByThread(String param, String format) throws InterruptedException {
    ExecutorService threadPool = Executors.newFixedThreadPool(5);
    SimpleDateFormat sdf = new SimpleDateFormat(format);
    // 模拟并发环境，开启5个并发线程
    for (int i = 0; i < 5; i++) {
        threadPool.execute(() -> {
            for (int j = 0; j < 2; j++) {
                try {
                    System.out.println(sdf.parse(param));
                } catch (ParseException e) {
                    System.out.println(e);
                }
            }
        });
    }
    threadPool.shutdown();

    threadPool.awaitTermination(1, TimeUnit.HOURS);
}

public static void getDataByThreadLocal(String time, String format) throws InterruptedException {
    ExecutorService threadPool = Executors.newFixedThreadPool(5);

    ThreadLocal<SimpleDateFormat> sdf = new ThreadLocal<SimpleDateFormat>() {
        @Override
        protected SimpleDateFormat initialValue() {
            return new SimpleDateFormat(format);
        }
    };

    // 模拟并发环境，开启5个并发线程
    for (int i = 0; i < 5; i++) {
        threadPool.execute(() -> {
            for (int j = 0; j < 2; j++) {
                try {
                    System.out.println(sdf.get().parse(time));
                } catch (ParseException e) {
                    System.out.println(e);
                }
            }
        });
    }
    threadPool.shutdown();

    threadPool.awaitTermination(1, TimeUnit.HOURS);
}

public class SimpleDateFormat extends DateFormat {
	@Override
	public Date parse(String text, ParsePosition pos){
		CalendarBuilder calb = new CalendarBuilder();
		
		Date parsedDate;
		try {
		    parsedDate = calb.establish(calendar).getTime();
		    // If the year value is ambiguous,
		    // then the two-digit year == the default start year
		    if (ambiguousYear[0]) {
		        if (parsedDate.before(defaultCenturyStart)) {
		            parsedDate = calb.addYear(100).establish(calendar).getTime();
		        }
		    }
		}
	}
}

class CalendarBuilder {
	Calendar establish(Calendar cal) {
	    boolean weekDate = isSet(WEEK_YEAR)
	                        && field[WEEK_YEAR] > field[YEAR];
	    if (weekDate && !cal.isWeekDateSupported()) {
	        // Use YEAR instead
	        if (!isSet(YEAR)) {
	            set(YEAR, field[MAX_FIELD + WEEK_YEAR]);
	        }
	        weekDate = false;
	    }
	
	    cal.clear();
	    // Set the fields from the min stamp to the max stamp so that
	    // the field resolution works in the Calendar.
	    for (int stamp = MINIMUM_USER_STAMP; stamp < nextStamp; stamp++) {
	        for (int index = 0; index <= maxFieldIndex; index++) {
	            if (field[index] == stamp) {
	                cal.set(index, field[MAX_FIELD + index]);
	                break;
	            }
	        }
	    }
		...
	
	}
}