java 时间处理

//获取当前日期,和前一天的日期
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");
        System.out.println(df.format(new Date()));

        Calendar calendar = Calendar.getInstance();
        calendar.setTime(new Date());
        calendar.add(Calendar.DATE,-1);
        Date dateBefore = calendar.getTime();
        System.out.println(df.format(dateBefore));

        System.out.println(dateBefore.getClass());

        // 字符串解析成 Date
        String date = "2021-11-01 04:00:50";

        Date parse = df.parse(date);

        System.out.println(parse);


        // 获取当前时间和2个小时之前的时间
        System.out.println("获取当前时间和2个小时之前的时间");
        DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:00:00");
        // dateTime1 比 dateTime2 早2小时
        LocalDateTime dateTime2 = LocalDateTime.now();
        LocalDateTime dateTime1 = dateTime2.minusHours(2);

        System.out.println(dateTimeFormatter.format(dateTime1));  // 2021-11-09 12:00:00
        System.out.println(dateTimeFormatter.format(dateTime2));  // 2021-11-09 14:00:00

        // 从字符串生成
        LocalDateTime dateTime3 = LocalDateTime.parse("2021-11-09 01:00:00",dateTimeFormatter);
        // 时间前推2小时，跨天
        LocalDateTime dateTime4 = dateTime3.minusHours(2);
        System.out.println(dateTime3);  // 2021-11-09T01:00
        System.out.println(dateTime4);  // 2021-11-08T23:00