LocalDate和localDateTime常用方法

public class TestTime extends TestCase {
    private static final Logger log = LoggerFactory.getLogger(TestLocalDate.class);
    public void testDate() {
        //创建LocalDate，只会获取年月日
        //获取当前年月日
        LocalDate localDate = LocalDate.now();
        //构造指定的年月日
        LocalDate specifiedDate = LocalDate.of(2019, 2, 19);
        //年
        int year = specifiedDate.getYear(); //2019
        int year1 = specifiedDate.get(ChronoField.YEAR); //2019
        //月
        Month month = specifiedDate.getMonth();  //FEBRUARY
        int month1 = specifiedDate.get(ChronoField.MONTH_OF_YEAR); //2
        //日
        int day = specifiedDate.getDayOfMonth(); //19
        int day1 = specifiedDate.get(ChronoField.DAY_OF_MONTH); //19
        //星期几
        DayOfWeek dayOfWeek = specifiedDate.getDayOfWeek(); //TUESDAY
        int dayOfWeek1 = specifiedDate.get(ChronoField.DAY_OF_WEEK); //2
 
        //创建LocalTime，只会获取几时几分几秒
        LocalTime localTime = LocalTime.now();
        //构造指定LocalTime
        LocalTime specifiedTime = LocalTime.of(8, 18, 28);
        //获取小时
        int hour = specifiedTime.getHour(); //8
        int hour1 = specifiedTime.get(ChronoField.HOUR_OF_DAY);  //8
        //获取分
        int minute = specifiedTime.getMinute(); //18
        int minute1 = specifiedTime.get(ChronoField.MINUTE_OF_HOUR); //18
 
        //创建LocalDateTime，获取年月日时分秒，等于LocalDate+LocalTime
        LocalDateTime localDateTime = LocalDateTime.now(); //now
        LocalDateTime specifiedTimeLocalDateTime = LocalDateTime.of(2019, Month.FEBRUARY, 19, 8, 18, 28); //2019-02-19T08:18:28
        LocalDateTime localDateTime2 = LocalDateTime.of(localDate, localTime); //now
        LocalDateTime localDateTime3 = localDate.atTime(localTime); //now
        LocalDateTime localDateTime4 = localTime.atDate(localDate); //now
 
        log.info("{},{},{},{},{}", localDateTime, specifiedTimeLocalDateTime, localDateTime2, localDateTime3, localDateTime4);
        //获取LocalDate
        specifiedTimeLocalDateTime.toLocalDate(); //2019-02-19
        //获取LocalTime
        specifiedTimeLocalDateTime.toLocalTime(); //08:18:28
 
        //创建 Instant 对象 是不带时区的即时时间点
        Instant instant = Instant.now(); //2020-04-23T06:58:21.651Z
        //获取秒数
        long currentSecond = instant.getEpochSecond(); //1587625101
        //获取毫秒数
        long currentMilli = instant.toEpochMilli(); //1587625101651
 
        //修改LocalDate、LocalTime、LocalDateTime、Instant
        //LocalDate 、 LocalTime 、 LocalDateTime 、 Instant 为不可变对象，修改这些对象对象会返回一个副本
        LocalDateTime localDateTime5 = LocalDateTime.of(2019, Month.SEPTEMBER, 10, 14, 46, 56); //2019-09-10T14:46:56
        //增加一年
        localDateTime5 = localDateTime5.plusYears(1); //2020-09-10T14:46:56
        localDateTime5 = localDateTime5.plus(1, ChronoUnit.YEARS); //2021-09-10T14:46:56
        //减少一个月
        localDateTime5 = localDateTime5.minusMonths(1); //2021-08-10T14:46:56
        localDateTime5 = localDateTime5.minus(1, ChronoUnit.MONTHS); //2021-07-10T14:46:56
 
        //通过with修改某些值
        //修改年为2020
        localDateTime = localDateTime.withYear(2020); //2020-04-23T15:00:55.751
        //修改为2022
        localDateTime = localDateTime.with(ChronoField.YEAR, 2022); //2022-04-23T15:00:55.751
 
        //时间计算
        //比如有些时候想知道这个月的最后一天是几号、下个周末是几号，通过提供的时间和日期API可以很快得到答案
        LocalDate localDate3 = LocalDate.now();
        //第一天
        LocalDate f = localDate3.with(firstDayOfMonth());   //2020-04-01
 
        //格式化时间
        LocalDate localDate4 = LocalDate.of(2019, 9, 10);
        String s1 = localDate4.format(DateTimeFormatter.BASIC_ISO_DATE);   //20190910
        String s2 = localDate4.format(DateTimeFormatter.ISO_LOCAL_DATE);   //2019-09-10
 
        String s = localDateTime.format(DateTimeFormatter.ISO_LOCAL_DATE); //2022-04-23
 
        //DateTimeFormatter 默认提供了多种格式化方式，如果默认提供的不能满足要求，可以通过
        //DateTimeFormatter 的 ofPattern 方法创建自定义格式化方式
        String s3 = localDateTime.format(DateTimeFormatter.ofPattern("yyyy-MM-dd hh:mm")); //2022-04-23 03:00
 
        //解析时间
        LocalDate localDate5 = LocalDate.parse("20190910", DateTimeFormatter.BASIC_ISO_DATE);  //2019-09-10
        LocalDate localDate6 = LocalDate.parse("2019-09-10", DateTimeFormatter.ISO_LOCAL_DATE); //2019-09-10
    }
}

项目应用LocalDateTime

将LocalDateTime字段以时间戳的方式返回给前端

添加日期转化类

public class LocalDateTimeConverter extends JsonSerializer<LocalDateTime> {
    @Override
    public void serialize(LocalDateTime value, JsonGenerator gen, SerializerProvider serializer) throws IOException {
        gen.writeNumber(value.toInstant(ZoneOffset.of("+8")).toEpochMilli());
    }
}

并在 LocalDateTime 字段上添加 @JsonSerialize(using = LocalDateTimeConverter.class) 注解，
如下：

1 2	`@JsonSerialize(using = LocalDateTimeConverter.class)` `private` `LocalDateTime day;`

将LocalDateTime字段以指定格式化日期的方式返回给前端

在 LocalDateTime 字段上添加 @JsonFormat(shape=JsonFormat.Shape.STRING, pattern="yyyy-MM-dd HH:mm:ss") 注解即可，如下：

1 2	`@JsonFormat(shape=JsonFormat.Shape.STRING, pattern="yyyy-MM-dd HH:mm:ss")` `private` `LocalDateTime register;`

对前端传入的日期进行格式化

在 LocalDateTime 字段上添加 @DateTimeFormat(pattern = "yyyy-MM-dd HH:mm:ss") 注解即可，如下：

1 2	`@DateTimeFormat(pattern =` `"yyyy-MM-dd HH:mm:ss")` `private` `LocalDateTime login;`

与原来Date类对比优势

Date如果不格式化，打印出的日期可读性差
使用SimpleDateFormat对时间进行格式化，但SimpleDateFormat是线程不安全的

SimpleDateFormat的format方法最终调用代码：

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;
    }

　　可以看到，calendar是共享变量，并且这个共享变量没有做线程安全控制。当多个线程同时使用相同的SimpleDateFormat对象【如用static修饰的SimpleDateFormat】调用format方法时，多个线程会同时调用calendar.setTime方法，可能一个线程刚设置好time值另外的一个线程马上把设置的time值给修改了导致返回的格式化时间可能是错误的。在多并发情况下使用SimpleDateFormat需格外注意SimpleDateFormat除了format是线程不安全以外，parse方法也是线程不安全的。parse方法实际调用alb.establish(calendar).getTime()方法来解析，alb.establish(calendar)方法里主要完成了：

重置日期对象cal的属性值
使用calb中中属性设置cal
返回设置好的cal对象

但是这三步不是原子操作

　　多线程并发如何保证线程安全 - 避免线程之间共享一个SimpleDateFormat对象，每个线程使用时都创建一次SimpleDateFormat对象 => 创建和销毁对象的开销大 - 对使用format和parse方法的地方进行加锁 => 线程阻塞性能差 - 使用ThreadLocal保证每个线程最多只创建一次SimpleDateFormat对象 => 较好的方法