java8Time随笔

Instant：瞬时实例
LocalDate：本地日期，不包含具体时间。例如：2014-01-14可以用来记录生日、纪念日、加盟日等。
LocalTime：本地时间，不包含日期
LocalDateTime：组合了日期和时间，但不包含时差和时区信息
ZonedDateTime：最完整的日期时间，包含时区和相对UTC或格林威治的时差

1. 获取当前时间：LocalDateTime.now();
2. 日期格式化:

   DateTimeFormatter默认提供了多种格式化方式:
   LocalDate localDate = LocalDate.of(2019, 9, 10);  
   String s1 = localDate.format(DateTimeFormatter.BASIC_ISO_DATE);  
   String s2 = localDate.format(DateTimeFormatter.ISO_LOCAL_DATE); 
   
   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
   LocalDateTime.now().format(DateTimeFormatter.ofPattern("yyyy-MM-dd hh:mm:ss a"));

   3.解析时间

LocalDate localDate1 = LocalDate.parse("20190910", DateTimeFormatter.BASIC_ISO_DATE);  
LocalDate localDate2 = LocalDate.parse("2019-09-10", DateTimeFormatter.ISO_LOCAL_DATE);

1. 获取时间戳：Instant.now().toEpochMilli()
Instant.now() （获取的是格林威治时间/世界时）
   （Date.from(Instant)将Instant转换成java.util.Date，Date.toInstant()则是将Date类转换成Instant类。）
2. LocalDateTime转String: LocalDateTime.parse(str,dateTimeFormatter); String转LocalDateTime: localDateTime.format(dateTimeFormatter);
3. Date转LocalDateTime：LocalDateTime localDateTime = LocalDateTime.ofInstant(date.toInstant(), ZoneId.systemDefault());
4. LocalDateTime转Date：Date date = Date.from(LocalDateTime.now().atZone(ZoneId.systemDefault()).toInstant());

LocalDate转LocalDateTime：LocalDateTime.of(LocalDate.parse("2021-01-08", DateTimeFormatter.ofPattern("yyyy-MM-dd")),LocalTime.MIN)

LocalDateTime转LocalDate：localDateTime.toLocalDate()

1. 前端传过来的日期字符串怎么转换并放到数据库
2. 获取某天的最开始那一秒（最开始时间）：LocalDateTime.of(oneDayLocalDate,LocalTime.MIN)
3. 通过with修改某些值

//修改年为2020
localDateTime = localDateTime.withYear(2020);  
//修改为2022  
localDateTime = localDateTime.with(ChronoField.YEAR, 2022);  

1. 通过自定义数据类型转换器返回时间戳给前端

// 新建一个类数据类型转换器
public class LocalDateTimeConverter extends JsonSerializer<LocalDateTime> {  

    @Override  
    public void serialize(LocalDateTime value, JsonGenerator gen, SerializerProvider serializers) throws IOException {  
    gen.writeNumber(value.toInstant(ZoneOffset.of("+8")).toEpochMilli());  
    }  
} 

// 然后在LocalDateTime类型的字段上，加上注解：
@JsonSerialize(using = LocalDateTimeConverter.class)

1. 一些例子

LocalDate localDate = LocalDate.now();
      System.out.println("LocalDate.now():"+localDate);
      System.out.println("localDate.getYear():"+localDate.getYear());
      System.out.println("localDate.getMonth():"+localDate.getMonth());
      System.out.println("localDate.getMonthValue():"+localDate.getMonthValue());
      System.out.println("localDate.getDayOfMonth():"+localDate.getDayOfMonth());
      System.out.println("localDate.getDayOfWeek():"+localDate.getDayOfWeek());
      System.out.println("localDate.getDayOfWeek().getValue():"+localDate.getDayOfWeek().getValue());

      LocalTime localTime = LocalTime.now();
      System.out.println("当前时间LocalTime.now(): " + localTime);
      System.out.println("当前时间+1小时 localTime.plusHours(1): " + localTime.plusHours(1));
      System.out.println("当前时间-1小时 localTime.plusHours(1): " + localTime.minusHours(1));

      Clock clock = Clock.systemUTC();
      Clock clock2 = Clock.systemDefaultZone();
      System.out.println("Clock : " + clock.millis());
      System.out.println("clock2 : " + clock2.millis());
      System.out.println("= 获取当前的时间戳 =");
      Instant timestamp = Instant.now();
      System.out.println("获取当前的时间戳: "+timestamp);
      System.out.println("= 格式化日期 =");
      DateTimeFormatter format = DateTimeFormatter.ofPattern("yyyy-MM-dd hh:mm:ss a");
      LocalDateTime localDateTime = LocalDateTime.now();
      System.out.println(" LocalDateTime.now(): "+localDateTime);
      System.out.println(" 格式化后的日期："+localDateTime.format(format));

      System.out.println("= 检查生日等周期性事件 =");
      LocalDate dateOfBirth = LocalDate.of(2019, 7, 25);
      MonthDay birthday = MonthDay.of(dateOfBirth.getMonth(), dateOfBirth.getDayOfMonth());
      MonthDay currentMonthDay = MonthDay.from(localDate);

      if(currentMonthDay.equals(birthday)){
          System.out.println(" Many Many happy returns of the day !!");
      }else{
          System.out.println(" Sorry, today is not your birthday");
      }