使用Jackson将LocalDateTime序列化为仅带毫秒的格式

Question

我想将LocalDateTime序列化为文本格式，仅显示毫秒。 我尝试了以下示例：

final ObjectMapper mapper = new ObjectMapper();

mapper.registerModule(new Jdk8Module());
mapper.registerModule(new JavaTimeModule());
mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
mapper.disable(SerializationFeature.WRITE_DATE_TIMESTAMPS_AS_NANOSECONDS);

final LocalDateTime t = LocalDateTime.of(2014, 3, 30, 12, 30, 23, 123456789);

System.out.println(mapper.writeValueAsString(t));

这将输出：

"2014-03-30T12:30:23.123456789"

所以，精度仍然是纳秒级别的，尽管不会显示它们，这是因为使用了mapper.disable(SerializationFeature.WRITE_DATE_TIMESTAMPS_AS_NANOSECONDS)。

我的期望是：

"2014-03-30T12:30:23.123"

为什么会出现这种情况？我该如何解决？

- TheBrizzler

---

我目前正在使用Jackson的2.9.8版本。 - TheBrizzler

Answer 1

由于您禁用了WRITE_DATES_AS_TIMESTAMPS，启用或禁用WRITE_DATE_TIMESTAMPS_AS_NANOSECONDS不起任何作用，因为localDateTime不再处于时间戳表示中。

如果您启用WRITE_DATES_AS_TIMESTAMPS，然后禁用WRITE_DATE_TIMESTAMPS_AS_NANOSECONDS，您将获得所需的结果，但以另一种形式呈现([2014, 3, 30, 12, 30, 23, 123])，因此这也不是一个选择。

因此，基本上只有两种选择来实现预期的结果：

1. The easiest - use this: System.out.println(mapper.writeValueAsString(t.truncatedTo(ChronoUnit.MILLIS)));

   (Remember that you can safely remove mapper.disable(SerializationFeature.WRITE_DATE_TIMESTAMPS_AS_NANOSECONDS);, as it does nothing in this case).

2. A more complicated way (without truncating the time):

   DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS");
   SimpleModule simpleModule = new SimpleModule();
   simpleModule.addSerializer(new LocalDateTimeSerializer(dateTimeFormatter));
   
   final ObjectMapper mapper = new ObjectMapper();
   mapper.registerModule(simpleModule);
   
   final LocalDateTime t = LocalDateTime.of(2014, 3, 30, 12, 30, 23, 123456789);
   
   System.out.println(mapper.writeValueAsString(t));
   

   (In that case you don't even need to use JavaTimeModule).

- amseager