使用 java 8 DateTime API 解析日期和 AM/PM

Question

我正在尝试使用 java 8 DateTime API.

解析日期时间字符串

要解析的字符串包含日期、月份、年份和AM/PM 标记，例如17/02/2015 PM。我使用以下模式：dd/MM/yyyy aa 并且我希望解析的 LocalDateTime 时间部分设置为 12:00:00 或 00:00:00，具体取决于 AM/PM 标记。

使用前者 java.text.SimpleDateFormat API，使用此模式解析按预期工作。

但是当我使用 java 8 DateTime API 和运行以下代码时：

LocalDateTime.parse("17/02/2015 PM", DateTimeFormatter.ofPattern("dd/MM/yyyy aa"));

我遇到以下异常：

Exception in thread "main" java.lang.IllegalArgumentException: Too many pattern letters: a
at java.time.format.DateTimeFormatterBuilder.parseField(DateTimeFormatterBuilder.java:1765)
at java.time.format.DateTimeFormatterBuilder.parsePattern(DateTimeFormatterBuilder.java:1604)
at java.time.format.DateTimeFormatterBuilder.appendPattern(DateTimeFormatterBuilder.java:1572)
at java.time.format.DateTimeFormatter.ofPattern(DateTimeFormatter.java:534)

如果我将模式切换为 dd/MM/yyyy a，则会出现以下异常：

Exception in thread "main" java.time.format.DateTimeParseException: Text '17/02/2015 PM' could not be parsed: Unable to obtain LocalDateTime from TemporalAccessor: {AmPmOfDay=1},ISO resolved to 2015-02-17 of type java.time.format.Parsed
    at java.time.format.DateTimeFormatter.createError(DateTimeFormatter.java:1918)
    at java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1853)
    at java.time.LocalDateTime.parse(LocalDateTime.java:492)
Caused by: java.time.DateTimeException: Unable to obtain LocalDateTime from TemporalAccessor: {AmPmOfDay=1},ISO resolved to 2015-02-17 of type java.time.format.Parsed
    at java.time.LocalDateTime.from(LocalDateTime.java:461)
    at java.time.LocalDateTime$$Lambda/474675244.queryFrom(Unknown Source)
    at java.time.format.Parsed.query(Parsed.java:226)
    at java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1849)
    ... 2 more
Caused by: java.time.DateTimeException: Unable to obtain LocalTime from TemporalAccessor: {AmPmOfDay=1},ISO resolved to 2015-02-17 of type java.time.format.Parsed
    at java.time.LocalTime.from(LocalTime.java:409)
    at java.time.LocalDateTime.from(LocalDateTime.java:457)
    ... 5 more

同样奇怪的是，当我进行反向操作时，格式化，它工作正常:

 System.out.println(LocalDateTime.of(2015, 02, 17, 0, 0, 0).format(DateTimeFormatter.ofPattern("dd/MM/yyyy a")));
 System.out.println(LocalDateTime.of(2015, 02, 17, 12, 0, 0).format(DateTimeFormatter.ofPattern("dd/MM/yyyy a")));

分别打印17/02/2015 AM和17/02/2015 PM.

java.time.DateTimeFormatter 的 java 文档说（§Resolving，步骤 #6）：

A LocalTime is formed if there is at least an hour-of-day available. This involves providing default values for minute, second and fraction of second.

我的理解是字段 hour-of-day 是解析 LocalDateTime 的必填项...难道没有任何方法可以仅使用 AM/PM 字段来解析时间部分吗？

Answer 1

您可以通过 DateTimeFormatterBuilder.parseDefaulting:

设置不可用字段的默认值

DateTimeFormatter formatter = new DateTimeFormatterBuilder()
    .appendPattern("dd/MM/yyyy a")
    .parseDefaulting(ChronoField.HOUR_OF_AMPM, 0) // this is required
    .parseDefaulting(ChronoField.MINUTE_OF_HOUR, 0) // optional, but you can set other value
    .parseDefaulting(ChronoField.SECOND_OF_MINUTE, 0) // optional as well
    .toFormatter();
System.out.println(LocalDateTime.parse("17/02/2015 PM", formatter)); // 2015-02-17T12:00
System.out.println(LocalDateTime.parse("17/02/2015 AM", formatter)); // 2015-02-17T00:00

使用 java 8 DateTime API 解析日期和 AM/PM

Parsing date and AM/PM using java 8 DateTime API

java

java-8

java-time