Parsing RFC 2822 date in JAVA

问题:

I need to parse an RFC 2822 string representation of a date in Java. An example string is here:

Sat, 13 Mar 2010 11:29:05 -0800

It looks pretty nasty so I wanted to make sure I was doing everything right and would run into weird problems later with the date being interpreted wrong either through AM-PM/Military time problems, UTC time problems, problems I don't anticipate, etc...

Thanks!

回答1:

This is quick code that does what you ask (using SimpleDateFormat)

String rfcDate = "Sat, 13 Mar 2010 11:29:05 -0800";
String pattern = "EEE, dd MMM yyyy HH:mm:ss Z";
SimpleDateFormat format = new SimpleDateFormat(pattern);
Date javaDate = format.parse(rfcDate);

//Done.

PS. I've not dealt with exceptions and concurrency here (as SimpleDateFormat is not synchronized when parsing date).

回答2:

If your application is using another language than English, you may want to force the locale for the date parsing/formatting by using an alternate SimpleDateFormat constructor:

String pattern = "EEE, dd MMM yyyy HH:mm:ss Z";
SimpleDateFormat format = new SimpleDateFormat(pattern, Locale.ENGLISH);

回答3:

Please keep in mind that the [day-of-week ","] is optional in RFC-2822, hence the suggested examples are not covering all RFC-2822 date formats. Additional, the RFC-822 date type allowed many different time zone notations(obs-zone), which are not covered by the "Z" format specifier.

I guess there is no easy way out, other than looking for "," and "-|+" to determine which pattern to use.

回答4:

Since Java 8 new datetime classes were implemented: java.time.ZonedDateTime and java.time.LocalDateTime. ZonedDateTime supports the parsing of RFC strings nearly out of the box:

String rfcDate = "Tue, 4 Dec 2018 17:37:31 +0100 (CET)";  
if (rfcDate.matches(".*[ ]\\(\\w\\w\\w\\)$")) {
    //Brackets with time zone are added sometimes, for example by JavaMail
    //This must be removed before parsing
    //from: "Tue, 4 Dec 2018 17:37:31 +0100 (CET)"
    //  to: "Tue, 4 Dec 2018 17:37:31 +0100"
    rfcDate = rfcDate.substring(0, rfcDate.length() - 6);
}

//and now parsing... 
DateTimeFormatter dateFormat = DateTimeFormatter.RFC_1123_DATE_TIME;
try {
    ZonedDateTime zoned = ZonedDateTime.parse(rfcDate, dateFormat);
    LocalDateTime local = zoned.toLocalDateTime();        
} catch (DateTimeParseException e) { ... }