Module jakarta.mail
Package jakarta.mail.internet

Class MimeUtility

java.lang.Object
jakarta.mail.internet.MimeUtility
public class MimeUtility extends Object
This is a utility class that provides various MIME related functionality.

There are a set of methods to encode and decode MIME headers as per RFC 2047. Note that, in general, these methods are not needed when using methods such as setSubject and setRecipients; Jakarta Mail will automatically encode and decode data when using these "higher level" methods. The methods below are only needed when maniuplating raw MIME headers using setHeader and getHeader methods. A brief description on handling such headers is given below:

RFC 822 mail headers must contain only US-ASCII characters. Headers that contain non US-ASCII characters must be encoded so that they contain only US-ASCII characters. Basically, this process involves using either BASE64 or QP to encode certain characters. RFC 2047 describes this in detail.

In Java, Strings contain (16 bit) Unicode characters. ASCII is a subset of Unicode (and occupies the range 0 - 127). A String that contains only ASCII characters is already mail-safe. If the String contains non US-ASCII characters, it must be encoded. An additional complexity in this step is that since Unicode is not yet a widely used charset, one might want to first charset-encode the String into another charset and then do the transfer-encoding.

Note that to get the actual bytes of a mail-safe String (say, for sending over SMTP), one must do 


        byte[] bytes = string.getBytes("iso-8859-1");

The setHeader and addHeader methods on MimeMessage and MimeBodyPart assume that the given header values are Unicode strings that contain only US-ASCII characters. Hence the callers of those methods must insure that the values they pass do not contain non US-ASCII characters. The methods in this class help do this.

The getHeader family of methods on MimeMessage and MimeBodyPart return the raw header value. These might be encoded as per RFC 2047, and if so, must be decoded into Unicode Strings. The methods in this class help to do this.

Several System properties control strict conformance to the MIME spec. Note that these are not session properties but must be set globally as System properties.

The mail.mime.decodetext.strict property controls decoding of MIME encoded words. The MIME spec requires that encoded words start at the beginning of a whitespace separated word. Some mailers incorrectly include encoded words in the middle of a word. If the mail.mime.decodetext.strict System property is set to "false", an attempt will be made to decode these illegal encoded words. The default is true.

The mail.mime.encodeeol.strict property controls the choice of Content-Transfer-Encoding for MIME parts that are not of type "text". Often such parts will contain textual data for which an encoding that allows normal end of line conventions is appropriate. In rare cases, such a part will appear to contain entirely textual data, but will require an encoding that preserves CR and LF characters without change. If the mail.mime.encodeeol.strict System property is set to "true", such an encoding will be used when necessary. The default is false.

In addition, the mail.mime.charset System property can be used to specify the default MIME charset to use for encoded words and text parts that don't otherwise specify a charset. Normally, the default MIME charset is derived from the default Java charset, as specified in the file.encoding System property. Most applications will have no need to explicitly set the default MIME charset. In cases where the default MIME charset to be used for mail messages is different than the charset used for files stored on the system, this property should be set.

The current implementation also supports the following System property.

The mail.mime.ignoreunknownencoding property controls whether unknown values in the Content-Transfer-Encoding header, as passed to the decode method, cause an exception. If set to "true", unknown values are ignored and 8bit encoding is assumed. Otherwise, unknown values cause a MessagingException to be thrown.