javax.mail.internet

Class MimeUtility

java.lang.Object

javax.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; JavaMail 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.

Author:

John Mani, Bill Shannon

Field Summary

Fields

Modifier and Type	Field and Description
static int	ALL

Method Summary

Methods

Modifier and Type	Method and Description
static InputStream	decode(InputStream is, String encoding) Decode the given input stream.
static String	decodeText(String etext) Decode "unstructured" headers, that is, headers that are defined as '*text' as per RFC 822.
static String	decodeWord(String eword) The string is parsed using the rules in RFC 2047 and RFC 2231 for parsing an "encoded-word".
static OutputStream	encode(OutputStream os, String encoding) Wrap an encoder around the given output stream.
static OutputStream	encode(OutputStream os, String encoding, String filename) Wrap an encoder around the given output stream.
static String	encodeText(String text) Encode a RFC 822 "text" token into mail-safe form as per RFC 2047.
static String	encodeText(String text, String charset, String encoding) Encode a RFC 822 "text" token into mail-safe form as per RFC 2047.
static String	encodeWord(String word) Encode a RFC 822 "word" token into mail-safe form as per RFC 2047.
static String	encodeWord(String word, String charset, String encoding) Encode a RFC 822 "word" token into mail-safe form as per RFC 2047.
static String	fold(int used, String s) Fold a string at linear whitespace so that each line is no longer than 76 characters, if possible.
static String	getDefaultJavaCharset() Get the default charset corresponding to the system's current default locale.
static String	getEncoding(DataHandler dh) Same as getEncoding(DataSource) except that instead of reading the data from an InputStream it uses the writeTo method to examine the data.
static String	getEncoding(DataSource ds) Get the Content-Transfer-Encoding that should be applied to the input stream of this DataSource, to make it mail-safe.
static String	javaCharset(String charset) Convert a MIME charset name into a valid Java charset name.
static String	mimeCharset(String charset) Convert a java charset into its MIME charset name.
static String	quote(String word, String specials) A utility method to quote a word, if the word contains any characters from the specified 'specials' list.
static String	unfold(String s) Unfold a folded header.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

ALL

public static final int ALL

See Also:

Constant Field Values

Method Detail

getEncoding

public static String getEncoding(DataSource ds)

Get the Content-Transfer-Encoding that should be applied to the input stream of this DataSource, to make it mail-safe.

The algorithm used here is:

• If the DataSource implements EncodingAware, ask it what encoding to use. If it returns non-null, return that value.
• If the primary type of this datasource is "text" and if all the bytes in its input stream are US-ASCII, then the encoding is "7bit". If more than half of the bytes are non-US-ASCII, then the encoding is "base64". If less than half of the bytes are non-US-ASCII, then the encoding is "quoted-printable".
• If the primary type of this datasource is not "text", then if all the bytes of its input stream are US-ASCII, the encoding is "7bit". If there is even one non-US-ASCII character, the encoding is "base64".

Parameters:

ds - the DataSource

Returns:

the encoding. This is either "7bit", "quoted-printable" or "base64"

getEncoding

public static String getEncoding(DataHandler dh)

Same as getEncoding(DataSource) except that instead of reading the data from an InputStream it uses the writeTo method to examine the data. This is more efficient in the common case of a DataHandler created with an object and a MIME type (for example, a "text/plain" String) because all the I/O is done in this thread. In the case requiring an InputStream the DataHandler uses a thread, a pair of pipe streams, and the writeTo method to produce the data.

Since:

JavaMail 1.2

decode

public static InputStream decode(InputStream is,
                 String encoding)
                          throws MessagingException

Decode the given input stream. The Input stream returned is the decoded input stream. All the encodings defined in RFC 2045 are supported here. They include "base64", "quoted-printable", "7bit", "8bit", and "binary". In addition, "uuencode" is also supported.

In the current implementation, if the mail.mime.ignoreunknownencoding system property is set to "true", unknown encoding values are ignored and the original InputStream is returned.

Parameters:

is - input stream

encoding - the encoding of the stream.

Returns:

decoded input stream.

Throws:

MessagingException - if the encoding is unknown

encode

public static OutputStream encode(OutputStream os,
                  String encoding)
                           throws MessagingException

Wrap an encoder around the given output stream. All the encodings defined in RFC 2045 are supported here. They include "base64", "quoted-printable", "7bit", "8bit" and "binary". In addition, "uuencode" is also supported.

Parameters:

os - output stream

encoding - the encoding of the stream.

Returns:

output stream that applies the specified encoding.

Throws:

MessagingException - if the encoding is unknown

encode

public static OutputStream encode(OutputStream os,
                  String encoding,
                  String filename)
                           throws MessagingException

Wrap an encoder around the given output stream. All the encodings defined in RFC 2045 are supported here. They include "base64", "quoted-printable", "7bit", "8bit" and "binary". In addition, "uuencode" is also supported. The filename parameter is used with the "uuencode" encoding and is included in the encoded output.

Parameters:

os - output stream

encoding - the encoding of the stream.

filename - name for the file being encoded (only used with uuencode)

Returns:

output stream that applies the specified encoding.

Throws:

MessagingException

Since:

JavaMail 1.2

encodeText

public static String encodeText(String text)
                         throws UnsupportedEncodingException

Encode a RFC 822 "text" token into mail-safe form as per RFC 2047.

The given Unicode string is examined for non US-ASCII characters. If the string contains only US-ASCII characters, it is returned as-is. If the string contains non US-ASCII characters, it is first character-encoded using the platform's default charset, then transfer-encoded using either the B or Q encoding. The resulting bytes are then returned as a Unicode string containing only ASCII characters.

Note that this method should be used to encode only "unstructured" RFC 822 headers.

Example of usage:

  MimePart part = ...
  String rawvalue = "FooBar Mailer, Japanese version 1.1"
  try {
    // If we know for sure that rawvalue contains only US-ASCII 
    // characters, we can skip the encoding part
    part.setHeader("X-mailer", MimeUtility.encodeText(rawvalue));
  } catch (UnsupportedEncodingException e) {
    // encoding failure
  } catch (MessagingException me) {
   // setHeader() failure
  }

 

Parameters:

text - Unicode string

Returns:

Unicode string containing only US-ASCII characters

Throws:

UnsupportedEncodingException - if the encoding fails

encodeText

public static String encodeText(String text,
                String charset,
                String encoding)
                         throws UnsupportedEncodingException

Encode a RFC 822 "text" token into mail-safe form as per RFC 2047.

The given Unicode string is examined for non US-ASCII characters. If the string contains only US-ASCII characters, it is returned as-is. If the string contains non US-ASCII characters, it is first character-encoded using the specified charset, then transfer-encoded using either the B or Q encoding. The resulting bytes are then returned as a Unicode string containing only ASCII characters.

Note that this method should be used to encode only "unstructured" RFC 822 headers.

Parameters:

text - the header value

charset - the charset. If this parameter is null, the platform's default chatset is used.

encoding - the encoding to be used. Currently supported values are "B" and "Q". If this parameter is null, then the "Q" encoding is used if most of characters to be encoded are in the ASCII charset, otherwise "B" encoding is used.

Returns:

Unicode string containing only US-ASCII characters

Throws:

UnsupportedEncodingException

decodeText

public static String decodeText(String etext)
                         throws UnsupportedEncodingException

Decode "unstructured" headers, that is, headers that are defined as '*text' as per RFC 822.

The string is decoded using the algorithm specified in RFC 2047, Section 6.1. If the charset-conversion fails for any sequence, an UnsupportedEncodingException is thrown. If the String is not an RFC 2047 style encoded header, it is returned as-is

Example of usage:

  MimePart part = ...
  String rawvalue = null;
  String  value = null;
  try {
    if ((rawvalue = part.getHeader("X-mailer")[0]) != null)
      value = MimeUtility.decodeText(rawvalue);
  } catch (UnsupportedEncodingException e) {
      // Don't care
      value = rawvalue;
  } catch (MessagingException me) { }

  return value;

 

Parameters:

etext - the possibly encoded value

Throws:

UnsupportedEncodingException - if the charset conversion failed.

encodeWord

public static String encodeWord(String word)
                         throws UnsupportedEncodingException

Encode a RFC 822 "word" token into mail-safe form as per RFC 2047.

The given Unicode string is examined for non US-ASCII characters. If the string contains only US-ASCII characters, it is returned as-is. If the string contains non US-ASCII characters, it is first character-encoded using the platform's default charset, then transfer-encoded using either the B or Q encoding. The resulting bytes are then returned as a Unicode string containing only ASCII characters.

This method is meant to be used when creating RFC 822 "phrases". The InternetAddress class, for example, uses this to encode it's 'phrase' component.

Parameters:

word - Unicode string

Returns:

Array of Unicode strings containing only US-ASCII characters.

Throws:

UnsupportedEncodingException - if the encoding fails

encodeWord

public static String encodeWord(String word,
                String charset,
                String encoding)
                         throws UnsupportedEncodingException

Encode a RFC 822 "word" token into mail-safe form as per RFC 2047.

The given Unicode string is examined for non US-ASCII characters. If the string contains only US-ASCII characters, it is returned as-is. If the string contains non US-ASCII characters, it is first character-encoded using the specified charset, then transfer-encoded using either the B or Q encoding. The resulting bytes are then returned as a Unicode string containing only ASCII characters.

Parameters:

word - Unicode string

charset - the MIME charset

encoding - the encoding to be used. Currently supported values are "B" and "Q". If this parameter is null, then the "Q" encoding is used if most of characters to be encoded are in the ASCII charset, otherwise "B" encoding is used.

Returns:

Unicode string containing only US-ASCII characters

Throws:

UnsupportedEncodingException - if the encoding fails

decodeWord

public static String decodeWord(String eword)
                         throws ParseException,
                                UnsupportedEncodingException

The string is parsed using the rules in RFC 2047 and RFC 2231 for parsing an "encoded-word". If the parse fails, a ParseException is thrown. Otherwise, it is transfer-decoded, and then charset-converted into Unicode. If the charset-conversion fails, an UnsupportedEncodingException is thrown.

Parameters:

eword - the encoded value

Throws:

ParseException - if the string is not an encoded-word as per RFC 2047 and RFC 2231.

UnsupportedEncodingException - if the charset conversion failed.

quote

public static String quote(String word,
           String specials)

A utility method to quote a word, if the word contains any characters from the specified 'specials' list.

The HeaderTokenizer class defines two special sets of delimiters - MIME and RFC 822.

This method is typically used during the generation of RFC 822 and MIME header fields.

Parameters:

word - word to be quoted

specials - the set of special characters

Returns:

the possibly quoted word

See Also:

HeaderTokenizer.MIME, HeaderTokenizer.RFC822

fold

public static String fold(int used,
          String s)

Fold a string at linear whitespace so that each line is no longer than 76 characters, if possible. If there are more than 76 non-whitespace characters consecutively, the string is folded at the first whitespace after that sequence. The parameter used indicates how many characters have been used in the current line; it is usually the length of the header name.

Note that line breaks in the string aren't escaped; they probably should be.

Parameters:

used - characters used in line so far

s - the string to fold

Returns:

the folded string

Since:

JavaMail 1.4

unfold

public static String unfold(String s)

Unfold a folded header. Any line breaks that aren't escaped and are followed by whitespace are removed.

Parameters:

s - the string to unfold

Returns:

the unfolded string

Since:

JavaMail 1.4

javaCharset

public static String javaCharset(String charset)

Convert a MIME charset name into a valid Java charset name.

Parameters:

charset - the MIME charset name

Returns:

the Java charset equivalent. If a suitable mapping is not available, the passed in charset is itself returned.

mimeCharset

public static String mimeCharset(String charset)

Convert a java charset into its MIME charset name.

Note that a future version of JDK (post 1.2) might provide this functionality, in which case, we may deprecate this method then.

Parameters:

charset - the JDK charset

Returns:

the MIME/IANA equivalent. If a mapping is not possible, the passed in charset itself is returned.

Since:

JavaMail 1.1

getDefaultJavaCharset

public static String getDefaultJavaCharset()

Get the default charset corresponding to the system's current default locale. If the System property mail.mime.charset is set, a system charset corresponding to this MIME charset will be returned.

Returns:

the default charset of the system's default locale, as a Java charset. (NOT a MIME charset)

Since:

JavaMail 1.1