Character Sets and Encodings - The J2EE 1.4 Tutorial

Character Sets and Encodings

Character Sets

A character set is a set of textual and graphic symbols, each of which is mapped to a set of nonnegative integers.

The first character set used in computing was US-ASCII. It is limited in that it can represent only American English. US-ASCII contains upper- and lower-case Latin alphabets, numerals, punctuation, a set of control codes, and a few miscellaneous symbols.

Unicode defines a standardized, universal character set that can be extended to accommodate additions. When the Java program source file encoding doesn't support Unicode, you can represent Unicode characters as escape sequences by using the notation \uXXXX, where XXXX is the character's 16-bit representation in hexadecimal. For example, the Spanish version of the Duke's Bookstore message file uses Unicode for non-ASCII characters:

{"TitleCashier", "Cajero"},
{"TitleBookDescription", "Descripci" + "\u00f3" + "n del 
Libro"},
{"Visitor", "Es visitanten" + "\u00fa" + "mero "},
{"What", "Qu" + "\u00e9" + " libros leemos"},
{"Talk", " describe como componentes de software de web pueden 
transformar la manera en que desrrollamos aplicaciones para el 
web. Este libro es obligatorio para cualquier programador de 
respeto!"},
{"Start", "Empezar a Comprar"}, 

Character Encoding

A character encoding maps a character set to units of a specific width and defines byte serialization and ordering rules. Many character sets have more than one encoding. For example, Java programs can represent Japanese character sets using the EUC-JP or Shift-JIS encodings, among others. Each encoding has rules for representing and serializing a character set.

The ISO 8859 series defines 13 character encodings that can represent texts in dozens of languages. Each ISO 8859 character encoding can have up to 256 characters. ISO 8859-1 (Latin-1) comprises the ASCII character set, characters with diacritics (accents, diaereses, cedillas, circumflexes, and so on), and additional symbols.

UTF-8 (Unicode Transformation Format, 8-bit form) is a variable-width character encoding that encodes 16-bit Unicode characters as one to four bytes. A byte in UTF-8 is equivalent to 7-bit ASCII if its high-order bit is zero; otherwise, the character comprises a variable number of bytes.

UTF-8 is compatible with the majority of existing web content and provides access to the Unicode character set. Current versions of browsers and email clients support UTF-8. In addition, many new web standards specify UTF-8 as their character encoding. For example, UTF-8 is one of the two required encodings for XML documents (the other is UTF-16).

See Appendix A for more information on character encodings in the Java 2 platform.

Web components usually use PrintWriter to produce responses; PrintWriter automatically encodes using ISO 8859-1. Servlets can also output binary data using OutputStream classes, which perform no encoding. An application that uses a character set that cannot use the default encoding must explicitly set a different encoding.

For web components, three encodings must be considered:

Request
Page (JSP pages)
Response

Request Encoding

The request encoding is the character encoding in which parameters in an incoming request are interpreted. Currently, many browsers do not send a request encoding qualifier with the Content-Type header. In such cases, a web container will use the default encoding--ISO-8859-1--to parse request data.

If the client hasn't set character encoding and the request data is encoded with a different encoding from the default, the data won't be interpreted correctly. To remedy this situation, you can use the ServletRequest.setCharacterEncoding(String enc) method to override the character encoding supplied by the container. To control the request encoding from JSP pages, you can use the JSTL fmt:requestEncoding tag. You must call the method or tag before parsing any request parameters or reading any input from the request. Calling the method or tag once data has been read will not affect the encoding.

Page Encoding

For JSP pages, the page encoding is the character encoding in which the file is encoded.

For JSP pages in standard syntax, the page encoding is determined from the following sources:

The page encoding value of a JSP property group (see Setting Properties for Groups of JSP Pages) whose URL pattern matches the page.
The pageEncoding attribute of the page directive of the page. It is a translation-time error to name different encodings in the pageEncoding attribute of the page directive of a JSP page and in a JSP property group.
The CHARSET value of the contentType attribute of the page directive.

If none of these is provided, ISO-8859-1 is used as the default page encoding.

For JSP pages in XML syntax (JSP documents), the page encoding is determined as described in section 4.3.3 and appendix F.1 of the XML specification.

The pageEncoding and contentType attributes determine the page character encoding of only the file that physically contains the page directive. A web container raises a translation-time error if an unsupported page encoding is specified.

Response Encoding

The response encoding is the character encoding of the textual response generated by a web component. The response encoding must be set appropriately so that the characters are rendered correctly for a given locale. A web container sets an initial response encoding for a JSP page from the following sources:

The CHARSET value of the contentType attribute of the page directive
The encoding specified by the pageEncoding attribute of the page directive
The page encoding value of a JSP property group whose URL pattern matches the page

If none of these is provided, ISO-8859-1 is used as the default response encoding.

The setCharacterEncoding, setContentType, and setLocale methods can be called repeatedly to change the character encoding. Calls made after the servlet response's getWriter method has been called or after the response is committed have no effect on the character encoding. Data is sent to the response stream on buffer flushes (for buffered pages) or on encountering the first content on unbuffered pages.

Calls to setContentType set the character encoding only if the given content type string provides a value for the charset attribute. Calls to setLocale set the character encoding only if neither setCharacterEncoding nor setContentType has set the character encoding before. To control the response encoding from JSP pages, you can use the JSTL fmt.setLocale tag.

To obtain the character encoding for a locale, the setLocale method checks the locale encoding mapping for the web application. For example, to map Japanese to the Japanese-specific encoding Shift_JIS, follow these steps:

Select the WAR.
Click the Advanced Settings button.
In the Locale Character Encoding table, Click the Add button.
Enter ja in the Extension column.
Enter Shift_JIS in the Character Encoding column.

If a mapping is not set for the web application, setLocale uses a Application Server mapping.

The first application in Chapter 12 allows a user to choose an English string representation of a locale from all the locales available to the Java 2 platform and then outputs a date localized for that locale. To ensure that the characters in the date can be rendered correctly for a wide variety of character sets, the JSP page that generates the date sets the response encoding to UTF-8 by using the following directive:

<%@ page contentType="text/html; charset=UTF-8" %> 

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