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Character Sets and Encodings

The following sections describe character sets and character encodings.

Character Sets

Acharacter set is a set of textual and graphic symbols, each of whichis mapped to a set of nonnegative integers.

The first character set used in computing was US-ASCII. It is limited inthat it can represent only American English. US-ASCII contains uppercase and lowercase Latinalphabets, numerals, punctuation, a set of control codes, and a few miscellaneous symbols.

Unicode defines a standardized, universal character set that can be extended to accommodateadditions. When the Java program source file encoding doesn’t support Unicode, you canrepresent Unicode characters as escape sequences by using the notation\uXXXX, whereXXXX isthe character’s 16-bit representation in hexadecimal. For example, the Spanish version of theDuke’s Bookstore message file uses Unicode for non-ASCII characters:

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

Character Encoding

Acharacter encoding maps a character set to units of a specific widthand defines byte serialization and ordering rules. Many character sets have more than oneencoding. For example, Java programs can represent Japanese character sets using theEUC-JPorShift-JIS encodings, among others. Each encoding has rules for representing and serializinga character set.

The ISO 8859 series defines 13 character encodings that can represent texts indozens of languages. Each ISO 8859 character encoding can have up to 256characters. 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 encodes16-bit Unicode characters as one to four bytes. A byte in UTF-8 isequivalent to 7-bit ASCII if its high-order bit is zero; otherwise, the charactercomprises a variable number of bytes.

UTF-8 is compatible with the majority of existing web content and provides accessto the Unicode character set. Current versions of browsers and email clients supportUTF-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 AppendixFigure 37-6 for more information on character encodings in the Java 2platform.

Web components usually usePrintWriter to produce responses;PrintWriter automatically encodes using ISO-8859-1. Servletscan also output binary data usingOutputStream classes, which perform no encoding. Anapplication that uses a character set that cannot use the default encoding mustexplicitly set a different encoding.

For web components, three encodings must be considered:

Request
Page (JSP pages)
Response

Request Encoding

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

If the client hasn’t set character encoding and the request data is encodedwith a different encoding from the default, the data won’t be interpreted correctly. Toremedy this situation, you can use theServletRequest.setCharacterEncoding(String enc) method to override thecharacter encoding supplied by the container. To control the request encoding from JSPpages, you can use the JSTLfmt:requestEncoding tag. You must call the method ortag before parsing any request parameters or reading any input from the request.Calling the method or tag once data has been read will notaffect the encoding.

Page Encoding

For JSP pages, thepage encoding is the character encoding in which the fileis encoded.

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

The page encoding value of a JSP property group (seeSetting Properties for Groups of JSP Pages) whose URL pattern matches the page.
ThepageEncoding attribute of thepage directive of the page. It is a translation-time error to name different encodings in thepageEncoding attribute of the page directive of a JSP page and in a JSP property group.
TheCHARSET value of thecontentType attribute of thepage directive.

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

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

ThepageEncoding andcontentType attributes determine the page character encoding of only thefile that physically contains thepage directive. A web container raises a translation-time errorif an unsupported page encoding is specified.

Response Encoding

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

TheCHARSET value of thecontentType attribute of thepage directive
The encoding specified by thepageEncoding attribute of thepage 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 defaultresponse encoding.

Thejavax.servlet.ServletResponse.setCharacterEncoding,javax.servlet.ServletResponse.setContentType, andjavax.servlet.ServletResponse.setLocale methods can be called repeatedly to change the characterencoding. Calls made after the servlet response’sgetWriter method has been called or afterthe response is committed have no effect on the character encoding. Data issent to the response stream on buffer flushes (for buffered pages) or onencountering the first content on unbuffered pages.

Calls tosetContentType set the character encoding only if the given content typestring provides a value for thecharset attribute. Calls tosetLocale set thecharacter encoding only if neithersetCharacterEncoding norsetContentType has set the character encodingbefore. To control the response encoding from JSP pages, you can use theJSTLfmt.setLocale tag.

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

Select the WAR.
Click the Advanced Settings button.
In the Locale Character Encoding table, Click the Add button.
Enterja in the Extension column.
EnterShift_JIS in the Character Encoding column.

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

The first application inChapter 5, JavaServer Pages Technology allows a user to choose an Englishstring representation of a locale from all the locales available to the Java2 platform and then outputs a date localized for that locale. To ensurethat the characters in the date can be rendered correctly for a widevariety of character sets, the JSP page that generates the date sets theresponse encoding to UTF-8 by using the following directive:

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