Internationalization Tag Library

Chapter 22 covers how to design web applications so that they conform to the language and formatting conventions of client locales. This section describes tags that support the internationalization of JSP pages.

JSTL defines tags for setting the locale for a page, creating locale-sensitive messages, and formatting and parsing data elements such as numbers, currencies, dates, and times in a locale-sensitive or customized manner. Table 14-7 lists the tags.

Table 14-7 Internationalization Tags 
Area
Function
Tags
Prefix
I18n
Setting Locale
setLocale
requestEncoding
fmt
Messaging
bundle
message
  param
setBundle
Number and Date Formatting
formatNumber
formatDate
parseDate
parseNumber
setTimeZone
timeZone

JSTL i18n tags use a localization context to localize their data. A localization context contains a locale and a resource bundle instance. To specify the localization context at deployment time, you define the context parameter javax.servlet.jsp.jstl.fmt.localizationContext, whose value can be a javax.servlet.jsp.jstl.fmt.LocalizationContext or a String. A String context parameter is interpreted as a resource bundle base name. For the Duke's Bookstore application, the context parameter is the String messages.BookstoreMessages. When a request is received, JSTL automatically sets the locale based on the value retrieved from the request header and chooses the correct resource bundle using the base name specified in the context parameter.

Setting the Locale

The setLocale tag is used to override the client-specified locale for a page. The requestEncoding tag is used to set the request's character encoding, in order to be able to correctly decode request parameter values whose encoding is different from ISO-8859-1.

Messaging Tags

By default, the capability to sense the browser locale setting is enabled in JSTL. This means that the client determines (via its browser setting) which locale to use, and allows page authors to cater to the language preferences of their clients.

The setBundle and bundle Tags

You can set the resource bundle at runtime with the JSTL fmt:setBundle and fmt:bundle tags. fmt:setBundle is used to set the localization context in a variable or configuration variable for a specified scope. fmt:bundle is used to set the resource bundle for a given tag body.

The message Tag

The message tag is used to output localized strings. The following tag from bookcatalog.jsp is used to output a string inviting customers to choose a book from the catalog. 

<h3><fmt:message key="Choose"/></h3>

The param subtag provides a single argument (for parametric replacement) to the compound message or pattern in its parent message tag. One param tag must be specified for each variable in the compound message or pattern. Parametric replacement takes place in the order of the param tags.

Formatting Tags

JSTL provides a set of tags for parsing and formatting locale-sensitive numbers and dates.

The formatNumber tag is used to output localized numbers. The following tag from bookshowcart.jsp is used to display a localized price for a book. 

<fmt:formatNumber value="${book.price}" type="currency"/>

Note that because the price is maintained in the database in dollars, the localization is somewhat simplistic, because the formatNumber tag is unaware of exchange rates. The tag formats currencies but does not convert them.

Analogous tags for formatting dates (formatDate) and for parsing numbers and dates (parseNumber, parseDate) are also available. The timeZone tag establishes the time zone (specified via the value attribute) to be used by any nested formatDate tags.

In bookreceipt.jsp, a "pretend" ship date is created and then formatted with the formatDate tag: 

<jsp:useBean id="now" class="java.util.Date" />
<jsp:setProperty name="now" property="time" 
  value="${now.time + 432000000}" />
<fmt:message key="ShipDate"/> 
<fmt:formatDate value="${now}" type="date"
  dateStyle="full"/>.