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# JSTL Formatting
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The JSTL Formatting tag library provides internationalization and localization capabilities, including number formatting, date formatting, and message localization for building multilingual web applications.
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## Tag Library Declaration
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```jsp
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<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>
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```
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## Capabilities
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### Number Parsing
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Parses string representations of numbers into numeric values with locale-aware formatting and error handling.
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```jsp { .api }
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<fmt:parseNumber var="variableName" value="${stringValue}" />
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<fmt:parseNumber var="variableName" value="${stringValue}" type="number|currency|percent" />
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<fmt:parseNumber var="variableName" value="${stringValue}" pattern="customPattern" />
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<fmt:parseNumber var="variableName" value="${stringValue}" parseLocale="${locale}" />
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```
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**Usage Example:**
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```jsp
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<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>
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<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
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<!-- Parse a currency string -->
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<fmt:parseNumber var="price" value="$1,234.56" type="currency" />
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<c:out value="Parsed price: ${price}" />
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<!-- Parse with custom pattern -->
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<fmt:parseNumber var="percentage" value="85.5%" pattern="#.#%" />
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<c:out value="Parsed percentage: ${percentage}" />
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<!-- Parse with error handling -->
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<c:catch var="parseError">
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    <fmt:parseNumber var="number" value="${userInput}" />
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</c:catch>
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<c:if test="${parseError != null}">
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    <div class="error">Failed to parse number: ${parseError.message}</div>
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</c:if>
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```
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### Number Formatting
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Formats numeric values as strings with locale-appropriate formatting, currency symbols, and percentage representation.
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```jsp { .api }
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<fmt:formatNumber value="${numericValue}" />
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<fmt:formatNumber value="${numericValue}" type="number|currency|percent" />
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<fmt:formatNumber value="${numericValue}" pattern="customPattern" />
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<fmt:formatNumber value="${numericValue}" maxFractionDigits="digits" />
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<fmt:formatNumber value="${numericValue}" minFractionDigits="digits" />
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<fmt:formatNumber value="${numericValue}" var="variableName" />
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```
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**Usage Example:**
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```jsp
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<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>
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<!-- Format as currency -->
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<fmt:formatNumber value="${product.price}" type="currency" />
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<!-- Format as percentage -->
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<fmt:formatNumber value="${successRate}" type="percent" maxFractionDigits="1" />
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<!-- Custom pattern -->
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<fmt:formatNumber value="${distance}" pattern="#,##0.00 km" />
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```
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### Date Parsing
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Parses string representations of dates into Date objects with locale and pattern support.
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```jsp { .api }
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<fmt:parseDate var="variableName" value="${dateString}" />
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<fmt:parseDate var="variableName" value="${dateString}" type="date|time|both" />
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<fmt:parseDate var="variableName" value="${dateString}" pattern="customPattern" />
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<fmt:parseDate var="variableName" value="${dateString}" parseLocale="${locale}" />
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```
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### Date Formatting
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Formats Date objects as strings with locale-appropriate formatting and customizable patterns.
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```jsp { .api }
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<fmt:formatDate value="${dateValue}" />
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<fmt:formatDate value="${dateValue}" type="date|time|both" />
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<fmt:formatDate value="${dateValue}" pattern="customPattern" />
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<fmt:formatDate value="${dateValue}" dateStyle="default|short|medium|long|full" />
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<fmt:formatDate value="${dateValue}" timeStyle="default|short|medium|long|full" />
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<fmt:formatDate value="${dateValue}" var="variableName" />
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```
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**Usage Example:**
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```jsp
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<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>
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<!-- Format current date -->
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<fmt:formatDate value="${now}" type="date" dateStyle="long" />
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<!-- Format with custom pattern -->
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<fmt:formatDate value="${event.startTime}" pattern="MMM dd, yyyy 'at' HH:mm" />
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<!-- Format time only -->
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<fmt:formatDate value="${appointment.time}" type="time" timeStyle="short" />
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```
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### Message Localization
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Retrieves localized messages from resource bundles based on the current locale.
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```jsp { .api }
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<fmt:message key="messageKey" />
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<fmt:message key="messageKey" var="variableName" />
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<fmt:message key="messageKey" bundle="${bundleVar}" />
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<fmt:message key="messageKey">
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    <fmt:param value="${parameter1}" />
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    <fmt:param value="${parameter2}" />
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</fmt:message>
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```
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### Locale Management
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Sets and manages the current locale for formatting operations.
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```jsp { .api }
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<fmt:setLocale value="${localeString}" />
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<fmt:setLocale value="${localeString}" scope="page|request|session|application" />
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```
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### Resource Bundle Management
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Loads and manages resource bundles for message localization.
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```jsp { .api }
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<fmt:bundle basename="bundleName">
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    <!-- Scoped bundle usage -->
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</fmt:bundle>
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<fmt:setBundle basename="bundleName" var="bundleVar" />
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```
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**Usage Example:**
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```jsp
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<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>
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<!-- Set locale -->
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<fmt:setLocale value="${user.preferredLocale}" />
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<!-- Load resource bundle -->
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<fmt:setBundle basename="messages" var="msgs" />
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<!-- Display localized messages -->
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<fmt:message key="welcome.message" bundle="${msgs}">
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    <fmt:param value="${user.name}" />
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</fmt:message>
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<!-- Conditional locale formatting -->
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<c:choose>
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    <c:when test="${user.locale == 'en_US'}">
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        <fmt:formatDate value="${order.date}" pattern="MM/dd/yyyy" />
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    </c:when>
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    <c:otherwise>
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        <fmt:formatDate value="${order.date}" pattern="dd/MM/yyyy" />
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    </c:otherwise>
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</c:choose>
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```
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## Error Handling
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JSTL formatting tags can throw the following exceptions:
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- **ParseException**: When parsing fails due to invalid format
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- **IllegalArgumentException**: Invalid pattern or parameter values
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- **MissingResourceException**: When resource bundles or keys are not found
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- **JspException**: General JSP processing errors
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Common error scenarios:
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- Invalid date/number formats in parsing operations
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- Missing or malformed resource bundle files
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- Incorrect locale specifications
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- Invalid pattern strings for custom formatting