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# JSTL Core Tags
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The JSTL Core tag library provides fundamental JSP functionality including conditionals, loops, URL manipulation, variable management, and exception handling. These tags form the foundation of server-side logic in JSP pages.
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## Tag Library Declaration
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```jsp
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<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
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```
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## Capabilities
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### URL Generation and Manipulation
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Creates and manipulates URLs with automatic encoding and session handling, supporting both absolute and relative URL generation with parameter addition.
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```jsp { .api }
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<!-- Basic URL generation -->
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<c:url value="/path/to/resource" />
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<!-- URL with variable -->
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<c:url value="/path/to/resource" var="urlVar" />
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<!-- URL with parameters -->
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<c:url value="/servlet">
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    <c:param name="param1" value="value1" />
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    <c:param name="param2" value="value2" />
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</c:url>
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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/core" prefix="c"%>
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<!-- Create a URL with parameters -->
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<c:url value="/user/profile" var="profileUrl">
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    <c:param name="userId" value="${currentUser.id}" />
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    <c:param name="tab" value="settings" />
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</c:url>
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<!-- Use the generated URL -->
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<a href="${profileUrl}">User Profile</a>
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<!-- Direct inline usage -->
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<form action="<c:url value='/submit'/>">
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    <!-- form content -->
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</form>
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```
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### Parameter Management
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Adds parameters to parent URL tags with automatic encoding and proper URL formatting.
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```jsp { .api }
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<c:param name="parameterName" value="parameterValue" />
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<c:param name="parameterName" value="${dynamicValue}" />
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```
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### Exception Handling
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Catches and handles exceptions that occur within the tag body, providing graceful error handling for JSP operations.
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```jsp { .api }
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<c:catch var="exceptionVariable">
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    <!-- Code that might throw exceptions -->
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</c:catch>
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```
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**Usage Example:**
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```jsp
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<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
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<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>
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<c:catch var="parseError">
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    <fmt:parseNumber var="number" value="${userInput}" />
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</c:catch>
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<c:if test="${parseError != null}">
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    <div class="error">
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        Invalid number format: ${parseError.message}
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    </div>
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</c:if>
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<c:if test="${parseError == null}">
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    <div class="success">
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        Parsed number: ${number}
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    </div>
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</c:if>
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```
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### Conditional Processing
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Evaluates conditions and conditionally includes content based on boolean expressions using EL (Expression Language).
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```jsp { .api }
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<c:if test="${condition}">
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    <!-- Content to include if condition is true -->
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</c:if>
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<c:if test="${condition}" var="resultVariable">
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    <!-- Content with result stored in variable -->
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</c:if>
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```
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**Usage Example:**
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```jsp
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<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
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<!-- Simple condition -->
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<c:if test="${user.isAdmin}">
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    <div class="admin-panel">Admin Controls</div>
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</c:if>
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<!-- Complex condition with operators -->
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<c:if test="${user.age >= 18 && user.verified}">
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    <div class="adult-content">Adult Content Available</div>
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</c:if>
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<!-- Null checking -->
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<c:if test="${sessionScope.cart != null && not empty sessionScope.cart.items}">
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    <div class="cart-summary">Items in cart: ${fn:length(sessionScope.cart.items)}</div>
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</c:if>
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```
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### Variable Assignment
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Sets and manages variables in various JSP scopes (page, request, session, application).
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```jsp { .api }
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<c:set var="variableName" value="value" />
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<c:set var="variableName" value="${expression}" />
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<c:set var="variableName" value="value" scope="page|request|session|application" />
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<c:set var="variableName">
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    <!-- Body content as value -->
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</c:set>
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```
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### Output with Escaping
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Outputs values with automatic XML/HTML escaping to prevent XSS attacks.
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```jsp { .api }
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<c:out value="${expression}" />
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<c:out value="${expression}" default="defaultValue" />
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<c:out value="${expression}" escapeXml="true|false" />
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```
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## Error Handling
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JSTL core tags can throw the following types of exceptions:
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- **JspException**: General JSP processing errors
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- **NumberFormatException**: When used with formatting operations
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- **IllegalArgumentException**: Invalid parameter values
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- **ELException**: Expression Language evaluation errors
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Common error scenarios:
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- Invalid EL expressions in test conditions
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- Null pointer exceptions when accessing undefined variables
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- Type conversion errors in parameter values
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- Invalid scope specifications in variable assignments