EL Expresions in JSP

Basic Syntax and Usage

EL (Expression Language) provides a simplified way to access data in JSP pages with out scriptlets. The syntax is ${expression}.

<% request.setAttribute("greeting","Hello World"); %>
Traditional JSP: <%= request.getAttribute("greeting") %>
EL Expression: ${greeting}

Accessing Data Structures

EL can navigate through objects, lists, and maps:

public class User {
    private String username = "testUser";
    private Address address = new Address();
    // getters/setters
}

${user.username}
${user.address.city}
${userList[0].username}
${userMap["key"].username}

Servlet Filters

Filter Implementation

Filters intercept requests/responses for preprocessing and postprocessing:

public class EncodingFilter implements Filter {
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) 
        throws IOException, ServletException {
        req.setCharacterEncoding("UTF-8");
        resp.setContentType("text/html;charset=UTF-8");
        chain.doFilter(req, resp);
    }
}

Filter Configuration

Filters are configured in web.xml:

<filter>
    <filter-name>EncodingFilter</filter-name>
    <filter-class>com.example.EncodingFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>EncodingFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

Servlet Listeners

Context Listener Example

Listeners respond to application events:

public class AppListener implements ServletContextListener {
    public void contextInitialized(ServletContextEvent sce) {
        // Application startup logic
    }
    
    public void contextDestroyed(ServletContextEvent sce) {
        // Application shutdown logic
    }
}

Listener Configuration

<listener>
    <listener-class>com.example.AppListener</listener-class>
</listener>

JSTL Tags

Conditional Logic with JSTL

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<c:if test="${user.role == 'admin'}">
    Admin controls
</c:if>

<c:choose>
    <c:when test="${score >= 90}">A</c:when>
    <c:when test="${score >= 80}">B</c:when>
    <c:otherwise>C</c:otherwise>
</c:choose>

Looping with JSTL

<c:forEach items="${userList}" var="user" varStatus="status">
    ${status.index}. ${user.username}
</c:forEach>