Java Series #12: JSP

JavaServer Pages (JSP) is a server-side technology used to build dynamic web pages. Internally, every JSP is converted into a Servlet, which is why understanding JSP syntax and lifecycle is critical.

JSP Syntax Tags Explained

<%= %> — Expression Tag

Used to output data directly to the response.

<%= username %>

Equivalent to:

out.print(username);

<% %> — Scriptlet

Used to write Java logic inside JSP.

<%
int count = 10;
System.out.println(count);
%>

Avoid heavy logic here — it hurts readability and maintainability.

<%! %> — Declaration

Used to declare methods or variables at class level.

<%!
int add(int a, int b) {
    return a + b;
}
%>

These become part of the generated servlet class.

<%@ %> — Directive

Used for configuration and settings.

<%@ page contentType="text/html" %>
<%@ include file="header.jsp" %>

JSP vs Servlets

JSP focuses on presentation, Servlets handle request processing.

JSP Lifecycle Explained

Internally, JSP follows this lifecycle:

1. Translation
   JSP is converted into a Servlet (.java file)

2. Compilation
   Servlet is compiled into .class

3. Class Loading
   JVM loads the servlet class

4. Instantiation
   Servlet object is created

5. Initialization
   jspInit() is called once

6. Request Processing
   _jspService() handles every request

7. Destruction
   jspDestroy() is called before unloading

Key point: _jspService() is called for every request, others only once.

JSTL Core Tags

JSTL removes Java code from JSP and makes pages cleaner.

Variable Handling

<c:set var="name" value="John" scope="session" />
<c:remove var="name" />

Output

<c:out value="${user}" default="Guest" />

Loops

<c:forEach var="item" items="${list}">
    ${item}
</c:forEach>

Conditional Logic

<c:if test="${age > 18}">
    Adult
</c:if>

<c:choose>
  <c:when test="${role == 'admin'}">Admin</c:when>
  <c:otherwise>User</c:otherwise>
</c:choose>

URL & Redirect

<c:url value="/login.jsp" />
<c:redirect url="home.jsp" />

Exception Handling

<c:catch var="error">
   ${1 / 0}
</c:catch>

Expression Language (EL)

Used to access request, session, and application data.

${param.name}
${sessionScope.user}
${requestScope.data}

EL keeps JSP declarative and readable.

Session Tracking in Servlets

HTTP is stateless, so session tracking is required.

Mechanisms

• Cookies: Small key–value data stored in the user’s browser and sent with every request to help identify the user across multiple requests.

• Hidden Form Fields: Session data is stored inside invisible form inputs and sent back to the server only when the form is submitted.

• URL Rewriting: Session information (like a session ID) is appended to the URL when cookies are disabled, allowing the server to track the user.

• HttpSession: Server-side storage that maintains user-specific data across multiple requests using a unique session ID.

Cookies vs Sessions

HttpSession Interface

Create / Fetch Session

HttpSession session = request.getSession();

• true → create if not exists (default)

• false → return null if not exists

Store & Retrieve Data

session.setAttribute("user", userObj);
session.getAttribute("user");
session.removeAttribute("user");

Session Timeout

session.setMaxInactiveInterval(100);

(Time in seconds)

Destroy Session (Logout)

session.invalidate();

Key Takeaways

• JSP is not separate from Servlets — it compiles into one

• Avoid scriptlets, prefer JSTL + EL

• Sessions are server-side and safer than cookies

• JSP is best for views, Servlets for control logic

Thanks to gfg!