JSP and Servlets

October 3, 2020

JSP

Basic concept

JSP: similar to html page, with some Java code. It’s processed on the server, results of Java code included in HTML returned to browser

jsp expression<%= new java.util.Date() %>
- Obj, <%= new String(“hello”).toUpperCase() %>
- number, <%= 25*4 %>
- boolean, <%= 5>6 %>
jsp scriptlet <% some Java %> out.println() to print. Minimisze the use here.

jsp declaration <%! variable or method declaration%>

<% String lower(String data){
  return data.toLowerCase();
  }%>

call Java class from JSP

<%@ page import="com.hello" %>

include files

<jsp:include page="header.html"/>

form/radio/checkbox action:response.jsp, this is where we response the input form. Use ${param.firstName} in response page to output/match the input name in form.

Session object

created once for user’s browser session, unique for this user. Keep track of user’s actions.

add data to session object

List<String> items = new ArrayList<>();
session.SetAttribute("myTodoList", items);
...
List<String> myStuff = (List<String>) session.getAttribute("myTodoList);

commonly used: isNew, getID, invalidate, setMaxInactiveInterval(long mills)
cookies
1. form, submit name favoriteLanguage
2. response.jsp, String favL = request.getParameter("favoriteLanguage")
3. Cookie theCookie = new Cookie("myapp.ck", favL) theCookie.setMaxAge
4. response.addCookie(theCookie) Notice this step, send cookie to browser, with response.
5. homepage, Cookie[] cookies = resquest.getCookies();
6. for loop, tempCk.getName().equals("myapp.ck"), get tempCk.getValue()

JSTL, jsp standard tag library

Choose

<c:choose>, <c:when test=>,<c:otherwise>

<c:if test="${not tempStudent.goldCustomer}">

forEach/forTokens

pageContext.setAttribute("myStudents",data);
<c:forEach var="tempStu" items="${myStudnets}">
${ tempStu.firstName }
</c:forEach>

function

<c:set var="data" value="haha" />
Length of ${data}: ${fn: length(data)}

I18N/Formatting

Servlets

Basics

Java class that processed on the server
Java class generates HTML that is returned to browser
can read HTML form data, use cookies and sessions etc.
At a high level, simialr to JSPs

@WebServlet("/funny")
public class HelloWorldServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public HelloWorldServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

		// Step 1: set the content type
		response.setContentType("text/html");
		
		// Step 2: get the printwriter
		PrintWriter out = response.getWriter();
		
		// Step 3: generate HTML content
		out.println("<html><body>");
		
		out.println("<h2>Hello World</h2>");
		out.println("<hr>");
		out.println("Time on the server is: " + new java.util.Date());
		
		out.println("</body></html>");
	}   
	
	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

form action=“StudentServlet”, method=“GET”. doGet method then use request.getParameter(“firstName”) to get the form data.
in doGet, response.setContentType("text/html")
PrintWriter out = response.getWriter.getWriter()
out.println("<html><body>")

Get/Post difference

GET: url?field=1?… good for debug, bookmark/email. limitation on data length.
Post: won’t see in url. No limitations on data length. can send binary data.

Servlet with JSP

Servlet can call JSP using a request dispatcher

RequestDispatcher dispatcher = request.getRequestDispatcher("/view_student.jsp");
Forward to JSP
dispatcher.forward(request, response);