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EL plus Scope confusion

Greenhorn

Posts: 4

posted 14 years ago

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Hello everyone, I am preparing for exam and would ask you some help in clarifying a few issues. But, before anything, some code is inbound first - it's a really simple login example I've constructed.
Let's assume correct username and pass combination is entered; questions are at the bottom.

LoginPage.jsp (entrypoint - view)

LoginServlet.java (controller)

UserDAO.java ("service" class)

//snipped imports and such

public class UserDAO {

static Connection currConn = null;
	static ResultSet rs = null;

public static UserBean login(UserBean userBean) {
		Statement stmt = null;

String username = userBean.getUsername();
		String password = userBean.getPassword();

String searchQuery = "SELECT * FROM users x WHERE x.email = '" + username + "' AND x.password = '" + password + "'";

System.out.println("Your user name is " + username);
		System.out.println("Your password is " + password);
		System.out.println("Query: " + searchQuery);

try {
			currConn = ConnectionManager.getConnection();
			stmt = currConn.createStatement();
			rs = stmt.executeQuery(searchQuery);
			boolean more = rs.next();

if (!more) {
				System.out.println("Sorry, you are not a registered user! Please sign up first");
				userBean.setValid(false);
			} else {
				String firstName = rs.getString("FIRST_NAME");
				String lastName = rs.getString("LAST_NAME");
				
				System.out.println("Welcome " + firstName);
				
				userBean.setFirstName(firstName);
				userBean.setLastName(lastName);
				userBean.setValid(true);
			}
	
			rs.close();
			stmt.close();
			currConn.close();
			
		} catch (Exception ex) {
			System.out.println("Log In failed: An Exception has occurred! " + ex);
			ex.printStackTrace();
		}

return userBean;
	}
}

UserBean.java (model, a classic POJO/bean used as a DTO)

userLogged.jsp (exitpoint - view) --never mind the div elements, they're there to center the view, but I won't spam you needlessly with CSS, it's irrelevant anyhow--

<%@ page language="java" contentType="text/html; charset=windows-1250" pageEncoding="windows-1250"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
	<link rel="stylesheet" type="text/css" href="mystyle.css" />
	<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
	<title>Successful login!</title>
</head>
<body>
	<div id="page">
		<div id="content_container">
			<div id="content">
			<jsp:useBean id="currentSessionUser" class="examplePackage.UserBean" scope="application">
				Welcome, <jsp:getProperty name="currentSessionUser" property="username"/> <br>
				********<br>
				Test 0 -> ${param.name}<br>
				Test 1 -> ${paramValues.name[0]}<br>
				Test 2 -> ${paramValues[name[0]]}<br>
				Test 3 -> ${param["name"]}<br>
				Test 4 -> ${param.username}<br>
				Test 5 -> ${param["username"]}<br>
				Test 6 -> ${sessionScope.currentSessionUser.username}<br>		
				*******<br>
				Test 7 -> ${header.host}<br>
				Test 8 -> ${header["host"]}<br>
				Test 9 -> ${pageContext.request.method}<br>
			</jsp:useBean>
			</div>
		</div>
	</div> 
</body>
</html>

Webpage output is as follows (c/p directly from FireFox):
Welcome, USER_X
********
Test 0 ->
Test 1 ->
Test 2 ->
Test 3 ->
Test 4 ->
Test 5 ->
Test 6 -> USER_X
*******
Test 7 -> localhost:8080
Test 8 -> localhost:8080
Test 9 -> GET

1) My first question is regarding the scope - which scope is actually applicable?? If you checkout userLogged.jsp, lines 13 and 22, you'll see my dilemma - if I use any other scope than "application" in L13, line 14 returns null value. On the other hand, should I use applicationScope on L22, it returns null (as it damn well should, since I am setting a SESSION attribute, not a context attribute!).
So, the question is - why should I use application scope on L13 anyway?? I'd expect nothing other than session scope, as can be seen from my controller.

2) The other question is regarding EL - why can't I fetch request parameters in Tests 0-5? Other stuff works fine (as can be seen from output), but I can't understand how to make these request parameters printed out as I inteded (via request EL implicit objects).

Thank you very much in advance

dileep keely

Ranch Hand

Posts: 112

posted 14 years ago

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The below are my findings:
To answer the second part
2) response.sendRedirect("userLogged.jsp");
and
${param.name} in userLogged.jsp

Here the response is already comitted and you are trying to fetch the parameters(request).

Description:
request,response objects are thread safe and synchronous in nature(Apache tomcat6.0).Once response is
given to the client we cann't retrive.

(go through sendRedirect/forward)

EL:Elmination to java syntaxs in JSP

It checks the syntax vaildity not the existence of variable.
-->Translation (which involves compilation):
The abstract class javax.servlet.jsp.el.ExpressionEvaluator has a method evaluate(java.lang.String, java.lang.Class,
javax.servlet.jsp.el.VariableResolver, javax.servlet.jsp.el.FunctionMapper) checks for syntax error.
--->RunTime:
At runtime the String representing the expression is sent to a method called resolveVariable()
which uses javax.servlet.jsp.JspContext 's public abstract java.lang.Object findAttribute(java.lang.String);

the returned object is sent to the outputstream via an out.print();
Note:Even if the variable doesn't exists sensible defaults are provided.

Adding on this finAttribute checks through the scopes in the following order.
page,request,session,application.

To answer the first part about scopes:
1)session.setAttribute("currentSessionUser", user);
and
<jsp:useBean id="currentSessionUser" class="examplePackage.UserBean" scope="application">

${param["username"]}-->Map of ServletRequest paramter.(You have already comitted response check forward/sendredirect you will get solution.)

${sessionScope.currentSessionUser.username}-->you are targeting the session scope,Which you have already set so is the result.

Conclusion:
-->sendRedirect will evades

up the request objects.

Please let me know if I am missing something.

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