JSP basics

JSP Scripting Elements:

  • JSP expression: Object(new String("adf")), Math expression(3 * 4), boolean(4 < 5)
  • JSP Scriptlet: insert 1 to many lines of java code; use out.println(...) to include content in the page. this would return the code to HTML page. So for example: "<br/> I really Iuv2code: ", do not forget the <br/> HTML tags.
  • JSP Declaration: Declare a method in the JSP page. And then you can use this method in other place in JSP page.

best practice:

Both for JSP Scriptlet and JSP Declaration, you should minimize the number of lines in those modules. Refactor this into a separate Java class, and make use of MVC.

how to implement the above best practice?

  • call Java Class from JSP

    • create java package in the java resources / src, establish a java file, then in the JSP file, we can invoke the method:

      fun: <%= com.luv2code.jsp.FunUtils.makeItLower("FUN FUN FUN") %> 
or
<%@ page import="com.luv2code.jsp.FunUtils, java.util.ArrayList" %>
fun: <%= FunUtils.makeItLower("FUN FUN FUN") %>

JSP built-in objects

  • request / response / out/ session / application 直接用
  • eg. <%= request.getHeader("User-Agent") %> -> information about the browser
  • eg. <%= request.getLocale() %> -> information about the language

how to include files in the JSP?

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

  • <jsp:include page="my-footer.jsp"/>

Reading HTML forms with JSP (form tag)

  • how to read basic HTML forms with JSP

first way:

HTML file:

First name: <input type="text" name="firstName" /> 
 Last name: <input type="text" name="lastName" />

JSP file:

The student is confirmed:
 <%= request.getParameter(“firstName”) %> <%= request.getParameter(“lastName”) %>

alternative way: (preferred one)

The student is confirmed: ${param.firstName} ${param.lastName}
  • Dropdown list (select tag) 下拉单选
<select name="country">  <!-- select tag for dropdown list -->
    <option>Brazil</option>
    <option>France</option>
    <option>Germany</option>
    <option>India</option>
    <option>Turkey</option>
    <option>United Kingdom</option>
    <option>United States of America</option>
</select>

The student's country: ${param.country}
  • radio button (input type="radio" ... ) 单选
HTML file:
<input type="radio" name="favoriteLanguage" value="Java"> Java
...

JSP file:
The student's favorite programming language: ${param.favoriteLanguage}   // param.name
  • check boxes (input type="checkbox" ... ) 多选
HTML file:
<input type="checkbox" name="favoriteLanguage" value="Java"> Java
...

JSP file:  // pay attention: different from previous one
String[] langs = request.getParameterValues("favoriteLanguage");
...

note: <!-- ...--> is the comment of the HTML

<!-- display list of "favoriteLanguage" -->
<ul>
    <%
        String[] langs = request.getParameterValues("favoriteLanguage");
        if (langs != null) {   // pay attention: should check if customer didn't check any option.
            for (String tempLang : langs) {
                out.println("<li>" + tempLang + "</li>");
            }
        }
    %>
</ul>

note: "ul" is short for "unordered list" "ol" is short for "ordered list", they are both used together with li tag to list some things.

State Management with JSP

difference between sessions and cookies

Most modern sites use the second approach, i.e. saving the identifier in a Cookie instead of passing it in a URL (which poses a security risk). Translating that to Web Servers: The server will store the pertinent information in the session object, and create a session ID which it will send back to the client in a cookie. When the client sends back the cookie, the server can simply look up the session object using the ID. So, if you delete the cookie, the session will be lost.

  • session tracking (server-side files)
List<String> items = new ArrayList<>();
session.setAttribute(“myToDoList”, items);

List<String> myStuff = (List<String>)session.getAttribute(“myToDoList”);

getId(): return String // return the id of the session

invalidate(): void // clear all the session information

<!-- step 1: create html form -->
<form action="todo-demo.jsp">     // point to the file itself
    Add new item: <input type="text" name="theItem" />

    <input type="submit" value="Submit" />
</form>


<!-- step 2: add new item to  "to do" list -->
<%
    // get the to do items from the session
    List<String> items = (List<String>) session.getAttribute("myToDoList");

    // if the to do items doesn't exist, then create a new one
    if (items == null) {
        items = new ArrayList<String>();
        session.setAttribute("myToDoList", items);
    }

    // see if there is form data to add
    String theItem = request.getParameter("theItem");
    if (theItem != null && theItem.length() != 0) {
        items.add(theItem);
    }
%>


<!-- step 3: display all "to do" item from session -->
<hr>
<b>To List Items:</b> <br/>

<ol>
<%
    for (String temp : items) {
        out.println("<li>" + temp + "</li>");
    }
%>
</ol>

PageContext has a set of attributes that are different from the Session object.

The attributes set on PageContext are only available for a given page. The attributes are not available to other pages or servlets in the application.

Session attributes are created per each user's session. The session attributes are unique to a given session id. Session attributes are available to other pages and servlets in the application for a given session id.

  • Cookie (client-side files)

Cookie class defined in package:javax.servlet.http

Cookie is simply a key/value pair and Browser will only send cookies that match the server’s domain name.

create cookie:

<html>
<head>
    <title>Confirmation</title>
</head>

<%
    // read form data
    String favLang = request.getParameter("favoriteLanguage");

    // create the cookie
    Cookie theCookie = new Cookie("myApp.favoriteLanguage", favLang);

    // set the life span   total number of seconds
    theCookie.setMaxAge(60*60*24*365);  //  for one year

    // send cookie to the browse
    response.addCookie(theCookie);    

%>

<body>
    Thanks! we set your favorite language to: ${param.favoriteLanguage}

    <br/>
    <br/>

    <a href="cookies-homepage.jsp"></a>
</body>
</html>

read cookie:

<html>
<body>
<h3>Training Portal</h3>

<!-- read the favorite programming language cookie -->
<% 
    // the default ... if there are no cookies
    String favLang = "Java";

    // get the cookies from the browser request
    Cookie[] theCookies = request.getCookies();

    // find our favorite language cookie
    if (theCookies != null) {
        for (Cookie t : theCookies) {
            if ("myApp.favoriteLanguage".equals(t.getName())) {
                favLang = t.getValue();
                break;
            }
        }
    }
%>

<!-- now show a personalized page .. use -->

<h4>New Books for <%= favLang %></h4>
<ul>
    <li>balasdasdf</li>
    <li>balasdasdf</li>
    <li>balasdasdf</li>
</ul>

<hr>   // pay attention: a horitontal line
<a href="cookies-personalize-form.html">Personalize this page</a>

</body>
</html>

think further: How to deal with the situation where cookie values exist whitespace?

to support cookie version 0

Basically, with Version 0 cookies, values should not contain white space, brackets, parentheses, equals signs, commas, double quotes, slashes, question marks, at signs, colons, and semicolons. Empty values may not behave the same way on all browsers. To support MSIE as well, you really need to URL-encode and URL-decode the cookie value yourself if it contains possibly characters which are invalid for version 0.

JSP Tags --> JSP Standard Tag Library (JSTL)

  • Oracle create a specification for standard tags

    • Core
    • Message Formatting I18N
    • Function
    • XML

  • JSTL setup and a simple test

    Note: This answer is for downloading the jars directly from maven without any scripts 
[That is how Google directed me here]

Assuming mvn dependency is like this:

<dependency>
    <groupId>com.zaxxer</groupId>
    <artifactId>HikariCP</artifactId>
    <version>2.4.7</version>
</dependency>

Goto http://search.maven.org and search for       g:"com.zaxxer" AND a:"HikariCP" AND v:"2.4.7" 
(simply searching for HikariCP also works. You may need to select the appropriate GroupId 
and Version from the results)

In the Search Results -> Download column, you should see:
jar  javadoc.jar  sources.jar available for direct download

  • JSTL core

    • looping with forEach
    <%
    // just create some sample data ... normally provided by MVC
    String[] cities = {"Mumbai", "Singapore", "Philadelphia"}; pageContext.setAttribute("myCities", cities);
%>
<body>
    <c:forEach var="tempCity" items="${myCities}"> ${tempCity} <br/>
    </c:forEach> 
</body>

    ：building a HTML table

    <table border="1">

<tr>
    <th>First Name</th>
    <th>Last Name</th>
    <th>Gold Customer</th>
</tr>

<c:forEach var="tempStudent" items="${myStudents}">
    <tr>
        <td>${tempStudent.firstName}</td>
        <td>${tempStudent.lastName}</td>
        <td> ${tempStudent.goldCustomer}</td>
    </tr>
</c:forEach>

</table>
    • note that, An HTML table is defined with the<table>tag. Each table row is defined with the<tr>tag. A table header is defined with the<th>tag. By default, table headings are bold and centered. A table data/cell is defined with the<td>tag

other tag: JSTL if tag (no else if)

simple if expression, but no else if ....

<td>
    <c:if test="${tempStudent.goldCustomer}">
        Special Discount
    </c:if>
    <c:if test="${not tempStudent.goldCustomer}">
        -
    </c:if>
</td>

other tag: JSTL choose tag similar to the switch expression. and

<c:choose>
    <c:when test="${tempStudent.goldCustomer}"> 
        Special Discount
    </c:when>
    // can add other when... 
    <c:otherwise> 
    -
    </c:otherwise>
</c:choose>

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