Servlets: Servlet / Web Browser Communication I Ethan Cerami New York University

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Road Map      

Overview of Browser/Servlet Communication Reading Form Data from Servlets Example 1: Reading three parameters Example 2: Reading all parameters Case Study: Resume Posting Service Security: Filtering User Input

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Note Change in Syllabus Core Servlets, Chapter 4 (skip sections 4.7 - 4.8)  Chapter 5 (skip sections 5.4 and 5.6). 

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Overview of Browser/Servlet Communication

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Overview 

This lecture is the first in two lectures that discuss the interaction between web browsers and servlets. Request Web Browser

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Response

Web Server

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Client Request Data 

When a user submits a browser request to a web server, it sends two categories of data: 

Form Data: Data that the user explicitly typed into an HTML form. 

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HTTP Request Header Data: Data that is automatically appended to the HTTP Request from the client. 

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For example: registration information.

For example: cookies, browser type, browser IP address.

This lecture examines Form Data; the next lecture examines HTTP Data.

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Form Data

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Form Data  

 

Based on our understanding of HTML, we now know how to create user forms. We also know how to gather user data via all the form controls: text, password, select, checkbox, radio buttons, etc. Now, the question is: if I submit form data to a servlet, how do I extract this form data? Figuring this out forms the basis of creating interactive web applications that respond to user requests.

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Reading Form Data from Servlets 

The HttpServletRequest object contains three main methods for extracting form data:   

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getParameter(): used to retrieve a single form parameter. getParameterValues(): used to retrieve a list of form values, e.g. a list of selected checkboxes. getParameterNames(): used to retrieve a full list of all parameter names submitted by the user.

We will examine each of these and then explore several examples.

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Reading Form Data All these methods work the same way regardless of whether the browser uses HTTP GET or HTTP POST.  Remember that form elements are case sensitive. Therefore, “userName” is not the same as “username.” 

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getParameter() Method Used to retrieve a single form parameter.  Possible return values: 

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String: corresponds to the form parameter. Empty String: parameter exists, but has no value. null: parameter does not exist.

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getParameterValues() Method  

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Used to retrieve multiple form parameters with the same name. For example, a series of checkboxes all have the same name, and you want to determine which ones have been selected. Returns an Array of Strings. 

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An array with a single empty string indicates that the form parameter exists, but has no values. null: indicates that the parameter does not exist.

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getParameterNames() method Returns an Enumeration object.  By cycling through the enumeration object, you can obtain the names of all parameters submitted to the servlet.  Note that the Servlet API does not specify the order in which parameter names appear. 

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Example 1: Reading three explicit parameters

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Example 1   



Our first example consists of one HTML page, and one servlet. The HTML page contains three form parameters: param1, param2, and param3. The Servlet extracts these specific parameters and echoes them back to the browser. Before we examine the code, let’s try it out…

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HTML> HEAD> <TITLE>Collecting Three Parameters HEAD> BODY BGCOLOR="#FDF5E6"> H1 ALIGN="CENTER">Collecting Three Parameters

ORM ACTION="/servlet/coreservlets.ThreeParams"> irst Parameter:

FORM>

BODY> HTML> 10/17/08

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package coreservlets; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; /** Simple servlet that reads three parameters from the * form data. */ public class ThreeParams extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); String title = "Reading Three Request Continued…. Parameters"; 10/17/08 Servlet / Web Browser Communication I 17

out.println(ServletUtilities.headWithTitle(title) + "\n" + "

" + title + "

\n" + "

\n" + "
• param1: " + request.getParameter("param1") + "\n" + "
• param2: " + request.getParameter("param2") + "\n" + "
• param3: " + request.getParameter("param3") + "\n" + "

\n" + ""); } }

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Example 2: Reading all Parameters

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Example 2 Example 1 will only read explicit parameters.  Now, let’s look at a Servlet that echoes back all the form parameters you send it. 

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Example 2   

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The Example works by first calling getParameterNames(). By cycling through the returned Enumeration, the servlet can access all form names. For each form name, we call getParameterValues() to extract the form values. By cycling through the returned array of strings, we then print out all the associated values.

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package coreservlets; import import import import

java.io.*; javax.servlet.*; javax.servlet.http.*; java.util.*;

public class ShowParameters extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); Output a PrintWriter out = response.getWriter(); simple HTML table for String title = "Reading All Request Parameters"; out.println(ServletUtilities.headWithTitle(title) + displaying the "\n" form + "

" + title parameters. + "

\n" + "\n" + Continued…. "\n" +

Enumeration paramNames = request.getParameterNames(); while(paramNames.hasMoreElements()) { String paramName = (String)paramNames.nextElement(); out.print("

" + paramName + "\n

"); String[] paramValues = 1. First, use request.getParameterValues(paramName); getParameterNa if (paramValues.length == 1) { mes() to String paramValue = paramValues[0]; retrieve an if (paramValue.length() == 0) Enumeration of out.println("No Value"); all form else parameters. out.println(paramValue); 2. Then, iterate } else { through each out.println(" "); element within for(int i=0; i<paramValues.length; i++) { the out.println(" " + paramValues[i]); } Enumeration. Continued…. out.println("

\n"); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } }

doPost calls doGet(). Therefore the servlet will work just as well for HTTP POSTs or GETs.

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Case Study: Resume Posting Service

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Resume Posting Service 

Our next servlet receives a series of parameters:  

Name, title, email address, programming languages. Font, font size, etc.

Based on these parameters, the user is able to post his/her resume online.  Let’s first try it out… 

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Cascading Style Sheets The Resume servlet utilizes Cascading Style Sheets (CSS).  We have not covered CSS but, we will cover the very basics right now.  Let’s begin with a brief description of CSS. 

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CSS Defined CSS: a simple mechanism for adding style (e.g. fonts, colors, spacing) to Web documents.  Two Step process for using CSS: 

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Step 1: Create your “styles” Step 2: Apply your styles to your HTML document.

Let’s look at an example…

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TML> ODY> TYLE TYPE="text/css"> ADING1 { color: blue; font-size: 64px;

ADING2 { color: gray; font-size: 22px;

First, you create your styles Within a <STYLE> tag.

Then, you apply your styles By using the SPAN tag.

STYLE> PAN CLASS="HEADING1">Resume Posting Service > PAN CLASS="HEADING2">Provided by hotcomputerjobs.com HTML>

Defining Styles Each Style has a name, and a set of properties.  For example, the heading1 tag is set to blue, 64 pixels big: 

.HEADING1 { color: blue; font-size: 64px; }



Lots of properties exist: color, font-size, text-align, font-family, etc.

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Applying Styles Once you have created your styles, you apply a style to your text via the SPAN tag.  For example, to apply the heading1 style: 

<SPAN CLASS="HEADING1">Resume Posting Service

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SubmitResume.java 

Three major sections to SubmitResume.java   

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Retrieve all the form parameters. Make the style sheet Output the HTML for the resume.

We will examine each piece. For the full code, let’s view it in JCreator.

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1.Retrieving Form Parameters First, the showPreview() method retrieves the form parameters.  If a parameter is missing, we supply a default: 

String fgColor = request.getParameter("fgColor"); fgColor = replaceIfMissing(fgColor, "BLACK"); String bgColor = request.getParameter("bgColor"); replaceIfMissing(bgColor, 10/17/08 bgColor =Servlet / Web Browser Communication I

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2. Make the Style Sheet 

Based on the form parameters, we create an appropriate stylesheet via the makeStyleSheet() method:

String styleSheet = "<STYLE TYPE=\"text/css\">\n" + "