Servlets

Advanced Java Programming

Topic: Servlet

ByRavi Kant Sahu

Asst. Professor, LPU

Introduction Servlets are java programs that run on a Web or Application

server.

Servlets are used to process client requests or produce dynamic Web pages.

Servlet are designed to work within a request/response processing model.

It act as a middle layer between a request coming from a Web browser or other HTTP client and databases on the HTTP server.

Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

Web Pages


Static Web Contents Web pages are stored as files in Web Server. A user types a URL for the web-page from a Web browser. The browser contacts the Web server and requests the file or web-page. The server finds the file and returns it to the browser. The browser then displays the file to the user.


Dynamic Web Contents

To view up-to-date information on the Web.

HTML pages for displaying recent information must be generated dynamically.

Dynamic Web pages are generated by Web servers.

The Web server needs to run certain programs to process user requests in order to produce a customized response.


Common Gateway Interface (CGI) CGI was proposed to generate dynamic Web content.

CGI provides a standard framework for Web servers to interact with external programs, known as CGI programs.

The Web server receives a request from a Web browser and passes it to the CGI program.

The CGI program processes the request and generates a response at runtime.

CGI programs are written in C, C++ or Perl (widely used).


Common Gateway Interface


GET and POST Methods The two most common HTTP requests, also known as methods,

are GET and POST.

The Web browser issues a request using a URL or an HTML form to trigger the Web server to execute a CGI program.

When issuing a CGI request directly from a URL, the GET method is used. This URL is known as a query string.

Example:http://www.webserverhost.com/cgi-bin/getBalance.cgi?accountId=scott+smith&password=tiger


The GET and POST methods both send requests to the Web server. The POST method always triggers the execution of the

corresponding CGI program.

The GET method may not cause the CGI program to be executed, if the previous same request is cached in the Web browser.

Use the GET method to speed up performance and use a POSTmethod if the request will actually update the database.

POST method is more secure than the GET method.

GET and POST Methods


Query String http://www.webserverhost.com/cgi-bin/

getBalance.cgi?accountId=scott+smith&password=tiger

The ? symbol separates the program from the parameters.

The parameter name and value are associated using the =symbol.

Parameter pairs are separated using the & symbol.

The + symbol denotes a space character.


Speed: CGI is very slow when handling a large number of requests simultaneously, because the Web

server spawns a process for executing each CGI program. Each process has its own runtime environment that contains and runs the CGI program.

Server Crash: If many CGI programs are executed simultaneously, System resource would be quickly exhausted, potentially causing the server to crash.

CGI Vs Servlets


ServletS


Servlet Advantages Performance is significantly better.

Servlets execute within the address space of a Web server.

Servlets are platform-independent.

Java security manager on the server enforces a set of restrictions to protect the resources on a server machine.

The full functionality of the Java class libraries is available to a servlet.


Servlet Container All servlets run inside a servlet container, also referred to as a

servlet server or a servlet engine.

A servlet container is a single process that runs in a Java Virtual Machine.

The JVM creates a thread to handle each servlet.

Example: Tomcat (developed by Apache, www.apache.org) GlassFish(developed by Sun, glassfish.dev.java.net).


The Servlet API


The Servlet Interface public void init() throws ServletException;

public void service(ServletRequest request, ServletResponse Response) throws ServletException, IOException;

public void destroy();


Servlet Life Cycle The init method is called when the servlet is first created.

The service method is invoked each time the server receives a request for the servlet.

The destroy method is invoked after a timeout period has passed or as the Web server is terminated.


ServletConfig interface The ServletConfig interface allows a servlet to obtain

configuration data.

String getInitParameter(String name)

Enumeration getInitParameterNames()

ServletContext getServletContext()

String getServletName()


GenericServlet Class

GenericServlet is an abstract class that defines a generic, protocol independent servlet.

It implements javax.servlet.Servlet and javax.servlet.ServletConfig.

All the methods in Servlet and ServletConfig are implemented in GenericServlet except service().


HttpServlet Class

It is an abstract class that extends GenericServlet and implements the service method.

Methods to process HTTP request: doGet() doPost() doDelete() doPut() doOptions() doTrace()

protected void doXxx(HttpServletRequest req, HttpServletResponse res) throws ServletException, java.io.IOException


ServletRequest Interface

String getParameter(String name)

String[] getParameterValues(String name)

String getRemoteAddr()

String getRemoteHost()


HttpServletRequest Interface

String getHeader(String name)

String getMethod()

String getQueryString()

javax.servlet.http.Cookies[] getCookies()

HttpSession getSession(boolean create)


ServletResponse Interface

java.io.PrintWriter getWriter()

void setContentType(type: String)


HttpServletResponse Interface

void addCookie(Cookie cookie)


Types of ServletGeneric servlets Extend javax.servlet.GenericServlet. Are protocol independent. They contain no inherent HTTP

support or any other transport protocol.

HTTP servlets Extend javax.servlet.HttpServlet. Have built-in HTTP protocol support and are more useful in a

Sun Java System Web Server environment.


Servlet Context Interface An object of ServletContext is created by the web-container at

the time of deploying the project.

This object is used to get configuration-information from web.xml file.

There is only one ServletContext object for any web-application.

If any information is to be shared with many servlets, its better to provide it in the web.xml file using <context-param> element.


HTML FORMs


HTML Forms

HTML forms enable us to submit data to the Web server.

It can contain text fields, text area, check boxes, combo boxes,lists, radio buttons, and buttons.


HTML tags to create HTML Form

<form> ... </form> tag is used to define the body of a form.

The attributes for the <form> tag are action and method.

The action attribute specifies the server program to be executedon the Web server when the form is submitted.

The method attribute is either get or post.

<label> ... </label> is used to define a label.


HTML tags to create HTML Form <input> defines an input field. Attributes for <input> are type, name, value, checked, size, and

maxlength.

The type attribute specifies the input type. Possible types are textfor a one-line text field, radio for a radio button, and checkboxfor a check box.

The name attribute gives a formal name for the attribute which isused by the servlet program to retrieve its associated value.

The names of the radio buttons in a group must be identical.


HTML tags to create HTML Form

The value attribute specifies a default value for a text field andtext area.

The checked attribute indicates whether a radio button or acheckbox is initially checked.

The size attribute specifies the size of a text field, and themaxlength attribute specifies the maximum length of a text field.


HTML tags to create HTML Form <select> ... </select> defines a combo box or a list.

The attributes for this tag are name, size, and multiple.

The size attribute specifies the number of rows visible in the list.

The multiple attribute specifies that multiple values can beselected from a list.

Set size to 1 and do not use a multiple for a combo box.


HTML tags to create HTML Form <option> ... </option> defines a selection list within a <select>

... </select> tag. This tag may be used with the value attribute to specify a value

for the selected option (e.g., <option value = "CS"> ComputerScience).

If no value is specified, the selected option is the value.

<textarea> ... </textarea> defines a text area. The attributes are name, rows, and cols. The rows and cols attributes specify the number of rows and

columns in a text area.


Obtaining Parameter Values fromHTML Forms


Retrieving Parameter Values Each GUI component in the form has a name attribute. The servlet uses the name attribute in the following method to

obtain the parameter value as a string. String getParameter(attributeName)

In case of a list with multiple values, use the following method to return the parameter values in an array of strings:

String[] getParameterValues(attributeName)

NOTE: If an attribute doesn’t exist, the getParameter method returns null. If an empty value of the parameter is passed to the servlet, the

getParameter method returns a string with an empty value.


Retrieving Parameter Values

We can specify the value attribute in a text field, text area, combo box, list, check box, or radio button in an HTML form.

For text field and text area, the value attribute specifies a default value to be displayed in the text field and text area.

The user can type in new values to replace it.


Retrieving Parameter Values


For combo box, list, check box, and radio button, the value attribute specifies the parameter value to be returned from the getParameter and getParameterValues methods.

If the value attribute is not specified for a combo box or a list, it returns the selected string from the combo box or the list.

If the value attribute is not specified for a radio button or a check box, it returns string on for a checked radio button or a checked check box, and returns null for an unchecked check box.

Session Tracking


Session Tracking A session can be defined as a series of related interactions

between a single client and the Web server over a period of time.

Tracking data among requests in a session is known as sessiontracking.

Techniques for session tracking Session Tracking Using Hidden Values Session Tracking Using Cookies Session Tracking Using the Servlet API


Session Tracking Using Cookies

Cookies are small text files that store sets of name/value pairs on the disk in the client’s computer.

Cookies are sent from the server through the instructions in the header of the HTTP response.

The javax.servlet.http.Cookie is used to create and manipulate cookies


Cookies


Session Tracking Using Cookies Cookie(String name, String value)

String getName() String getValue() Void setValue(String newValue) int getMaxAge() void setMaxAge(int expiration) boolean getSecure() void setSecure(boolean flag) String getComment() Void setComment(String purpose)


Using Cookies Cookie stores a name/value pair and other information about the

cookie.

To send a cookie to the browser, use the addCookie() method in the HttpServletResponse class.

response.addCookie(cookie);

To obtain cookies from a browser, use getCookie() method.request.getCookies();

where request/response is an instance of HttpServletRequest/ HttpServletResponse.


Example using Cookies


Session Tracking using Cookies(Servlet1)

public void doPost(HttpServletRequest request, HttpServletResponse response){ try{

response.setContentType("text/html");PrintWriter out = response.getWriter();String n=request.getParameter("userName");out.print("Welcome "+n);Cookie ck=new Cookie("uname",n); //creating cookie objectresponse.addCookie(ck); //adding cookie in the response//creating submit buttonout.print("<form action='servlet2'>");out.print("<input type='submit' value='go'>");out.print("</form>");out.close(); } catch(Exception e){}}


public void doPost(HttpServletRequest request, HttpServletResponse response)

{ try{response.setContentType("text/html");PrintWriter out = response.getWriter();Cookie ck[]=request.getCookies();out.print("Hello "+ck[0].getValue());out.close();

}catch(Exception e){System.out.println(e);} }


Session Tracking using Cookies (Servlet 2)

Session Tracking Using Hidden Values

We can track a session by passing data from the servlet to the client as hidden values in a dynamically generated HTML form by including a field like this one:

<input type= "hidden” name= “sessionid" value=“12345">

The next request will submit the data back to the servlet.

The servlet retrieves this hidden value just like any otherparameter value, using the getParameter method.


Hidden Form Fields


Servlet 1

public void doGet(HttpServletRequest request, HttpServletResponse response){ try{

response.setContentType("text/html");PrintWriter out = response.getWriter();String n=request.getParameter("userName");out.print("Welcome "+n);

//creating form that have invisible textfieldout.print("<form action='servlet2'>");out.print("<input type='hidden' name="uname" value='"+n+"'>");out.print("<input type='submit' value='go'>");out.print("</form>");out.close(); }catch(Exception e){System.out.println(e);} }


Servlet 2

public void doGet(HttpServletRequest request, HttpServletResponse response)

try{response.setContentType("text/html");PrintWriter out = response.getWriter();

//Getting the value from the hidden fieldString n=request.getParameter("uname");out.print("Hello "+n);out.close();

}catch(Exception e){System.out.println(e);} }


Session Tracking using HTTP Session

The Container creates a session id for each user.

The container uses this id to identify the particular user.

An object of HttpSession can be used to perform two tasks: 1. Bind objects2. View and manipulate information about a session, such as the session identifier, creation time, and last accessed time.


Session Tracking using HTTP Session


Creating Session Object

The HttpServletRequest interface provides two methods to get the object of HttpSession:

public HttpSession getSession()Returns the current session associated with this request, or if the request does not have a session, creates one.

public HttpSession getSession(boolean create)Returns the current HttpSession associated with this request or, if there is no current session and create is true, returns a new session.


Methods of HTTP Session Interface

public String getId():Returns a string containing the unique identifier value.

public long getCreationTime():Returns the time when this session was created, measured in milliseconds since midnight January 1, 1970 GMT.

public long getLastAccessedTime():Returns the last time the client sent a request associated with this session, as the number of milliseconds since midnight January 1, 1970 GMT.

public void invalidate():Invalidates this session then unbinds any objects bound to it.


Servlets

Technology

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