JAVA: Servlets Lifecycle

<< JAVA: Creating a Simple Web Application in Tomcat

JAVA: More on Servlets >>

Web Design & Development CS506

Lesson 28

Servlets Lifecycle

In the last handout, we have seen how to write a simple servlet. In this handout we will look more

specifically on how servlets get created and destroyed. What different set of method are invoked during the

lifecycle of a typical servlet.

The second part consists on reading HTML form data through servlet technology. This will be explored in

detail using code example

Stages of Servlet Lifecycle

A servlet passes through the following stages in its life.

Initialize

Service

Destroy

As you can conclude from the diagram below, that with the passage of time a servlet passes through these

stages one after another.

1. Initialize

When the servlet is first created, it is in the initialization stage. The webserver invokes he init() method of

the servlet in this stage. It should be noted here that init() is only called once and is not called for each

request. Since there is no constructor available in Servlet so this urges its use for one time initialization

(loading of resources, setting of parameters etc) just as the init() method of applet.

Initialize stage has the following characteristics and usage

Executed once, when the servlet gets loaded for the first time

Not called for each client request

The above two points make it an ideal place to perform the startup tasks which are done in

constructor in a normal class.

2. Service

The service() method is the engine of the servlet, which actually processes the client's request. On every

request from the client, the server spawns a new thread and calls the service() method as shown in the

figure below. This makes it more efficient as compared to the technologies that use single thread to respond

to requests.

204

Web Design & Development CS506

The figure below show both versions of the implementation of service cycle. In the upper part of diagram,

we assume that servlet is made by sub-classing from GenericServlet. (Remember, GenericServlet is used

for constructing protocol independent servlets.). To provide the desired functionality, service() method is

overridden. The client sends a request to the web server; a new thread is created to serve this request

followed by calling the service() method. Finally a response is prepared and sent back to the user according

to the request.

The second part of the figure illustrates a situation in which servlet is made using HttpServlet class. Now,

this servlet can only serves the HTTP type requests.

205

Web Design & Development CS506

Summary

A Servlet is constructed and initialized. The initialization can be performed inside of init() method.

Servlet services zero or more requests by calling service() method that may decide to call further

methods depending upon the Servlet type (Generic or HTTP specific)

Server shuts down, Servlet is destroyed and garbage is collected

The following figure can help to summarize the life cycle of the Servlet

The web sever creates a servlet instance. After successful creation, the servlet enters into initialization

phase. Here, init() method is invoked for once. In case web server fails in previous two stages, the servlet

instance is unloaded from the server.

After initialization stage, the Servlet becomes available to serve the clients requests and to generate

response accordingly. Finally, the servlet is destroyed and unloaded from web server.

Reading HTML Form Data Using Servlets

In the second part, the required concepts and servlet technology is explored in order to read HTML form

data. To begin with, let's first identify in how many ways a client can send data

HTML & Servlets

Generally HTML is used as a Graphics User Interface for a Servlet. In the figure below, HTML form is

being used as a GUI interface for MyServlet. The data entered by the user in HTML form is transmitted to

the MyServlet that can process this data once it read out. Response may be generated to fulfil the

application requirements.

206

Web Design & Development CS506

207

Web Design & Development CS506

Example Code: Reading Form Data using Servlet

This example consists of one HTML page (index.html), one servlet (MyServlet.java) and one xml file

(web.xml) file. The HTML page contains two form parameters: firstName and surName. The Servlet

extracts these specific parameters and echoes them back to the browser after appending "Hello".

Note: The example given below and examples later in coming handouts are built using netBeans®4.1. It's

important to note that tomcat server bundled with netBeans® runs on 8084 port by default.

index.html

Let's have a look on the HTML code used to construct the above page.

<html>

<head>

<title> Reading Two Parameters </title>

</head>

<body>

<H2> Please fill out this form: </H2>

<FORM METHOD="GET"

ACTION="http://localhost:8084/paramapp/formservlet"NAME="myform" >

<BR> Firstname:

<BR> Surname:

<BR> <INPUT TYPE="submit" value="Submit Form"> <INPUT TYPE="reset"

value="Reset">

</FORM>

</body>

</html>

Let's discuss the code of above HTML form. As you can see in the <FORM> tag, the attribute METHODis

set to "GET". The possible values for this attribute can be GET and POST. Now what do these values

208

Web Design & Development CS506

mean?

Setting the method attribite to "GET" means that we want to send the HTTP request using the GET

method which will evantually activate the doGet() method of the servlet. In the GET method the

information in the input fields entered by the user, merges with the URL as the query string and are

visible to the user.

Setting METHOD value to "POST" hides the entered information from the user as this information

becomes the part of request body and activates doPost() method of the servlet.

Attribute ACTION of <FROM> tag is set to http://localhost:8084/paramapp/formservlet. The form data

will be transmitted to this URL. paramapp is the name of web application created using netBeans.

formservlet is the value of <url-pattern> defined in the web.xml. The code of web.xml is given at the end.

The NAME attribute is set to "myform" that helps when the same page has more than one forms.

However, here it is used only for demonstration purpose.

To create the text fields where user can enter data, following lines of code come into play

Each text field is distinguished on the basis of name assigned to them. Later these names also help in

extracting the values entered into these text fields.

MyServlet.java

Now lets take a look at the servlet code to which HTML form data is submitted.

import java.io.*;import javax.servlet.*;import

javax.servlet.http.*;

public class MyServlet extends HttpServlet{

public void doGet(HttpServletRequest req,

HttpServletResponse res)throws ServletException,

IOException{

// reading first name parameter/textfield

String fName = req.getParameter("firstName");

// reading surname parameter/textfield

String sName = req.getParameter("surName");

// gettting stream from HttpServletResponse objectPrintWriter out = res.getWriter();

out.println("Hello: " + fName + " " +sName ");

out.close();

}

}// end FormServlet

We started the code with importing three packages.

import java.io.*,

import javax.servlet.*;

import javax.servlet.http.*;

These packages are imported to have the access on PrintWriter, HttpServlet, HttpServletRequest,

HttpServletResponse, ServletException and IOException classes.

The class MySevlet extends from HttpServlet to inherit the HTTP specific functionality. If you recall

HTML code (index.html) discussed above, the value of mehtod attribute was set to "GET". So in this case,

we only need to override doGet()Method.

Entering inside doGet() method brings the crux of the code. These are:

String fName = req.getParameter("firstName");

209

Web Design & Development CS506

String sName = req.getParameter("surName");

Two String variables fName and sName are declared that receive String values returned by getParameter()

method. As discussed earlier, this method returns Stringcorresponds to the form parameter. Note that the

values of name attributes of input tags used in index.html have same case with the ones passed to

getParameter() methods as parameters. The part of HTML code is reproduced over here again:

In the last part of the code, we get the object of PrintWriter stream from the object of

HttpServletResponse. This object will be used to send data back the response. Using PrintWriter object

(out), the names are printed with appended "Hello" that becomes visible in the browser.

web.xml

<?xml version="1.0" encoding="ISO-8859-1"?>

<web-app>

<servlet-name> FormServlet </servlet-name>

<servlet-class> MyServlet </servlet-class>

</servlet>

<servlet-mapping>

<servlet-name> FormServlet </servlet-name>

<url-pattern> /formservlet </url-pattern>

</servlet-mapping>

</web-app>

The <servlet-mapping> tag contains two tags <servlet-name> and <urlpatteren> containing name and

pattern of the URL respectively. Recall the value of action attribute of the <form> element in the HTML

page. You can see it is exactly the same as mentioned in <url-pattern> tag.

http://localhost:8084/paramapp/formservlet

References:

JAVA a Lab Course by Umair Javed

Java API documentation

Core Servlets and JSP by Marty Hall

210

Table of Contents: