Introducing Servlets

Servlets are small programs that execute on the server side of a web connection. Just as applets dynamically extend the functionality of a web browser, servlets dynamically extend the functionality extend the functionality of a web server. We will focus on the core concepts, interfaces, and classes. The Life Cycle of a Servlet: 
Three methods are central to the life cycle of a servlet. These are init(), service(), destroy(). They are implemented by every servlet and are invoked at specific times by the server. Let us consider a typical user scenario to understand when these methods are called. First, assume that a user enters a Uniform Resource Locator (URL) to a web browser. The browser then generates an HTTP request for this URL. This request is then sent to the appropriate server. Second, the HTTP request is received by the web server. The server maps this request to a particular servlet. The servlet is dynamically retrieved and loaded into the address space if the server. Third, the server invokes the init() method of the servlet. This method invoked only when the servlet is first loaded into memory. It is possible to pass initialization parameters to the servlet so it may configure itself. Fourth, the server invokes service() method of the servlet. This method is called to process the HTTP request. You will see that it is possible for the servlet to read data that has been provided in the HTTP request. It may also formulate an HTTP response for the client. The servlet remains in the server’s address space and is available to process any other HTTP requests received from clients. The service() method is called for each HTTP request. Finally, the server may decide to unload the servlet from its memory. The algorithms by which this determination is made are specific to each server. The server calls the destroy() method to relinquish any resources such as file handles that are allocated for the servlet and its objects can then be garbage collected.
Servlet Development Options: 
To create servlet, you will need access to a servlet container/server. Two popular ones are Glassfish is form Oracle and is provided by the java EE SDK. It is supported by NetBeans. Tomcat is an open-source product maintained by the Apache Software Foundation. It can also be used by NetBeans. Both Tomcat and Glassfish can also be used with other IDEs, such as Eclipse. Although IDEs such as NetBeans and Eclipse are very useful and can streamline the creation of servlets.
Using Tomcat:
Once installed, you start Tomcat by selecting startup.bat form the bin directly under the apache-tomcat directory. To stop Tomcat, execute shutdown.bat, also in the bin directory.
A Simple Servlet:
To become familiar with the key servlet concepts, we will begin by building and testing a simple servlet.  
The basic steps are the following:
1. Create and compile the servlet source code. Then, copy the servlet’s class file to the proper directory, and add the servlet’s name and mappings to the proper web.xml file.
2. Start Tomcat.
3. Start a web browser and request the servlet.
Create and Compile the Servlet source Code.

import java.io.*; import javax.servlet.*; public class HelloServlet extends HenericServlet{ public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException{ response.setContentType("text/html"); printWriter pw=response.getWriter(); pw.println("<b>Hello Manoj!"); pw.close(); }}

First, note that it imports the javax.servlet package. This package contains the classes and interfaces required to build servlets. The program defines HelloServlet as a subclass of GenericServlet. The GenericServlet class provides functionality that simplifies the creation of a servlet. For Example, it provides versions of init() and destroy(), which may be used as is. You need supply only the service() Method.
The GenericServlet Class:
The GenericServlet class provides implementations of the basic life cycle methods for a servlet. GenericServlet implementation the servlet and ServletConfig interfaces. In addition, a method to append a string to the server log file is available.
The signatures of this method are show here:
Void log(String s) Void log(String s, Throwable e)

Here, s is the String to be appended to the log, and e is an exception that occurred.
The servletInputStream Class:
The servletInputStream class extends InputStream. It is implemented by the servlet container and provides an input stream that a servlet developer cane uses to read the date from the client request. In addition to the input methods. Inherited from InputStream, a method is provided to read bytes from the stream.
It is shown here:
int readLine(byte[] buffer, int offset, int size) throws IOException
Here, buffer is the array into which size bytes are placed string at offset. The method returns the actual number of bytes of read or -1 if an end-of-stream condition is encountered.
The serverOutputStream Class:
The serverOutputStream class extends OutputStream. It is implemented by the servlet container and provides an output stream that a servlet developer can use to write the data to a client response. In addition to the output method provided by outputStream. It also defines the print() and println() methods, which output data to this stream.
The Servlet Exception Classes:
javax.servlet defines two exceptions. The first is ServletException, which indicates that servlet problem has occurred. The second is UnavailableException, which extends ServletException. It indicates that a servlet is unavailable.
Reading Servlet Parameters:
The servletRequest interface includes method allow you to read the name and values of parameters that are included in a client request. We will develop a servlet that illustrates their use. The example contains two files. A web page is defined in PostParameters.html and a servlet is defined in postParametersServlet.java.

Code of PostParameters.html

<html> <head> <title>Servlet Demo</title> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> </head> <body> <center> <form name="Form1" method="POST" action="http://localhost:8080/examples/servlets/postParametersServlet"> <table> <tr><th>Empolyee</th> <td><input type="text" name="e" size="25" value=""/></td></tr> <tr><th>Phone</th> <td><input type="textbox" name="p" size="25" value="" /></td></tr> </table> <input type="submit" value="submit"> </form> </center> </body> </html>

Code of postParametersServlet.java

import java.io.IOException; import java.io.PrintWriter; import java.util.Enumeration; import javax.servlet.GenericServlet; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; /** * * @author Manoj */ public class postParametersServlet extends GenericServlet{ private static final long serialVersionUID = 1L; @Override public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { //Get Print Writer. PrintWriter pw=res.getWriter(); //Get enumeration of parameter names. Enumeration e=req.getParameterNames(); //Display parameter names and values. while(e.hasMoreElements()){ String pname=(String)e.nextElement(); pw.print(pname + "="); String pvalue=req.getParameter(pname); pw.println(pvalue);} pw.close(); } }

Compile the servlet, Next, copy it to the appropriate directory,and update the web.xml file, as previously described. then, perform these steps to text this example.
 1. Start Tomcat (if is not already running).
 2. Display the web page in a browser.
 3. Enter an Employee name and Phone Number in the text field.
 4. Submit the web page.
After following these steps, the browser will display a response that is dynamically generated by the servlet.