Servlets - Overview

What are Servlets?

Java Servlets are programs that run on a Web or Application server and act as a middle layer between a request coming from a Web browser or other HTTP client and databases or applications on the HTTP server.

Servlets - Environment Setup

A development environment is where you would develop your Servlet, test them and finally run them.
Like any other Java program, you need to compile a servlet by using the Java compiler javac and after compilation the servlet application, it would be deployed in a configured environment to test and run.
This development environment setup involves following steps:

Servlets - Life Cycle

A servlet life cycle can be defined as the entire process from its creation till the destruction. The following are the paths followed by a servlet

• The servlet is initialized by calling the init () method.
• The servlet calls service() method to process a client's request.
• The servlet is terminated by calling the destroy() method.
  

Servlets - Examples

Servlets are Java classes which service HTTP requests and implement the javax.servlet.Servlet interface. Web application developers typically write servlets that extend javax.servlet.http.HttpServlet, an abstract class that implements the Servlet interface and is specially designed to handle HTTP requests.

Servlets - Form Data

You must have come across many situations when you need to pass some information from your browser to web server and ultimately to your backend program. The browser uses two methods to pass this information to web server. These methods are GET Method and POST Method.

Servlets - Client HTTP Request

When a browser requests for a web page, it sends lot of information to the web server which can not be read directly because this information travel as a part of header of HTTP request. You can check HTTP Protocol for more information on this.

Servlets - Server HTTP Response

As discussed in previous chapter, when a Web server responds to a HTTP request to the browser, the response typically consists of a status line, some response headers, a blank line, and the document. A typical response looks like this:

Servlets - Http Status Codes

The format of the HTTP request and HTTP response messages are similar and will have following structure:

• An initial status line + CRLF ( Carriage Return + Line Feed ie. New Line )
• Zero or more header lines + CRLF
• A blank line ie. a CRLF
• An optioanl message body like file, query data or query output.
  

Servlets - Writing Filters

Servlet Filters are Java classes that can be used in Servlet Programming for the following purposes:

• To intercept requests from a client before they access a resource at back end.
• To manipulate responses from server before they are sent back to the client.
  

Servlets - Exception Handling

When a servlet throws an exception, the web container searches the configurations in web.xml that use the exception-type element for a match with the thrown exception type.
You would have to use the error-page element in web.xml to specify the invocation of servlets in response to certain exceptions or HTTP status codes.

Servlets - Cookies Handling

Cookies are text files stored on the client computer and they are kept for various information tracking purpose. Java Servlets transparently supports HTTP cookies.
There are three steps involved in identifying returning users:

Servlets - Session Tracking

HTTP is a "stateless" protocol which means each time a client retrieves a Web page, the client opens a separate connection to the Web server and the server automatically does not keep any record of previous client request.

Servlets - Database Access

This tutorial assumes you have understanding on how JDBC application works. Before starting with database access through a servlet, make sure you have proper JDBC environment setup along with a database.

Servlets - File Uploading

A Servlet can be used with an HTML form tag to allow users to upload files to the server. An uploaded file could be a text file or image file or any document.

Servlet - Handling Date

One of the most important advantages of using Servlet is that you can use most of the methods available in core Java. This tutorial would take you through Java provided Date class which is available in java.util package, this class encapsulates the current date and time.

Servlets - Page Redirection

Page redirection is generally used when a document moves to a new location and we need to send the client to this new location or may be because of load balancing, or for simple randomization.
The simplest way of redirecting a request to another page is using method sendRedirect() of response object. Following is the signature of this method:

Servlets - Hits Counter

Hit Counter for a Web Page:

Many times you would be interested in knowing total number of hits on a particular page of your website. It is very simple to count these hits using a servlet because the life cycle of a servlet is controlled by the container in which it runs.

Servlets - Auto Page Refresh

Consider a webpage which is displaying live game score or stock market status or currency exchange ration. For all such type of pages, you would need to refresh your web page regularly using referesh or reload button with your browser.

Servlets - Sending Email

To send an email using your a Servlet is simple enough but to start with you should have JavaMail API and Java Activation Framework (JAF) installed on your machine.

Servlets - Packaging

The web application structure involving the WEB-INF subdirectory is standard to all Java web applications and specified by the servlet API specification. Given a top-level directory name of myapp, Here is what this directory structure looks like: