Introduction to J2EE

 

 Web Application Model

Servlet

Servlets are Java programs that run on Web or application servers, acting as a middle layer between requests coming from Web browsers or other HTTP clients and databases or applications on the HTTP server

Tasks Performed

• Read the explicit data sent by the client
• Read the implicit HTTP request data sent by the browser
• Generate the results
• Send the explicit data (i.e., the document) to the client.
• Send the implicit HTTP response data

Advantages of Servlets

• Efficient
• If there are many requests to the same servlet, only a single copy of the servlet class would be loaded
• Convenient
• Have utilities like parsing, decoding HTML form data, cookie & session handling, etc.
• Powerful
• Servlets can communicate directly to the Web server
• Multiple servlets can also share data
• Portable
• Run on All operating systems and servers
• Inexpensive
• Secure
• Servlets are not executed by general-purpose operating system shells
• No buffer overflow attacks

What is a container? (Servlet Engine)

Servlets don’t have a main method. Because of that, they are under the control of another java application called container : Tomcat is an example of a container

Servlet Life Cycle

• init
• Executed once when the servlet is first loaded or at server start.
• Performs initializations such as setting up a database connection, loading a data file, etc.

• service
• Each time the server receives a request for a servlet, the server spawns a new thread and calls service(). 
• The service method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc., as appropriate.
• destroy
• Called when server deletes servlet instance.
• It will close database connections, halt background threads, write cookie lists or hit counts to disk, and perform other such cleanup activities

Basic Servlet Structure

Web Application Structure

Next Chapter : Servlet