1. Servlets

2. Introduction programs that run on a Web or Application server
technology is used to create web application (resides at server side and generates dynamic web page).
Servlet is a web component that is deployed on the server to create dynamic web page.
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
Using Servlets, you can collect input from users through web page forms, present records from a database or another source, and create web pages dynamically.
Servlet is a class that extend the capabilities of the servers and respond to the incoming request. It can respond to any type of requests.

3. CGI(Commmon Gateway Interface)
enables the web server to call an external program and pass HTTP request information to the external program to process the request.
For each request, it starts a new process.
There are many problems in CGI technology:
If number of clients increases, it takes more time for sending response.
For each request, it starts a process and Web server is limited to start processes.
It uses platform dependent language e.g. C, C++, perl.

4. Advantage of Servlet
The web container creates threads for handling the multiple requests to the servlet.
better performance: because it creates a thread for each request not process.
Portability: because it uses java language.
Robust: Servlets are managed by JVM so we don't need to worry about memory leak, garbage collection etc.
Secure: because it uses java language..

7. HTTP Requests

8. GET- It requests the data from a specified resource
GET and POST
Two common methods for the request-response between a server and client are:
GET- It requests the data from a specified resource
POST- It submits the processed data to a specified resource
GET
POST
1) In case of Get request, only limited amount of data can be sent because data is sent in header.
In case of post request, large amount of data can be sent because data is sent in body.
2) Get request is not secured because data is exposed in URL bar.
Post request is secured because data is not exposed in URL bar.
3) Get request can be bookmarked.
Post request cannot be bookmarked.
4) Get request is idempotent . It means second request will be ignored until response of first request is delivered
Post request is non-idempotent.

9. Servlets Tasks Read the explicit data sent by the clients (browsers).
Read the implicit HTTP request data sent by the clients (browsers).
Process the data and generate the results.
Send the explicit data (i.e., the document) to the clients (browsers).
Send the implicit HTTP response to the clients (browsers).

10. Servlets Packages
javax.servlet- contains many interfaces and classes that are used by the servlet or web container. These are not specific to any protocol.
javax.servlet.http - contains interfaces and classes that are responsible for http requests only.

11. The servlet is initialized by calling the init () method.
Servlets - Life Cycle
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.
init() method
called only once
It is called when the servlet is first created, and not called again for each user request
When a user invokes a servlet, a single instance of each servlet gets created, with each user request resulting in a new thread that is handed off to doGet or doPost
service() method
main method to perform the actual task.
handle requests coming from the client( browsers) and to write the formatted response back to the client.
checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete

12. destroy() method
called only once at the end of the life cycle of a servlet
chance to close database connections, halt background threads

13. provides common behaviour to all the servlets.
Servlet Interface
provides common behaviour to all the servlets.
Servlet interface needs to be implemented for creating any servlet (either directly or indirectly).
It provides 3 life cycle methods that are used to initialize the servlet, to service the requests, and to destroy the servlet
Methods of Servlet interface
Method
Description
public void init(ServletConfig config)
initializes the servlet. It is the life cycle method of servlet and invoked by the web container only once.
public void service(ServletRequest request,ServletResponse response)
provides response for the incoming request. It is invoked at each request by the web container.
public void destroy()
is invoked only once and indicates that servlet is being destroyed.

14. GenericServlet class
implements Servlet, ServletConfig and Serializable interfaces
protocol-independent.

15. HttpServlet class
extends the GenericServlet class and implements Serializable interface
provides http specific methods such as doGet, doPost, doHead, doTrace

16. Reading Form Data using Servlet:
getParameter(): You call request.getParameter() method to get the value of a form parameter.
getParameterValues(): Call this method if the parameter appears more than once and returns multiple values, for example checkbox.
getParameterNames(): Call this method if you want a complete list of all parameters in the current request.

17. GET method
sends the encoded user information appended to the page request
The page and the encoded information are separated by the ? character as follows:
default method to pass information from browser to web server
it produces a long string that appears in your browser's Location:box
POST method
more reliable method of passing information to a backend program
instead of sending it as a text string after a ? in the URL it sends it as a separate message.
This message comes to the backend program in the form of the standard input which you can parse and use for your processing. Servlet handles this type of requests using doPost() method.