JSP/servlet getting started-servlet getting started

Some of the current JSP books directly describe the use of JSP, and then explain the use of serverlet; some books first describe the use of serverlet, and then explain the use of JSP. I personally think the second one is better. You can learn the reason! So today we will continue to learn how to use serverlet!

Okay, let's talk about it. Start now!

2. What is Servlet?
   ① Servlet is a Java class
   ② Servlet is a class that inherits the httpservlet class
   ③ This operation is run on the server to process client requests.
3. Introduction to servlet-related packages
   -- Javax. servlet .* : Stores general Servlet classes unrelated to HTTP;
   -- Javax. servlet. http .* : In addition to inheriting javax. servlet. *, It also adds http-related functions.
   (Note: It is necessary to learn about the HTTP protocol, because Web development will involve it)
   All servlets must implement the javax. servlet. servlet interface ).
   If the Servlet Program It has nothing to do with HTTP, so it must inherit the javax. servlet. genericservlet class;
   If the servlet program is related to the HTTP protocol, it must inherit the javax. servlet. http. httpservlet class.
   -- Httpservlet : Provides an abstract class to create an HTTP servlet.
   Public void doget () method: Used for processing The client sends Get Request
   Public void dopost () method: Used for processing Post request
   there are several ways for you to view the API help file
   -- javax. servlet package interface:
   servletconfig interface used by the servlet container during initialization
   servletcontext interface: define the servlet method for obtaining information from its container
   servletrequest interface: request information from the server
   servletresponse interface: responds to client requests
   filter interface:
   -- javax. servlet package class:
   servletinputstream class : used to read binary data from the client
   servletoutputstream class : used to send binary data to the client
   -- javax. servlet. HTTP packet interface:
   httpservletrequest interface: provides HTTP Request Information
   httpservletresponse interface: provide HTTP Response
   
5. Servlet Lifecycle
   -- The servlet Life Cycle refers to the whole process of the Existence time and destruction after the servlet instance is created.
   -- Servlet has three methods in its lifecycle.
   Init () method:
   Service () method: Dispatches client requests to the protectedServiceMethod
   Destroy () method: Called by the servlet container to indicate to a servlet that the servlet is being taken out of service.
   -- Each stage of the servlet Lifecycle
   ----Instantiation: Servlet container create servlet instance
   ----Initialization: Call the init () method
   ----Service: If there is a request, call the service () method
   ----Destroy: Call the destroy () method before destroying an instance.
   ----Garbage Collection: Destroy an instance
6. Basic servlet Structure
   
   

   Package CN. Dragon. servlet; // Import the corresponding package Import java. Io. ioexception; Import java. Io. printwriter; Import javax. servlet. servletexception; Import javax. servlet. http. httpservlet; Import javax. servlet. http. httpservletrequest; Import javax. servlet. http. httpservletresponse; /** * This is the first servlet example. * @ Author CN. Dragon */ Public class servletdemofirst extends httpservlet { // Process get requests sent by the client Public void doget (httpservletrequest request, httpservletresponse response) Throws servletexception, ioexception { Response. setcontenttype ("text/html; charset = gb2312"); // This statement specifies the content format and character encoding used to send to the client. Printwriter out = response. getwriter (); Out. println ("Hello! "); // Use the printwriter object method to send data to the client Out. Close (); } // Process the POST request sent by the client Public void dopost (httpservletrequest request, httpservletresponse response) Throws servletexception, ioexception { Doget (request, response); // This statement is used to call the doget () method for processing when the client sends a POST request. } }

7. Servlet deployment
   
   

   The following screenshot   <Servlet> <Description> arbitrary </description> <Display-Name> arbitrary </display-Name> <Servlet-Name>Servletdemofirst</Servlet-Name> <Servlet-class>CN. Dragon. servlet. servletdemofirst</Servlet-class> </Servlet> <Servlet-mapping> <Servlet-Name>Servletdemofirst</Servlet-Name> <URL-pattern>/Servlet/servletdemofirst</Url-pattern> </Servlet-mapping>   Note] ① The two above<Servlet-Name>Must be the same ② <Servlet-class> next refers to the corresponding class. Tip: You can copy it directly in your servlet class to avoid errors! ③ <URL-pattern> must be/ServletAdd the servlet name. Let's record it now.

8. Servlet instance demonstration
   

   Package CN. Dragon. servlet; Import java. Io. ioexception; Import java. Io. printwriter; Import javax. servlet. servletexception; Import javax. servlet. http. httpservlet; Import javax. servlet. http. httpservletrequest; Import javax. servlet. http. httpservletresponse; Public class servletdemosecond extends httpservlet { // Initialization Public void Init () throws servletexception { System. Out. println ("I'm the init () method! Used for initialization "); } // Process get requests Public void doget (httpservletrequest request, httpservletresponse response) Throws servletexception, ioexception { System. Out. println ("I'm the doget () method! Used to process get requests "); Response. setcontenttype ("text/html; charset = gb2312 "); Printwriter out = response. getwriter (); Out. println ("<HTML> "); Out. println ("<body> "); Out. println ("this is an example of servlet "); Out. println ("</body> "); Out. println ("} // Process post requests Public void dopost (httpservletrequest request, httpservletresponse response) Throws servletexception, ioexception { Doget (request, response ); } // Destroy the instance Public void destroy (){ Super. Destroy (); System. Out. println ("I'm the destroy () method! Used to destroy an instance "); } }   Web. xml file <? XML version = "1.0" encoding = "UTF-8"?> <Web-app version = "2.4" Xmlns = "http://java.sun.com/xml/ns/j2ee" Xmlns: xsi = "http://www.w3.org/2001/XMLSchema-instance" Xsi: schemalocation = "http://java.sun.com/xml/ns/j2ee Http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd> <Servlet> <Servlet-Name> servletdemosecond </servlet-Name> <Servlet-class> CN. Dragon. servlet. servletdemosecond </servlet-class> </Servlet> <Servlet-mapping> <Servlet-Name> servletdemosecond </servlet-Name> <URL-pattern>/servlet/servletdemosecond </url-pattern> </Servlet-mapping>