Servlet principle

servlet server applet Small programs running on the server side
It's an interface , Specifies the java Class can be accessed by the browser ( By tomcat Server identification ) The rules of
Special that can be accessed by the browser java class , Must depend on web The server software can recognize and execute , It must abide by certain rules, that is, to achieve servlet Interface

servlet How it works
1. Browser access url http://localhost:8080/tomcat_1/demo1 , among localhost:8080 Find the corresponding server , tomcat_1 The corresponding items can be found for the virtual directory ,/demo1 For the resource path
2.tomcat The server will be from web.xml Search for url-pattern by /demo1 Configuration of
3. To find url-pattern by demo1 Of servlet-class( Full path of class )
4. Create objects of the class through reflection according to the full path of the class
5. Call its methods

servlet Life cycle of
1. establish init( Only once , By default, it is executed on the first access , It can also be executed when the server is started , Need to be revised web.xml In the label load-on-startup Value , Negative numbers are the default ,0 Or a positive number means that when the server starts )
2. service service( Each call will execute )
3. The destruction destroy ( Execute once before destruction )

servlet Annotation version configuration
There is no need for web.xml, Just add @WebServlet(“ Resource path ”) that will do . @WebServlet(“ Resource path ”) It's right @WebServlet(urlPatterns=“ Resource path ”) Abbreviation The premise is to write only this attribute , To configure multiple attributes, you need to write them all in sequence