What is a Servlet?

Servlet 工作原理解析

https://www.ibm.com/developerworks/cn/java/j-lo-servlet/index.html

你可以理解为，Spring MVC是基于servlet的，它有一个DispatherServlet，然后它负责处理请求，并且调用了你的controller。

打一个比方，web网站是应用程序么？你可以说浏览器是一个应用程序，而web网站是让浏览器这个应用程序作为基础，成为通用的应用的容器。

a small, server-resident program that typically runs automatically in response to user input.

What is a Servlet?

• A servlet is simply a class which responds to a particular type of network request - most commonly an HTTP request.
• Basically servlets are usually used to implement web applications - but there are also various frameworks which operate on top of servlets (e.g. Struts) to give a higher-level abstraction than the "here's an HTTP request, write to this HTTP response" level which servlets provide.
• Servlets run in a servlet container which handles the networking side (e.g. parsing an HTTP request, connection handling etc). One of the best-known open source servlet containers is Tomcat.
• In a request/response paradigm, a web server can serve only static pages to the client
• To serve dynamic pages, a we require Servlets.
• Servlet is nothing but a Java program
• This Java program doesn’t have a main method. It only has some callback methods.
• How does the web server communicate to the servlet? Via container or Servlet engine.
• Servlet lives and dies within a web container.
• Web container is responsible for invoking methods in a servlets. It knows what callback methods the Servlet has.

Flow of Request

• Client sends HTTP request to Web server
• Web server forwards that HTTP request to web container.
• Since Servlet can not understand HTTP, its a Java program, it only understands objects, so web container converts that request into valid request object
• Web container spins a thread for each request
• All the business logic goes inside doGet() or doPost() callback methods inside the servlets
• Servlet builds a Java response object and sends it to the container. It converts that to HTTP response again to send it to the client

How does the Container know which Servlet client has requested for?

• There’s a file called web.xml
• This is the master file for a web container
• You have information about servlet in this file-
• servlets
• Servlet-name
• Servlet-class
• servlet-mappings- the path like /Login or /Notifications is mapped here in
• Servlet-name
• url-pattern
• and so on
• Every servlet in the web app should have an entry into this file
• So this lookup happens like- url-pattern -> servlet-name -> servlet-class

How to "install" Servlets? * Well, the servlet objects are inherited from the library- javax.servlet.* . Tomcat and Spring can be used to utilize these objects to fit the use case.

Ref- Watch this on 1.5x- https://www.youtube.com/watch?v=tkFRGdUgCsE . This has an awesome explanation.

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A servlet is simply a class which responds to a particular type of network request - most commonly an HTTP request. Basically servlets are usually used to implement web applications - but there are also various frameworks which operate on top of servlets (e.g. Struts) to give a higher-level abstraction than the "here's an HTTP request, write to this HTTP response" level which servlets provide.

Servlets run in a servlet container which handles the networking side (e.g. parsing an HTTP request, connection handling etc). One of the best-known open source servlet containers is Tomcat.

https://stackoverflow.com/questions/7213541/what-is-java-servlet