我是否正确理解了servlet的工作原理？

Question

So, yeah, I summed up all what I understood and drew a simple diagram.

If I am not wrong, the servlet is the CGI (Common Gateway Interface) because the servelet is THE ONLY way you can get access to the resources on the server. So, in short, it is the COMMON GATEWAY.

The CONTAINER, like Apache Tomcat, is responsible for capturing the request sent by the user and sending it to the servlet.

What the user perceives is a dynamic webpage called web app.

This is what I learnt so far.

Have I learnt it correctly ?

Answer 1

You are almost right. Here are typical workflows you can follow when working with plain servlets:

Servlet renders page

1. Servlet container find servlet that matches request URL

2. doGet() or doPost() is called depending on HTTP method requested

3. Servlet does some processing

4. Response (HTML, XML, JSON, image...) is generated directly in the servlet and sent to the client using getOutputStream() or getWriter()

    PrintWriter out = response.getWriter(); out.println("Hello World"); 

JSP handles request

1. Servlet container finds JSP matching request. You must understand that underneath each JSP is translated to some internal servlet

2. This JSP is interpreted. Raw text is sent directly, Java code in scriptlets is executed

3. JSP ends, request is done

Servlet forwards to JSP

1. Same as 1-3 in first scenario

2. Servlet chooses JSP file and forwards to that JSP

3. JSP file is then evaluated, it has access to some context (request attributes, session) that was passed by servlet

    RequestDispatcher dispatcher = getServletContext() .getRequestDispatcher("foo.jsp"); dispatcher.forward(request, response); 

The last scenario is considered the best one as it does not mix business logic (servlet) and presentation (JSP).

Answer 2

A Servlet processes a request and generates a response.

A JSP gets compiled into a servlet, so JSPs are a subset of servlets.

It is not a Servlet who looks for the correct JSP, this is the container's job.