Describe the process of handling JSP pages, from…

Client request: The web browser sends an HTTP request to the server for a URL corresponding to a JSP page.
Web server processing: The web server (e.g., Apache) receives the request and forwards it to the servlet container (e.g., Tomcat).
JSP search and compilation: The servlet container checks whether the requested JSP page has been previously compiled.
- If compiled and up-to-date: The container uses the existing compiled servlet class.
- If not compiled or outdated (JSP file changed): The container searches for the JSP file on disk.
  - Translation phase: The JSP translator (part of the servlet container) converts the JSP code into servlet source code in Java. It analyzes directives, scriptlets, expressions, declarations, and static HTML/XML/text, creating the _jspService() method.
  - Compilation phase: The Java compiler compiles the generated servlet source code into bytecode (.class file).
Servlet loading and initialization: The servlet container loads the compiled servlet class into the JVM and initializes it (calls the jspInit() method).
Servlet execution: The servlet container invokes the _jspService() method of the compiled servlet. This method:
- Receives HttpServletRequest and HttpServletResponse objects.
- Executes the Java code within scriptlets and expressions.
- Inserts static HTML/XML/text into the output stream.
- Uses implicit objects (out, request, response, session, application, etc.) to interact with the environment and generate the response.
Response formation: The HTML/XML/text generated by the servlet is assembled into the HttpServletResponse object.
Response sending: The servlet container sends the formed HTTP response back to the client via the web server.
Client display: The web browser receives the HTTP response and displays its content to the user.
Servlet termination: When taken out of service or when the application stops, the servlet container calls the jspDestroy() method.

Here is a simplified diagram of the servlet lifecycle methods generated from JSP:

Method	Description	When called
jspInit()	Servlet initialization	Once, at the first load of the servlet.
_jspService()	Request processing and response formation	On each request to the JSP page.
jspDestroy()	Servlet termination	Once, when the servlet is unloaded (stopped).

Example of a simple JSP:

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Example from JSP</title>
</head>
<body>
    <h1>Hello from JSP!</h1>
    <p>Current time: <%= new java.util.Date() %></p>
</body>
</html>

Translated version (pseudo-servlet code):

// Generated servlet example
public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent,
                 org.apache.jasper.runtime.JspSourceImports {

    // ... (methods jspInit, jspDestroy, getDependants, getImports) ...

    public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
            throws java.io.IOException, javax.servlet.ServletException {

        // Implicit objects
        javax.servlet.jsp.PageContext pageContext = null;
        javax.servlet.ServletContext application = null;
        javax.servlet.ServletConfig config = null;
        javax.servlet.jsp.JspWriter out = null;
        java.lang.Object page = this;
        javax.servlet.jsp.JspWriter _jspx_out = null;
        javax.servlet.jsp.PageContext _jspx_page_context = null;

        try {
            // Initialize implicit objects
            response.setContentType("text/html; charset=UTF-8");
            pageContext = _jspxFactory.getPageContext(this, request, response,
                        null, true, 8192, true);
            _jspx_page_context = pageContext;
            application = pageContext.getServletContext();
            config = pageContext.getServletConfig();
            out = pageContext.getOut();
            _jspx_out = out;

            // Output static HTML
            out.write("<!DOCTYPE html>\n");
            out.write("<html>\n");
            out.write("<head>\n");
            out.write("<meta charset=\"UTF-8\">\n");
            out.write("<title>Example from JSP</title>\n");
            out.write("</head>\n");
            out.write("<body>\n");
            out.write("    <h1>Hello from JSP!</h1>\n");
            out.write("    <p>Current time: ");
            // Execute expression
            out.print( new java.util.Date() );
            out.write("</p>\n");
            out.write("</body>\n");
            out.write("</html>");

        } catch (java.lang.Throwable t) {
            // Error handling
            if (!(t instanceof javax.servlet.jsp.SkipPageException)){
                out = _jspx_out;
                if (out != null && out.getBufferSize() != 0)
                try {
                    if (response.isCommitted()) {
                        out.flush();
                    } else {
                        out.clearBuffer();
                    }
                } catch (java.io.IOException e) {}
                if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
                else throw new ServletException(t);
            }
        } finally {
            _jspxFactory.releasePageContext(_jspx_page_context);
        }
    }
}

Describe the process of handling JSP pages, from server request to user response.