JSP的MVC模式中，Model=JavaBean，View=JSP，Controller=Servlet。
简而言之：JavaBean用于存储数据，JSP用于显示页面，Servlet用于处理数据（脏活累活都给它）。
目录结构：
1.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"pageEncoding="UTF-8"%><!DOCTYPE html><html><head><meta charset="UTF-8"><title>Insert title here</title></head><body><form action="compute">Circle's radius:<input type="text" name="r"> <inputtype="submit" value="submit"></form></body></html>
2.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"pageEncoding="UTF-8"%><!DOCTYPE html><jsp:useBean id="data" type="bean.Circle" scope="request"></jsp:useBean><html><head><meta charset="UTF-8"><title>Insert title here</title></head><body><p>Circle's radius:<jsp:getProperty name="data" property="r" /></p><p>Circle's area:<jsp:getProperty name="data" property="area" /></p><p><jsp:getProperty name="data" property="mess" /></p></body></html>
web.xml
<?xml version="1.0" encoding="UTF-8"?><web-app><servlet><servlet-name>compute</servlet-name><servlet-class>pute</servlet-class></servlet><servlet-mapping><servlet-name>compute</servlet-name><url-pattern>/compute</url-pattern></servlet-mapping></web-app>
bean.Circle.java
package bean;public class Circle {private double r;private double area;private String mess="";public double getR() {return r;}public void setR(double r) {this.r = r;}public double getArea() {return area;}public void setArea(double area) {this.area = area;}public String getMess() {return mess;}public void setMess(String mess) {this.mess = mess;}}
pute.java
package servlet;import java.io.*;import javax.servlet.*;import javax.servlet.http.*;import bean.Circle;import java.math.*;public class Compute extends HttpServlet {public void init(ServletConfig config) throws ServletException {super.init(config);}public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {Circle circle = new Circle();request.setAttribute("data", circle);try {double area = Math.PI * Math.pow(Double.parseDouble(request.getParameter("r")), 2);circle.setArea(area);circle.setR(Double.parseDouble(request.getParameter("r")));} catch (Exception e) {circle.setArea(-1);circle.setMess("Error: " + e);}RequestDispatcher dispatcher = request.getRequestDispatcher("2.jsp");dispatcher.forward(request, response);}public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {doPost(request, response);}}
Demo:
