【Head First Servlets and JSP】笔记21：从有脚本到无脚本

可以建立多态的bean引用吗
使用type，但没有class
scope属性默认为“page”

从有脚本到无脚本

1、快速搭建一个测试环境：输入用户名，返回“Hello, 用户名”

index.html

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <title>Title</title>

    <style>

        body {

            font-family:'comic sans ms',sans-serif;

        }

    </style>

</head>

<body>

    <form action="checking" method="post">

        <p>Name:</p>

        <p><input type="text" name="name" value="admin"></p>

        <p>Comments: </p>

        <p><textarea name="comments" rows="7" cols="30">Your comments</textarea></p>

        <p><input type="submit"></p>

    </form>

</body>

</html>

web.xml

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"

         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"

         version="3.1">

        <servlet>

            <servlet-name>checking</servlet-name>

            <servlet-class>com.demo.checking</servlet-class>

        </servlet>

        <servlet-mapping>

            <servlet-name>checking</servlet-name>

            <url-pattern>/checking</url-pattern>

        </servlet-mapping>

</web-app>

com.demo.checking

package com.demo;

import javax.servlet.RequestDispatcher;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import java.io.IOException;

public class checking extends HttpServlet {

    @Override

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        doPost(req, resp);

    }

    @Override

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        String name = req.getParameter("name");

        req.setAttribute("name", name);

        RequestDispatcher view = req.getRequestDispatcher("/index.jsp");

        view.forward(req, resp);

    }

}

index.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>

<html>

  <head>

    <title>$Title$</title>

    <style>

      body {

        font-family:'comic sans ms',sans-serif;

      }

    </style>

  </head>

  <body>

    <p>Hello, <%=request.getAttribute("name")%></p>

    <%--<p>Hello, <%=request.getParameter("name")%></p>--%>

  </body>

</html>

2、把传递“值”改为传递“对象”

com.demo.Person

package com.demo;

public class Person implements java.io.Serializable {

    private String name;

    public Person() {

    }

    public String getName() {

        return name;

    }

    public void setName(String name) {

        this.name = name;

    }

}

com.demo.checking

package com.demo;

import javax.servlet.*;

import javax.servlet.http.*;

import java.io.IOException;

public class checking extends HttpServlet {

    @Override

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        doPost(req, resp);

    }

    @Override

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        Person person = new Person();

        person.setName(req.getParameter("name"));

        req.setAttribute("person", person);

        RequestDispatcher view = req.getRequestDispatcher("/index.jsp");

        view.forward(req, resp);

    }

}

index.jsp

<%@ page import="com.demo.Person" %>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>

<html>

  <head>

    <title>$Title$</title>

    <style>

      body {

        font-family:'comic sans ms',sans-serif;

      }

    </style>

  </head>

  <body>

    <p>Hello, <%=((Person)request.getAttribute("person")).getName()%></p>

  </body>

</html>

3、不过，还记得那个备忘录吗？可以用一句话来总结：“使用脚本则死”。所以我们需要另一种方法。

Person是一个JavaBean，所以我们使用与bean相关的标准动作

<%@ page import="com.demo.Person" %>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>

<html>

  <head>

    <title>$Title$</title>

    <style>

      body {

        font-family:'comic sans ms',sans-serif;

      }

    </style>

  </head>

  <body>

    <jsp:useBean id="person" class="com.demo.Person" scope="request" />

    <p>Person created by servlet: <jsp:getProperty name="person" property="name" /></p>

  </body>

</html>

<jsp:useBean>与<jsp:getProperty>的关系

<jsp:useBean>可以用来声明和初始化你在<jsp:getProperty>中使用的具体bean对象。

<jsp:getProperty>中的“name”值与<jsp:useBean>中的“id”值相对应；<jsp:useBean>中的“id”值与requst中的person属性对应。// req.setAttribute("person", person);

事实上，上述代码在_jspService()中将会变成这样

      com.demo.Person person = null;

      person = (com.demo.Person) _jspx_page_context.getAttribute("person", javax.servlet.jsp.PageContext.REQUEST_SCOPE);

      if (person == null){

        person = new com.demo.Person();

        _jspx_page_context.setAttribute("person", person, javax.servlet.jsp.PageContext.REQUEST_SCOPE);

      }

关于PageContext可以参考这里。

4、使用JavaBean标准动作设置对象的成员值。

<%@ page import="com.demo.Person" %>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>

<html>

  <head>

    <title>$Title$</title>

    <style>

      body {

        font-family:'comic sans ms',sans-serif;

      }

    </style>

  </head>

  <body>

    <%-- id值对应request的实例域中的person --%>

    <jsp:useBean id="person" class="com.demo.Person" scope="request" />

    <jsp:setProperty name="person" property="name" value="fuck_admin" />

    <p>Person created by servlet: <jsp:getProperty name="person" property="name" /></p>

  </body>

</html>

5、但是，上述代码有一个问题：覆盖了用户输入的name。而我们只想在查找不到用户输入的时候，自行创建bean并设置实例域的值。

也就是说，我们希望_jspService()中是这样的：

对应的JavaBean标准动作就是：

  <body>

    <%-- id值对应request的实例域中的person成员 --%>

    <jsp:useBean id="person" class="com.demo.Person" scope="request">

      <jsp:setProperty name="person" property="name" value="fuck_admin" />

    </jsp:useBean>

    <p>Person created by servlet: <jsp:getProperty name="person" property="name" /></p>

  </body>

可以建立多态的bean引用吗？——为<jsp:useBean>增加一个type属性

也就是说，我们希望用一个父类引用持有子类对象。类似于：

        Person p = new Employee();

首先，我们把Person改为抽象类（这样就无法创建Person类对象了），然后实现一个Employee类：

package com.demo;

public class Employee extends Person {

    private int empID;

    public int getEmpID() {

        return empID;

    }

    public void setEmpID(int empID) {

        this.empID = empID;

    }

}

接着修改servlet和JSP，如下所示：

    @Override

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        Person person = new Employee();

        person.setName(req.getParameter("name"));

        req.setAttribute("person", person);

        RequestDispatcher view = req.getRequestDispatcher("/index.jsp");

        view.forward(req, resp);

    }

<<<<——IDE报错，修改：

  <body>

    <%-- id值对应request的实例域中的person成员 --%>

    <jsp:useBean id="person" type="com.demo.Person" class="com.demo.Employee" scope="request">

      <jsp:setProperty name="person" property="name" value="fuck_admin" />

    </jsp:useBean>

    <p>Person created by servlet: <jsp:getProperty name="person" property="name" /></p>

  </body>

type不仅可以是抽象类，也可以是接口类型、普通类。（共同点是都可以作为实现类or子类的持有者）

使用type，但没有class

试一下就知道了，修改一下JSP;

  <body>

    <%-- id值对应request的实例域中的person成员 --%>

    <jsp:useBean id="person" type="com.demo.Person" scope="request">

      <jsp:setProperty name="person" property="name" value="fuck_admin" />

      <%-- 通过这个体内语句我们可以判断bean是不是新创建的 --%>

    </jsp:useBean>

    <p>Person created by servlet: <jsp:getProperty name="person" property="name" /></p>

  </body>

完全修改servlet；

    @Override

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        Employee employee = new Employee();

        employee.setName(req.getParameter("name"));

        req.setAttribute("person", employee);

        RequestDispatcher view = req.getRequestDispatcher("/index.jsp");

        view.forward(req, resp);

    }

重新部署，输出结果仍是admin，证明Person引用正确持有了Employee对象。因此“使用type，但没有class”是可行的。这个时候如果我把

上面的一句代码注释掉；

        // req.setAttribute("person", employee);

将无法正常运行（并不会输出fuck_admin），所以，只有type的前提是bean已经在指定作用域存在。

scope属性默认为page