标签：min 报错 建立 用户名 共同点 exce san void div
试图章节式的阅读《Head First Servlets and JSP》总感觉不对劲，这本书前后的联系性很强，有点类似于“连续剧”，而不是通常的“知识清单”。
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>
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）
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。
【Head First Servlets and JSP】实践记录01：从有脚本到无脚本
标签：min 报错 建立 用户名 共同点 exce san void div
原文地址：http://www.cnblogs.com/xkxf/p/7045532.html
