JavaBean 基础概念、使用实例及代码分析

JavaBean的概念

　　JavaBean是一种可重复使用的、且跨平台的软件组件。

　　JavaBean可分为两种：一种是有用户界面的（有UI的）；另一种是没有用户界面的（无UI的），无UI的JavaBean主要负责处理事务（如数据运算，操纵数据库）。

　　JSP通常访问的是后一种JavaBean。

JSP与JavaBean搭配使用的优点

　　使得HTML与Java程序分离，这样便于维护代码。

　　如果把所有的程序代码都写到JSP网页中，会使得代码繁杂，难以维护。

　　可以降低开发JSP网页人员对Java编程能力的要求。

　　JSP侧重于生成动态网页，事务处理由JavaBean来完成，这样可以充分利用JavaBean组件的可重用性特点，提高开发网站的效率。

　　MVC设计模式（Model View Controller）。

JavaBean的特征

　　一个标准的JavaBean有以下几个特征：

　　JavaBean是一个公共（public）的类。

　　JavaBean有一个不带参数的构造方法。

　　JavaBean通过setXXX方法设置属性，通过getXXX方法获取属性。

JSP访问JavaBean的语法

1.导入JavaBean类。

　　通过<%@ page import>指令导入JavaBean类，

　　例如：

<%@ page import="mypack.MyJavaBean" %>

2.声明JavaBean对象。

　　标签用来声明JavaBean对象，

　　例如：

<jsp:useBean id="myBean" class="mypack.MyJavaBean" scope="session"/>

3.访问JavaBean属性。

　　JSP提供了访问JavaBean属性的标签，如果要将JavaBean的某个属性输出到网页上，可以用标签。

　　例如：

<jsp:getProperty name="myBean" property="count"/>

　　如果要给JavaBean的某个属性赋值，可以用标签，

　　例如：

<jsp:setProperty name="myBean" property="count" value="0"/>

　　使用指定的值来设置Bean的属性，这个值可以使字符串，也可以是表达式。

　　如果是字符串，那么它就会被转换成Bean属性的类型。

　　如果是一个表达式，那么它的类型就必须和将要设定的属性值的类型一致。

　　不能在同一个中使用value和param参数。

　　其中param是指通过请求参数的值来给属性赋值。

一个JavaBean使用的例子

　　写一个Person类（JavaBean）：

package com.shengqishiwind.bean;

public class Person
{
    private String name = "zhangsan";
    private int age = 24;
    private String address = "beijing"; 
    public Person()
    {
        
    }
    public String getName()
    {
        return name;
    }
    public void setName(String name)
    {
        this.name = name;
    }
    public int getAge()
    {
        return age;
    }
    public void setAge(int age)
    {
        this.age = age;
    }
    public String getAddress()
    {
        return address;
    }
    public void setAddress(String address)
    {
        this.address = address;
    }

    
}

　　然后在JSP中导入这个类，并且声明该类的对象，然后获取其属性或者更改属性：

　　导入：

<%@ page language="java" import="java.util.*,com.shengqishiwind.bean.Person" pageEncoding="UTF-8"%>

　　声明：

      
    <jsp:useBean id="person" class="com.shengqishiwind.bean.Person" scope="session"/>

　　获取对象属性：

    
    name: <jsp:getProperty name="person" property="name" /><br>
    age: <jsp:getProperty name="person" property="age" /><br>
    address: <jsp:getProperty name="person" property="address" /><br>

　　设置对象属性：

    
    <jsp:setProperty name="person" property="name" value="lisi"/>
    
    <jsp:setProperty name="person" property="age" value="30"/>    
    <jsp:setProperty name="person" property="age" value="<%=20+20 %>"/>    
    <jsp:setProperty name="person" property="address" param="hello"/>

　　关于声明对象，也可以用脚本段来声明，其作用是一样的：

    
    <br>another person:
    <% Person person2 = new Person();//这里不能把对象叫做person，提示重复的局部变量，进一步说明这种方法和上面的方法，本质上是一样的
    out.println(person2.getName());
    %>

　　所以输出属性时也可以直接用表达式：

    
    
    <br>name again: <%= person.getName()%><br>

　　完整JSP代码如下：

<%@ page language="java" import="java.util.*,com.shengqishiwind.bean.Person" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">  
    
    <title>My JSP 'JavaBeanTest.jsp' starting pagetitle>
    
    <meta http-equiv="pragma" content="no-cache">
    <meta http-equiv="cache-control" content="no-cache">
    <meta http-equiv="expires" content="0">    
    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
    <meta http-equiv="description" content="This is my page">
    

  head>
  
  <body>
      
    <jsp:useBean id="person" class="com.shengqishiwind.bean.Person" scope="session"/>
    
    name: <jsp:getProperty name="person" property="name" /><br>
    age: <jsp:getProperty name="person" property="age" /><br>
    address: <jsp:getProperty name="person" property="address" /><br>
    
    
    <jsp:setProperty name="person" property="name" value="lisi"/>
    
    <jsp:setProperty name="person" property="age" value="30"/>    
    <jsp:setProperty name="person" property="age" value="<%=20+20 %>"/>    
    <jsp:setProperty name="person" property="address" param="hello"/>    
    
    
    name after change: <jsp:getProperty name="person" property="name" /><br>
    age after change: <jsp:getProperty name="person" property="age" /><br>
    address after change: <jsp:getProperty name="person" property="address" /><br>
    
    
    <br>name again: <%= person.getName()%><br>
    
    
    <br>another person:
    <% Person person2 = new Person();//这里不能把对象叫做person，提示重复的局部变量，进一步说明这种方法和上面的方法，本质上是一样的
    out.println(person2.getName());
    %>
    
    
    <jsp:useBean id="person3" class="com.shengqishiwind.bean.Person" scope="session"/>

  body>
html>

JavaBeanTest.jsp

　　运行时输入地址（注意加上请求参数）：http://localhost:8080/HelloWeb/JavaBeanTest.jsp?hello=shanghai

　　运行后如图：

生成的Java源文件

　　查看Tomcat work路径下该项目路径生成的源文件：（JSP会转化成Servlet）：

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.40
 * Generated at: 2013-06-23 14:41:47 UTC
 * Note: The last modified time of this file was set to
 *       the last modified time of the source file after
 *       generation to assist with modification tracking.
 */
package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import java.util.*;
import com.shengqishiwind.bean.Person;

public final class JavaBeanTest_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {

  private static final javax.servlet.jsp.JspFactory _jspxFactory =
          javax.servlet.jsp.JspFactory.getDefaultFactory();

  private static java.util.Map _jspx_dependants;

  private javax.el.ExpressionFactory _el_expressionfactory;
  private org.apache.tomcat.InstanceManager _jsp_instancemanager;

  public java.util.Map getDependants() {
    return _jspx_dependants;
  }

  public void _jspInit() {
    _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
    _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
  }

  public void _jspDestroy() {
  }

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

    final javax.servlet.jsp.PageContext pageContext;
    javax.servlet.http.HttpSession session = null;
    final javax.servlet.ServletContext application;
    final javax.servlet.ServletConfig config;
    javax.servlet.jsp.JspWriter out = null;
    final java.lang.Object page = this;
    javax.servlet.jsp.JspWriter _jspx_out = null;
    javax.servlet.jsp.PageContext _jspx_page_context = null;


    try {
      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();
      session = pageContext.getSession();
      out = pageContext.getOut();
      _jspx_out = out;

      out.write('\r');
      out.write('\n');

String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";

      out.write("\r\n");
      out.write("\r\n");
      out.write("\r\n");
      out.write("\r\n");
      out.write("  \r\n");
      out.write("    );
      out.print(basePath);
      out.write("\">  \r\n");
      out.write("    \r\n");
      out.write("    My JSP 'JavaBeanTest.jsp' starting page\r\n");
      out.write("    \r\n");
      out.write("\t\r\n");
      out.write("\t\r\n");
      out.write("\t    \r\n");
      out.write("\t\r\n");
      out.write("\t\r\n");
      out.write("\t\r\n");
      out.write("\r\n");
      out.write("  \r\n");
      out.write("  \r\n");
      out.write("  \r\n");
      out.write("  \t\r\n");
      out.write("    ");
      com.shengqishiwind.bean.Person person = null;
      synchronized (session) {
        person = (com.shengqishiwind.bean.Person) _jspx_page_context.getAttribute("person", javax.servlet.jsp.PageContext.SESSION_SCOPE);
        if (person == null){
          person = new com.shengqishiwind.bean.Person();
          _jspx_page_context.setAttribute("person", person, javax.servlet.jsp.PageContext.SESSION_SCOPE);
        }
      }
      out.write("\r\n");
      out.write("    \r\n");
      out.write("\tname: ");
      out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getName())));
      out.write("
\r\n");
      out.write("\tage: ");
      out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getAge())));
      out.write("
\r\n");
      out.write("\taddress: ");
      out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getAddress())));
      out.write("
\r\n");
      out.write("\t\r\n");
      out.write("\t\r\n");
      out.write("\t");
      org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(_jspx_page_context.findAttribute("person"), "name", "lisi", null, null, false);
      out.write("\r\n");
      out.write("\t\r\n");
      out.write("\t");
      org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(_jspx_page_context.findAttribute("person"), "age", "30", null, null, false);
      out.write("\t\r\n");
      out.write("\t");
      org.apache.jasper.runtime.JspRuntimeLibrary.handleSetProperty(_jspx_page_context.findAttribute("person"), "age",
20+20 );
      out.write("\t\r\n");
      out.write("\t");
      org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(_jspx_page_context.findAttribute("person"), "address", request.getParameter("hello"), request, "hello", false);
      out.write("\t\r\n");
      out.write("\t\r\n");
      out.write("\t\r\n");
      out.write("\tname after change: ");
      out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getName())));
      out.write("
\r\n");
      out.write("\tage after change: ");
      out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getAge())));
      out.write("
\r\n");
      out.write("\taddress after change: ");
      out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getAddress())));
      out.write("
\r\n");
      out.write("\t\r\n");
      out.write("\t\r\n");
      out.write("\t
name again: ");
      out.print( person.getName());
      out.write("
\r\n");
      out.write("\t\r\n");
      out.write("\t\r\n");
      out.write("\t
another person:\r\n");
      out.write("\t");
 Person person2 = new Person();//这里不能把对象叫做person，提示重复的局部变量，进一步说明这种方法和上面的方法，本质上是一样的
    out.println(person2.getName());
    
      out.write("\r\n");
      out.write("\t\r\n");
      out.write("\t\r\n");
      out.write("    ");
      com.shengqishiwind.bean.Person person3 = null;
      synchronized (session) {
        person3 = (com.shengqishiwind.bean.Person) _jspx_page_context.getAttribute("person3", javax.servlet.jsp.PageContext.SESSION_SCOPE);
        if (person3 == null){
          person3 = new com.shengqishiwind.bean.Person();
          _jspx_page_context.setAttribute("person3", person3, javax.servlet.jsp.PageContext.SESSION_SCOPE);
        }
      }
      out.write("\r\n");
      out.write("\r\n");
      out.write("  \r\n");
      out.write("\r\n");
    } catch (java.lang.Throwable t) {
      if (!(t instanceof javax.servlet.jsp.SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          try { 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);
    }
  }
}

JavaBeanTest_jsp.java

　　其中，关键代码对应如下：

　　导入语句对应：

import com.shengqishiwind.bean.Person;

　　生成对象的语句对应：

 com.shengqishiwind.bean.Person person = null;
      synchronized (session) {
        person = (com.shengqishiwind.bean.Person) _jspx_page_context.getAttribute("person", javax.servlet.jsp.PageContext.SESSION_SCOPE);
        if (person == null){
          person = new com.shengqishiwind.bean.Person();
          _jspx_page_context.setAttribute("person", person, javax.servlet.jsp.PageContext.SESSION_SCOPE);
        }
      }

　　获取name属性：

 out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getName())));

　　设置name属性：

  org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(_jspx_page_context.findAttribute("person"), "name", "lisi", null, null, false);

参考资料

　　圣思园张龙老师Java Web视频教程。

web JSP servlet JavaBean

JavaBean 基础概念、使用实例及代码分析

JavaBean 基础概念、使用实例及代码分析

JavaBean的概念

JSP与JavaBean搭配使用的优点

JavaBean的特征

JSP访问JavaBean的语法

一个JavaBean使用的例子

参考资料

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