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。

　　<jsp:useBean>标签用来声明JavaBean对象，

　　例如： 

 

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<jsp:getProperty>标签。

　　例如：

 

　　如果要给JavaBean的某个属性赋值，可以用<jsp:setProperty>标签，

　　例如：

  

　　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<jsp:setProperty>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：

　　获取对象属性：

         name: 
     
    age: 
     
    address: 
     

　　设置对象属性：

　　

　　关于声明对象，也可以用脚本段来声明，其作用是一样的：

         
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：

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

 

 　　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+"/";%>                        
          
          
              
          
          
                  
          
          
          name: 
      
    age: 
      
    address: 
      
        
          
          
          
          
          
          
          
          
              
          name after change: 
      
    age after change: 
      
    address after change: 
      
        
          
name again: <%= person.getName()%>
        
          
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());    %>        
          
        

 

　　运行时输入地址（注意加上请求参数）：

　　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("    
          \r\n");      out.write("    \r\n");      out.write("    
        \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);    }  }}
       
      

 

　　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。