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：
<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<jsp:getProperty>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<jsp:setProperty>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<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：
<!-- 声明JavaBean对象 --> <jsp:useBean id="person" class="com.shengqishiwind.bean.Person" scope="session"/>
获取对象属性：
<!-- 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：
<!-- 设置对象姓名属性 --> <jsp:setProperty name="person" property="name" value="lisi"/> <!-- 设置对象年龄属性 --> <jsp:setProperty name="person" property="age" value="30"/> <!-- 这里有自动的类型转换：String2int--> <jsp:setProperty name="person" property="age" value="<%=20+20 %>"/> <!-- value值还可以用表达式 --> <jsp:setProperty name="person" property="address" param="hello"/> <!-- 可以通过请求参数的值来赋值,如果不传递这个参数，则该属性仍是原值 -->
关于声明对象，也可以用脚本段来声明，其作用是一样的：
<!-- 脚本段声明对象 --> <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：
<!-- 用表达式输出对象属性 -->
<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 page</title> <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"> <!-- <link rel="stylesheet" type="text/css" href="styles.css"> --> </head> <body> <!-- 声明JavaBean对象 --> <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> <!-- 设置对象姓名属性 --> <jsp:setProperty name="person" property="name" value="lisi"/> <!-- 设置对象年龄属性 --> <jsp:setProperty name="person" property="age" value="30"/> <!-- 这里有自动的类型转换：String2int--> <jsp:setProperty name="person" property="age" value="<%=20+20 %>"/> <!-- value值还可以用表达式 --> <jsp:setProperty name="person" property="address" param="hello"/> <!-- 可以通过请求参数的值来赋值,如果不传递这个参数，则该属性仍是原值 --> <!-- 再次获取对象属性 --> 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> <!-- 用表达式输出对象属性 --> <br>name again: <%= person.getName()%><br> <!-- 脚本段声明对象 --> <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()); %> <!-- 再声明一个Person类对象，注意id必须和前面的不同，声明的是同一个类的不同的对象--> <jsp:useBean id="person3" class="com.shengqishiwind.bean.Person" scope="session"/> </body> </html>
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<java.lang.String,java.lang.Long> _jspx_dependants; private javax.el.ExpressionFactory _el_expressionfactory; private org.apache.tomcat.InstanceManager _jsp_instancemanager; public java.util.Map<java.lang.String,java.lang.Long> 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(' '); out.write(' '); String path = request.getContextPath(); String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; out.write(" "); out.write(" "); out.write("<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> "); out.write("<html> "); out.write(" <head> "); out.write(" <base href=""); out.print(basePath); out.write(""> "); out.write(" "); out.write(" <title>My JSP 'JavaBeanTest.jsp' starting page</title> "); out.write(" "); out.write(" <meta http-equiv="pragma" content="no-cache"> "); out.write(" <meta http-equiv="cache-control" content="no-cache"> "); out.write(" <meta http-equiv="expires" content="0"> "); out.write(" <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"> "); out.write(" <meta http-equiv="description" content="This is my page"> "); out.write(" <!-- "); out.write(" <link rel="stylesheet" type="text/css" href="styles.css"> "); out.write(" --> "); out.write(" "); out.write(" </head> "); out.write(" "); out.write(" <body> "); out.write(" <!-- 声明JavaBean对象 --> "); 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(" "); out.write(" <!-- 获取对象属性 --> "); out.write(" name: "); out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getName()))); out.write("<br> "); out.write(" age: "); out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getAge()))); out.write("<br> "); out.write(" address: "); out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getAddress()))); out.write("<br> "); out.write(" "); out.write(" <!-- 设置对象姓名属性 --> "); out.write(" "); org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(_jspx_page_context.findAttribute("person"), "name", "lisi", null, null, false); out.write(" "); out.write(" <!-- 设置对象年龄属性 --> "); out.write(" "); org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(_jspx_page_context.findAttribute("person"), "age", "30", null, null, false); out.write(" <!-- 这里有自动的类型转换：String2int--> "); out.write(" "); org.apache.jasper.runtime.JspRuntimeLibrary.handleSetProperty(_jspx_page_context.findAttribute("person"), "age", 20+20 ); out.write(" <!-- value值还可以用表达式 --> "); out.write(" "); org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(_jspx_page_context.findAttribute("person"), "address", request.getParameter("hello"), request, "hello", false); out.write(" <!-- 可以通过请求参数的值来赋值,如果不传递这个参数，则该属性仍是原值 --> "); out.write(" "); out.write(" <!-- 再次获取对象属性 --> "); out.write(" name after change: "); out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getName()))); out.write("<br> "); out.write(" age after change: "); out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getAge()))); out.write("<br> "); out.write(" address after change: "); out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getAddress()))); out.write("<br> "); out.write(" "); out.write(" <!-- 用表达式输出对象属性 --> "); out.write(" <br>name again: "); out.print( person.getName()); out.write("<br> "); out.write(" "); out.write(" <!-- 脚本段声明对象 --> "); out.write(" <br>another person: "); out.write(" "); 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(" "); out.write(" "); out.write(" <!-- 再声明一个Person类对象，注意id必须和前面的不同，声明的是同一个类的不同的对象--> "); 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(" "); out.write(" "); out.write(" </body> "); out.write("</html> "); } 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。