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。