JavaBean 基础概念、使用实例及代码分析 |
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JavaBean 基础概念、使用实例及代码分析
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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。 通过指令导入JavaBean类， 例如：
2.声明JavaBean对象。 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，可以用标签。 例如：
如果要给JavaBean的某个属性赋值，可以用标签， 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： 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:
所以输出属性时也可以直接用表达式： name again:
完整JSP代码如下：  
DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
My JSP 'JavaBeanTest.jsp' starting page
name:
age:
address:
name after change:
age after change:
address after change:
name again:
another person:
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;");
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(" 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("\tname again: ");
out.print( person.getName());
out.write("\r\n");
out.write("\t\r\n");
out.write("\t\r\n");
out.write("\tanother 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。
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