JavaBean 基礎概念、使用實例及代碼分析
JavaBean的概念
JavaBean是一種可重復使用的、且跨平台的軟件組件。
JavaBean可分為兩種：一種是有用戶界面的（有UI的）；另一種是沒有用戶界面的（無UI的），無UI的JavaBean主要負責處理事務（如數據運算，操縱數據庫）。
JSP通常訪問的是后一種JavaBean。
JSP與JavaBean搭配使用的優點
使得HTML與Java程序分離，這樣便於維護代碼。
如果把所有的程序代碼都寫到JSP網頁中，會使得代碼繁雜，難以維護。
可以降低開發JSP網頁人員對Java編程能力的要求。
JSP側重於生成動態網頁，事務處理由JavaBean來完成，這樣可以充分利用JavaBean組件的可重用性特點，提高開發網站的效率。
MVC設計模式（Model View Controller）。
JavaBean的特征
一個標准的JavaBean有以下幾個特征：
JavaBean是一個公共（public）的類。
JavaBean有一個不帶參數的構造方法。
JavaBean通過setXXX方法設置屬性，通過getXXX方法獲取屬性。
JSP訪問JavaBean的語法
1.導入JavaBean類。
通過<%@ page import>指令導入JavaBean類，
例如：
<%@ page import="mypack.MyJavaBean" %>
2.聲明JavaBean對象。
<jsp:useBean>標簽用來聲明JavaBean對象，
例如：
<jsp:useBean id="myBean" class="mypack.MyJavaBean" scope="session"/>
3.訪問JavaBean屬性。
JSP提供了訪問JavaBean屬性的標簽，如果要將JavaBean的某個屬性輸出到網頁上，可以用<jsp:getProperty>標簽。
例如：
<jsp:getProperty name="myBean" property="count"/>
如果要給JavaBean的某個屬性賦值，可以用<jsp:setProperty>標簽，
例如：
<jsp:setProperty name="myBean" property="count" value="0"/>
使用指定的值來設置Bean的屬性，這個值可以使字符串，也可以是表達式。
如果是字符串，那么它就會被轉換成Bean屬性的類型。
如果是一個表達式，那么它的類型就必須和將要設定的屬性值的類型一致。
不能在同一個<jsp:setProperty>中使用value和param參數。
其中param是指通過請求參數的值來給屬性賦值。
一個JavaBean使用的例子
寫一個Person類（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中導入這個類，並且聲明該類的對象，然后獲取其屬性或者更改屬性：
導入：
<%@ page language="java" import="java.util.*,com.shengqishiwind.bean.Person" pageEncoding="UTF-8"%>
聲明：
<!-- 聲明JavaBean對象 --> <jsp:useBean id="person" class="com.shengqishiwind.bean.Person" scope="session"/>
獲取對象屬性：
<!-- 獲取對象屬性 --> name: <jsp:getProperty name="person" property="name" /><br> age: <jsp:getProperty name="person" property="age" /><br> address: <jsp:getProperty name="person" property="address" /><br>
設置對象屬性：
<!-- 設置對象姓名屬性 --> <jsp:setProperty name="person" property="name" value="lisi"/> <!-- 設置對象年齡屬性 --> <jsp:setProperty name="person" property="age" value="30"/> <!-- 這里有自動的類型轉換：String2int--> <jsp:setProperty name="person" property="age" value="<%=20+20 %>"/> <!-- value值還可以用表達式 --> <jsp:setProperty name="person" property="address" param="hello"/> <!-- 可以通過請求參數的值來賦值,如果不傳遞這個參數，則該屬性仍是原值 -->
關於聲明對象，也可以用腳本段來聲明，其作用是一樣的：
<!-- 腳本段聲明對象 --> <br>another person: <% Person person2 = new Person();//這里不能把對象叫做person，提示重復的局部變量，進一步說明這種方法和上面的方法，本質上是一樣的 out.println(person2.getName()); %>
所以輸出屬性時也可以直接用表達式：
<!-- 用表達式輸出對象屬性 -->
<br>name again: <%= person.getName()%><br>
完整JSP代碼如下：
<%@ 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> <!-- 聲明JavaBean對象 --> <jsp:useBean id="person" class="com.shengqishiwind.bean.Person" scope="session"/> <!-- 獲取對象屬性 --> name: <jsp:getProperty name="person" property="name" /><br> age: <jsp:getProperty name="person" property="age" /><br> address: <jsp:getProperty name="person" property="address" /><br> <!-- 設置對象姓名屬性 --> <jsp:setProperty name="person" property="name" value="lisi"/> <!-- 設置對象年齡屬性 --> <jsp:setProperty name="person" property="age" value="30"/> <!-- 這里有自動的類型轉換：String2int--> <jsp:setProperty name="person" property="age" value="<%=20+20 %>"/> <!-- value值還可以用表達式 --> <jsp:setProperty name="person" property="address" param="hello"/> <!-- 可以通過請求參數的值來賦值,如果不傳遞這個參數，則該屬性仍是原值 --> <!-- 再次獲取對象屬性 --> 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> <!-- 用表達式輸出對象屬性 --> <br>name again: <%= person.getName()%><br> <!-- 腳本段聲明對象 --> <br>another person: <% Person person2 = new Person();//這里不能把對象叫做person，提示重復的局部變量，進一步說明這種方法和上面的方法，本質上是一樣的 out.println(person2.getName()); %> <!-- 再聲明一個Person類對象，注意id必須和前面的不同，聲明的是同一個類的不同的對象--> <jsp:useBean id="person3" class="com.shengqishiwind.bean.Person" scope="session"/> </body> </html>
運行時輸入地址（注意加上請求參數）：http://localhost:8080/HelloWeb/JavaBeanTest.jsp?hello=shanghai
運行后如圖：
生成的Java源文件
查看Tomcat work路徑下該項目路徑生成的源文件：（JSP會轉化成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('\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("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n"); out.write("<html>\r\n"); out.write(" <head>\r\n"); out.write(" <base href=\""); out.print(basePath); out.write("\"> \r\n"); out.write(" \r\n"); out.write(" <title>My JSP 'JavaBeanTest.jsp' starting page</title>\r\n"); out.write(" \r\n"); out.write("\t<meta http-equiv=\"pragma\" content=\"no-cache\">\r\n"); out.write("\t<meta http-equiv=\"cache-control\" content=\"no-cache\">\r\n"); out.write("\t<meta http-equiv=\"expires\" content=\"0\"> \r\n"); out.write("\t<meta http-equiv=\"keywords\" content=\"keyword1,keyword2,keyword3\">\r\n"); out.write("\t<meta http-equiv=\"description\" content=\"This is my page\">\r\n"); out.write("\t<!--\r\n"); out.write("\t<link rel=\"stylesheet\" type=\"text/css\" href=\"styles.css\">\r\n"); out.write("\t-->\r\n"); out.write("\r\n"); out.write(" </head>\r\n"); out.write(" \r\n"); out.write(" <body>\r\n"); out.write(" \t<!-- 聲明JavaBean對象 -->\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(" <!-- 獲取對象屬性 -->\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("<br>\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("<br>\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("<br>\r\n"); out.write("\t\r\n"); out.write("\t<!-- 設置對象姓名屬性 -->\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<!-- 設置對象年齡屬性 -->\r\n"); out.write("\t"); org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(_jspx_page_context.findAttribute("person"), "age", "30", null, null, false); out.write("\t<!-- 這里有自動的類型轉換：String2int-->\r\n"); out.write("\t"); org.apache.jasper.runtime.JspRuntimeLibrary.handleSetProperty(_jspx_page_context.findAttribute("person"), "age", 20+20 ); out.write("\t<!-- value值還可以用表達式 -->\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<!-- 可以通過請求參數的值來賦值,如果不傳遞這個參數，則該屬性仍是原值 -->\r\n"); out.write("\t\r\n"); out.write("\t<!-- 再次獲取對象屬性 -->\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("<br>\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("<br>\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("<br>\r\n"); out.write("\t\r\n"); out.write("\t<!-- 用表達式輸出對象屬性 -->\r\n"); out.write("\t<br>name again: "); out.print( person.getName()); out.write("<br>\r\n"); out.write("\t\r\n"); out.write("\t<!-- 腳本段聲明對象 -->\r\n"); out.write("\t<br>another person:\r\n"); out.write("\t"); Person person2 = new Person();//這里不能把對象叫做person，提示重復的局部變量，進一步說明這種方法和上面的方法，本質上是一樣的 out.println(person2.getName()); out.write("\r\n"); out.write("\t\r\n"); out.write("\t<!-- 再聲明一個Person類對象，注意id必須和前面的不同，聲明的是同一個類的不同的對象-->\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(" </body>\r\n"); out.write("</html>\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，關鍵代碼對應如下：
導入語句對應：
import com.shengqishiwind.bean.Person;
生成對象的語句對應：
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); } }
獲取name屬性：
out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.shengqishiwind.bean.Person)_jspx_page_context.findAttribute("person")).getName())));
設置name屬性：
org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(_jspx_page_context.findAttribute("person"), "name", "lisi", null, null, false);
參考資料
聖思園張龍老師Java Web視頻教程。