一、什麼是JavaBean？
JavaBean是一個遵循特定寫法的Java類，它通常具有如下的特點：
這個Java類必須具有一個無參數的構造方法。
屬性私有化。
私有化的屬性化必須通過public類型的方法暴露給其他程序，並且方法的命名也必須遵守一定的命名規範。
JavaBean在JavaEE開中中，通常用於封裝數據，對於遵循以上寫法的JavaBean組件，其他程序可以通過反射技術實例化JavaBean對象，並且通過反射哪些遵守命名規範的方法，從而獲取JavaBean的屬性，進而調用其屬性保存數據。
二、JavaBean的屬性
JavaBean的屬性可以是任意類型，並且一個JavaBean可以有多個屬性。每個屬性通常都需要具有對應的setter方法和getter方法，setter方法稱爲屬性修改器，getter方法稱爲屬性訪問器。
屬性修改器必須以小寫的set前綴開頭，後跟屬性名，並且屬性名的第一個字母必須要大寫。
屬性訪問器通常以小寫的get前綴開始，後跟屬性名，並且屬性名的第一個字母必須大寫。
一個JavaBean的某個屬性也可以只有setter方法或者getter方法，這樣的屬性通常也稱爲只寫、只讀屬性。
總結：是setter方法和getter方法，成就屬性，並不是field都是屬性。
package cn.vo;
public class User {
private String username;
private String password;
} 上面的類嚴格意義上是沒有任何屬性，但是如果要說有的話，那就只有class，爲什麼呢，因爲每個類都繼承自Object，而Object有一個getClass()方法。
package cn.vo;
public class User {
private String username;
private String password;
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
} 而此時的User類，有三個屬性啊。
三、JavaBean在Servlet的時代
我們知道Servlet在Javaweb體系中是首先出現的，所以下面我們來模擬場景。
User.java
package cn.vo;
public class User {
private String username;
private String password;
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
}login.jsp
<%@ page language="java" import="java.util.*" 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 'login.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>
<form action="${request.servletContext}/login" method="post">
<table>
<tr>
<td>用戶名</td>
<td>
<input type="text" name="username"/>
</td>
</tr>
<tr>
<td>密碼</td>
<td>
<input type="password" name="password"/>
</td>
</tr>
<tr>
<td colspan="2">
<input type="submit" value="登錄"/>
</td>
</tr>
</table>
</form>
</body>
</html>LoginServlet.java
package cn.Introspector;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import cn.vo.User;
@SuppressWarnings("serial")
public class LoginServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//設置請求的編碼
request.setCharacterEncoding("utf-8");
//設置響應的編碼
response.setContentType("text/html;charset=utf-8");
//獲取用戶名
String username = request.getParameter("username");
//獲取密碼
String password = request.getParameter("password");
//實例化User
User vo = new User();
vo.setUsername(username);
vo.setUsername(password);
response.getWriter().print("姓名:"+username+",密碼："+password);
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.doGet(request, response);
}
}這時，可能有人會想就兩個屬性，爲什麼封裝到對象中，這樣不是很煩，不是的哦，如果這個類有50個屬性，那麼我們一個一個接收，很煩的啊，所以，我們將數據封裝到JavaBean中，然後傳遞JavaBean，這樣是非常方便的。
四、JavaBean在jsp的時代
隨着時代的發展，我們知道Servlet有許多不足，所以，sun公司就推出了jsp技術。那麼JavaBean在jsp時代又有怎麼樣的變化呢？
login.jsp
<%@ page language="java" import="java.util.*" 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 'login.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> <form action="/day11/loginJsp.jsp" method="post"> <table> <tr> <td>用戶名</td> <td> <input type="text" name="username"/> </td> </tr> <tr> <td>密碼</td> <td> <input type="password" name="password"/> </td> </tr> <tr> <td colspan="2"> <input type="submit" value="登錄"/> </td> </tr> </table> </form> </body> </html>
loginJsp.jsp
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <title>My JSP 'loginJsp.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> <!-- class屬性：寫封裝數據類的全路徑，用於獲取反射的Class類，以便來實例化對象 --> <jsp:useBean id="u" class="cn.vo.User"></jsp:useBean> <!-- property屬性：要和表單中的對應的name相同，這樣才能將表單對應對應的數據封裝到對象之中 --> <jsp:setProperty property="username" name="u"/> <jsp:setProperty property="password" name="u"/> <jsp:getProperty property="username" name="u"/> <jsp:getProperty property="password" name="u"/> </body> </html>
其實，這個時候，我們就應該有點看出sun公司的意圖了，那就是將表單的數據封裝到對象之中，來傳遞。但是這種模式很快就要被淘汰了，因爲MVC出現了，MVC的V讓jsp來顯示，C是讓Servlet來充當了。但是，JavaBean從發展而來的種種表明，將數據封裝到JavaBean是一條正確之路。
五、JavaBean在MVC2.0時代
通過Introspector類獲取Bean對象的BeanInfo，然後通過BeanInfo類來獲取屬性的描述器（PropertyDescriptor），通過這個屬性描述器就可以獲取某個屬性對應的getter/setter方法，然後通過反射機制來調用這些方法。
login.jsp
<%@ page language="java" import="java.util.*" 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 'login.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> <form action="/day11/login" method="post"> <table> <tr> <td>用戶名</td> <td> <input type="text" name="username"/> </td> </tr> <tr> <td>密碼</td> <td> <input type="password" name="password"/> </td> </tr> <tr> <td colspan="2"> <input type="submit" value="登錄"/> </td> </tr> </table> </form> </body> </html>
LoginServlet.java
package cn.Introspector;
import java.beans.BeanInfo;
import java.beans.IntrospectionException;
import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Map;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import cn.vo.User;
@SuppressWarnings("serial")
public class LoginServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.setCharacterEncoding("utf-8");
response.setContentType("text/html;charset=utf-8");
Map<String, String[]> parameterMap = request.getParameterMap();
User user = new User();
try {
BeanInfo info = Introspector.getBeanInfo(user.getClass());
PropertyDescriptor[] propertyDescriptors = info.getPropertyDescriptors();
for (PropertyDescriptor propertyDescriptor : propertyDescriptors) {
if(!propertyDescriptor.getName().equals("class")){
if(parameterMap.containsKey(propertyDescriptor.getName())){
Method writeMethod = propertyDescriptor.getWriteMethod();
writeMethod.invoke(user, parameterMap.get(propertyDescriptor.getName())[0]);
}
}
}
} catch (IntrospectionException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalAccessException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalArgumentException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (InvocationTargetException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
response.getWriter().println("姓名:"+user.getUsername()+",密碼："+user.getPassword());
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.doGet(request, response);
}
}User.java
package cn.vo;
public class User {
private String username;
private String password;
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
}六、BeanUtils工具包
雖然，上面已經實現了功能，但是在開發中我們還會遇到許多問題，比如多選框等等，我們都沒有考慮。
Apache組織開發了一套用於操作JavaBean的API，這套API考慮到了很多實際開發中的應用場景，一次，在實際開發之中很多程序員使用這套API操作JavaBean，以簡化程序代碼的編寫。
BeanUtils工具包常用類。
BeanUtils：
populate(Object bean,Map properties)
自定義轉換器：
ConvertUtils.register(Converter convert,Class clazz)
傳入日期類型的Date.class
login.jsp
<%@ page language="java" import="java.util.*" 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 'login.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> <form action="/day11/login" method="post"> <table> <tr> <td>用戶名</td> <td> <input type="text" name="username"/> </td> </tr> <tr> <td>密碼</td> <td> <input type="password" name="password"/> </td> </tr> <tr> <td colspan="2"> <input type="submit" value="登錄"/> </td> </tr> </table> </form> </body> </html>
User.java
package cn.vo;
public class User {
private String username;
private String password;
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
}LoginServlet.java
package cn.Introspector;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.util.Map;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.commons.beanutils.BeanUtils;
import cn.vo.User;
@SuppressWarnings("serial")
public class LoginServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.setCharacterEncoding("utf-8");
response.setContentType("text/html;charset=utf-8");
Map<String, String[]> parameterMap = request.getParameterMap();
User user = new User();
try {
BeanUtils.populate(user, parameterMap);
} catch (IllegalAccessException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (InvocationTargetException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
response.getWriter().println("姓名:"+user.getUsername()+",密碼："+user.getPassword());
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.doGet(request, response);
}
}日期轉換器
login.jsp
<%@ page language="java" import="java.util.*" 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 'login.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> <form action="/day11/login" method="post"> <table> <tr> <td>用戶名</td> <td> <input type="text" name="username"/> </td> </tr> <tr> <td>密碼</td> <td> <input type="password" name="password"/> </td> </tr> <tr> <td>生日</td> <td> <input type="text" name="birthday"/> </td> </tr> <tr> <td colspan="2"> <input type="submit" value="登錄"/> </td> </tr> </table> </form> </body> </html>
User.java
package cn.vo;
import java.util.Date;
public class User {
private String username;
private String password;
private Date birthday;
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
}DateConverter.java
package cn.util;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import org.apache.commons.beanutils.Converter;
public class DateConverter implements Converter {
@Override
public Object convert(Class claza, Object obj) {
if(obj instanceof String){
String date = (String) obj;
try {
return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse(date);
} catch (ParseException e) {
e.printStackTrace();
}
}
return null;
}
}LoginServlet.java
package cn.Introspector;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.util.Date;
import java.util.Map;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.commons.beanutils.BeanUtils;
import org.apache.commons.beanutils.ConvertUtils;
import cn.util.DateConverter;
import cn.vo.User;
@SuppressWarnings("serial")
public class LoginServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.setCharacterEncoding("utf-8");
response.setContentType("text/html;charset=utf-8");
Map<String, String[]> parameterMap = request.getParameterMap();
User user = new User();
ConvertUtils.register(new DateConverter(), Date.class);
try {
BeanUtils.populate(user, parameterMap);
} catch (IllegalAccessException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (InvocationTargetException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
response.getWriter().println("姓名:"+user.getUsername()+",密碼："+user.getPassword()+"，生日："+user.getBirthday());
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.doGet(request, response);
}
}