Question

我目前正在学习JSP / Servelets。以下是包含2个JSP页面和servlet的代码，其工作方式如下：

JSP＃1请求用户信息，＆amp;将它传递给servlet。
servlet实例化一个JavaBean类，并使用 Session 对象将其传递给第二个JSP。
然后第二个JSP显示用户在第一个JSP中输入的信息;然后，用户可以点击返回按钮返回第一个JSP。

我的问题是：我已经阅读了各种教程（尤其是Murach的JSP，这段代码所基于的），如果将Javabean属性传递给会话对象而不是请求对象，应该在整个会话期间维护JavaBean的值。但是当我返回第一页时，JB字段为空。我做错了吗？我将不胜感激任何帮助！

以下是代码：

第一个JSP：

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"     "http://www.w3.org/TR/html4/loose.dtd">
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
        <title>Email Application</title>
    </head>

    <body>
    <h1>Join our Email list</h1>
    <p>To join our email list, enter your name and email address below.<br>
    Then, click on the Submit button</p>

    <form action="addToEmailList" method="post">
    <jsp:useBean id="user" scope="session" class="business.User"/>
        <table cellspacing="5" border="0">
            <tr>
                <td align="right">First name:</td>
                <td>
                    <input type="text" name="firstName"
                    value = "<jsp:getProperty name="user" property="firstName"/>">
                </td>
            </tr>
            <tr>
                <td align="right">Last name:</td>
                <td><input type="text" name="lastName"
                    value = "<jsp:getProperty name="user" property="lastName"/>">
                </td>
            </tr>
            <tr>
                <td align="right">Email address:</td>
                <td><input type="text" name="emailAddress"
                    value = "<jsp:getProperty name="user" property="emailAddress"/>">
                </td>
            </tr>
            <tr>
                <td>I'm interested in these types of music:</td>
                <td><select name="music" multiple>
                        <option value="rock">Rock</option>
                        <option value="country">Country</option>
                        <option value="bluegrass">Bluegrass</option>
                        <option value="folkMusic">Folk Music</option>
                    </select>
                </td>
            </tr>
            <tr>
                <td ></td>
                <td><input type="submit" name="Submit"></td>
            </tr>
        </table>
    </form>

</body>

的Servlet

public class AddToEmailListServlet extends HttpServlet
{
private static final long serialVersionUID = 1L;
private HttpServletRequest request;
private HttpServletResponse response;
private String firstName;
private String lastName;
private String emailAddress;
private String[] musicTypes;
private Music music;
private User user;

protected void doPost(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException
{
    this.request = request;
    this.response = response;
    setInstanceVariables();
    writeDataToFile();
    forwardResponseToJSPpage();
    System.err.println("Hello!");
}

private void setInstanceVariables()
{
    this.firstName = getFirstName();
    this.lastName = getLastName();
    this.emailAddress = getEmailAddress();
    this.musicTypes =  request.getParameterValues("music"); //getMusic();
    this.user = new User(firstName,lastName,emailAddress);
}

private String getFirstName()
{
    return  request.getParameter("firstName");
}

private String getLastName()
{
    return request.getParameter("lastName");
}

private String getEmailAddress()
{
    return request.getParameter("emailAddress");
}


private void writeDataToFile() throws IOException
{
    String path = getRelativeFileName();
    UserIO.add(user,path);
}

private String getRelativeFileName()
{
    ServletContext sc = getServletContext();
    String path = sc.getRealPath("/WEB-INF/EmailList.txt");
    return path;
}

private void forwardResponseToJSPpage() throws ServletException, IOException
{
    if(this.emailAddress.isEmpty()||this.firstName.isEmpty()||this.lastName.isEmpty())
    {
        String url = "/validation_error.jsp";
        RequestDispatcher dispatcher = 
                getServletContext().getRequestDispatcher(url);
        dispatcher.forward(request,response);
    }
    else
    {
    music = new Music(musicTypes); 

    HttpSession session = this.request.getSession();
    session.setAttribute("music", music);
    session.setAttribute("user", user);

    String url = "/display_email_entry.jsp";
    RequestDispatcher dispatcher = 
            getServletContext().getRequestDispatcher(url);
    dispatcher.forward(request,response);
    }
}

protected void doGet(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException
{
    doPost(request,response);
}

}

第二个JSP：

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"     "http://www.w3.org/TR/html4/loose.dtd">
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ include file="/header.html" %>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
    <title>Email Application</title>
</head>
    <body>
    <h1>Thanks for joining our email list</h1>

     <p>Here is the information that you entered:</p>

<jsp:useBean id="user" scope="session" class="business.User"/>
<table cellspacing="5" cellpadding="5" border="1">
    <tr>
        <td align="right">First name:</td>
        <td><jsp:getProperty name="user" property="firstName"/></td>
    </tr>
    <tr>
        <td align="right">Last name:</td>
        <td><jsp:getProperty name="user" property="lastName"/></td>
    </tr>
    <tr>
        <td align="right">Email Address:</td>
        <td><jsp:getProperty name="user" property="emailAddress"/></td>
    </tr>
</table>


<p>We'll use the Email to otify you whenever we have new releases of the following types of music:</p>

<c:forEach items="${music.musicTypes}" var="i">
    <c:out value="${i}"></c:out><br/>
</c:forEach>

<p>To enter another email address, click on the Back <br>
button in your browser or the Return button shown <br>
below.</p>

<form action="join_email_list.jsp" method="post">
    <input type="submit" value="Return">
</form>
</body>
<%@ include file="/footer.html" %>
</html>

整个会话期间都没有记住JavaBean

0 个答案: