JSP-javabean返回null，但是为什么呢? [英] JSP - javabean returns null, but why?
问题描述
当用户按下JSP上的登录按钮时，请求将提交到我的servlet，该服务器通过getParameter()从登录表单中获取输入的用户名和密码.
When an user presses the login button on a JSP, the request is submitted to my servlet which gets the entered username and password from the login form by getParameter().
用户名和密码存储在javabean中.如果我放置一个println()语句来显示bean.getUsername()和bean.getPassword()的值，那么它将在日志中打印用户名和密码.
The username and password are stored in a javabean. If I put a println() statement which shows the values of bean.getUsername() and bean.getPassword(), then it prints the username and password in the log.
但是我的母版页上的以下代码段
But the following code snippet on my master page
<jsp:useBean id="bean" class="package.Bean" scope="session" />
<jsp:getProperty name="bean" property="username" />
显示null而不是用户名.我在做什么错了?
shows null instead of the username. What am I doing wrong?
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
String username = request.getParameter("username");
String password = request.getParameter("password");
// Store username and password in bean from login.jsp
Bean bean = new bean();
bean.setUsername(username);
bean.setPassword(password);
// check login details with database
if (login success)
{
HttpSession session = request.getSession();
session.setAttribute("username", username);
// redirect user to welcome page
}
else
// login failed
}
推荐答案
可能您没有让servlet在会话范围内存储Bean.但是由于无论如何都在使用servlet，因此请忘记<jsp:useBean>.从技术上讲，它毫无价值.还将模型与视图紧密耦合.
Likely you haven't let the servlet store the bean in the session scope. But since you're using servlets anyway, forget the <jsp:useBean> thing. It's technically worthless. It also tight-couples the model with the view.
只需让servlet将bean置于会话范围内即可:
Just let the servlet put the bean in session scope:
User user = userDAO.find(username, password);
if (user != null) {
request.getSession().setAttribute("user", user); // Store logged-in user in session.
response.sendRedirect("home"); // Redirect to login home page or whatever.
} else {
request.setAttribute("message", "Unknown login, please try again"); // Store error message for redisplay in login page.
request.getRequestDispatcher("/WEB-INF/login.jsp").forward(request, response); // Forward to login page.
}
然后在用户主页中，只需使用EL(表达语言)来访问User bean.
Then in the user home page, just use EL (Expression Language) to access the User bean.
<p>Welcome, <c:out value="${user.username}" /></p>
${foo}基本上在页面，请求，会话和应用程序范围内搜索以名称foo存储的属性，并返回第一个非空值.在这种情况下，${user}将引用您存储在会话范围中的User bean. ${user.username}基本上将显示user.getUsername()的返回值.
The ${foo} basically searches for an attribute which is stored with the name foo in page, request, session and application scopes and returns the first non-null value. In this case, the ${user} will refer to your User bean which you've stored in the session scope. The ${user.username} will basically display the return value of user.getUsername().
<c:out>并不是必需的，但是它可以防止您的页面受到潜在的XSS攻击.
The <c:out> is by the way not necessary, but it prevents your page from potential XSS attacks.
- Our JSP wiki page
- Our Servlets wiki page
与具体问题无关，package是关键字，因此是非法的软件包名称.发布过分简化的代码示例时请多加注意；)如果您想混淆软件包名称，请使用com.example或其他名称.
Unrelated to the concrete problem, package is a keyword and thus an illegal package name. Please take care when posting oversimplified code examples ;) Rather use com.example or something if you'd like to obfuscate package names.
更新:根据您的更新，您是在会话作用域中设置用户名，而不是bean本身.即使您可以通过${username}显示它，但是为了能够使用${bean.username}，也需要按照上述答案中的说明进行相应的修复.
Update: as per your update, you're setting the username in the session scope, not the bean itself. Even though you would be able to display it by ${username}, in order to be able to use ${bean.username}, you need to fix it accordingly as described in above answer.
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