jsp - 如何在JSP中执行验证并以相同的形式显示错误消息？

Question

当用户提交错误的输入时，如何在同一个 JSP 中显示错误消息？我不打算抛出异常并显示错误页面。

Answer 1

最简单的方法是在 JSP 中为验证错误消息添加占位符。

JSP /WEB-INF/foo.jsp :

<form action="${pageContext.request.contextPath}/foo" method="post">
    <label for="foo">Foo</label>
    <input id="foo" name="foo" value="${fn:escapeXml(param.foo)}">
    <span class="error">${messages.foo}</span>
    <br />
    <label for="bar">Bar</label>
    <input id="bar" name="bar" value="${fn:escapeXml(param.bar)}">
    <span class="error">${messages.bar}</span>
    <br />
    ...
    <input type="submit">
    <span class="success">${messages.success}</span>
</form>

在 servlet您将表单提交到的位置，可以使用 Map<String, String>获取要在 JSP 中显示的消息。

Servlet @WebServlet("foo") :

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.getRequestDispatcher("/WEB-INF/foo.jsp").forward(request, response);
}

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    Map<String, String> messages = new HashMap<String, String>();
    request.setAttribute("messages", messages); // Now it's available by ${messages}

    String foo = request.getParameter("foo");
    if (foo == null || foo.trim().isEmpty()) {
        messages.put("foo", "Please enter foo");
    } else if (!foo.matches("\\p{Alnum}+")) {
        messages.put("foo", "Please enter alphanumeric characters only");
    }

    String bar = request.getParameter("bar");
    if (bar == null || bar.trim().isEmpty()) {
        messages.put("bar", "Please enter bar");
    } else if (!bar.matches("\\d+")) {
        messages.put("bar", "Please enter digits only");
    }

    // ...

    if (messages.isEmpty()) {
        messages.put("success", "Form successfully submitted!");
    }

    request.getRequestDispatcher("/WEB-INF/foo.jsp").forward(request, response);
}

如果您创建更多执行或多或少相同的 JSP 页面和 servlet，并开始注意到这毕竟是大量重复的样板代码，那么请考虑使用 MVC 框架。

另请参阅:

• Our Servlets wiki page
• What is the best practice for validating parameters in JSP?