JSP中无法显示对象值求助：登录后展示订单异常

Hey there! Let’s work through why your JSP isn’t showing the OrderSample values—this is a common snag, and we’ll break it down step by step to get your orders displaying properly.

1. Fix Your Database Query First #

Looking at your code snippet, your SQL is pulling from useradmin.users:

String SQL = "SELECT * from useradmin.users";

That’s fetching user data, not order data! You need to target your actual orders table instead (e.g., useradmin.orders). Update the query to select columns relevant to your OrderSample class, like:

String SQL = "SELECT order_id, order_date, total_amount FROM useradmin.orders";

2. Ensure OrderSample is JSP-Friendly #

JSP can only access class properties via public getter methods, so double-check your class setup:

• Make sure all order fields (e.g., orderId, orderDate, totalAmount) match the data types of your database columns.
• Add public getters for every field you want to display. For example:

  public class OrderSample {
      private int orderId;
      private Date orderDate;
      private double totalAmount;
  
      // Mandatory getters for JSP access
      public int getOrderId() { return orderId; }
      public Date getOrderDate() { return orderDate; }
      public double getTotalAmount() { return totalAmount; }
  
      // Setters to populate from ResultSet
      public void setOrderId(int orderId) { this.orderId = orderId; }
      public void setOrderDate(Date orderDate) { this.orderDate = orderDate; }
      public void setTotalAmount(double totalAmount) { this.totalAmount = totalAmount; }
  }
  

3. Correctly Populate the Order List in Your DAO #

Your showOrders() method needs to properly map the ResultSet to OrderSample objects. I noticed a potential variable name conflict (you named the list and the object both order—that’s confusing!). Fix it like this:

public static ArrayList<OrderSample> showOrders() throws ClassNotFoundException, SQLException, LoginSampleException {
    ArrayList<OrderSample> ordersList = new ArrayList<>(); // Rename list to avoid conflict
    // Use try-with-resources to auto-close connections/statements (JDBC best practice)
    try (Connection con = Connector.connection();
         Statement statement = con.createStatement();
         ResultSet rs = statement.executeQuery("SELECT order_id, order_date, total_amount FROM useradmin.orders")) {

        while (rs.next()) {
            OrderSample order = new OrderSample();
            order.setOrderId(rs.getInt("order_id")); // Match your database column name
            order.setOrderDate(rs.getDate("order_date"));
            order.setTotalAmount(rs.getDouble("total_amount"));
            ordersList.add(order); // Add the populated object to the list
        }
    }
    return ordersList;
}

4. Pass the Order List to JSP Correctly #

In your servlet/controller code (the one handling post-login logic), you need to add the order list to the request scope before forwarding to the JSP:

// After successful login
ArrayList<OrderSample> orders = YourDaoClass.showOrders();
request.setAttribute("orders", orders); // Store the list in request scope
request.getRequestDispatcher("/orders.jsp").forward(request, response); // Forward to JSP

⚠️ Don’t use response.sendRedirect() here—redirect creates a new request, so the orders attribute will be lost. Stick with forward() to keep the data available.

5. Display the Orders in JSP #

Use JSTL (cleaner than scriptlets) to loop through the list. First, add the JSTL core taglib at the top of your JSP:

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

Then render the orders in a table (or any format you prefer):

<h2>Your Orders</h2>
<table border="1">
    <tr>
        <th>Order ID</th>
        <th>Order Date</th>
        <th>Total Amount</th>
    </tr>
    <c:forEach items="${orders}" var="order">
        <tr>
            <td>${order.orderId}</td> <!-- Uses getOrderId() behind the scenes -->
            <td>${order.orderDate}</td>
            <td>${order.totalAmount}</td>
        </tr>
    </c:forEach>
</table>

If you can’t use JSTL, you can use scriptlets (though not recommended for modern JSP):

<%
    ArrayList<OrderSample> orders = (ArrayList<OrderSample>) request.getAttribute("orders");
    if (orders != null && !orders.isEmpty()) {
        for (OrderSample order : orders) {
%>
            <p>Order ID: <%= order.getOrderId() %> | Date: <%= order.getOrderDate() %></p>
<%
        }
    } else {
%>
            <p>No orders found.</p>
<%
    }
%>

6. Quick Debugging Tips #

• Check if the orders list is empty: Add a log statement in your servlet to print orders.size()—if it’s 0, your query isn’t returning any data.
• Catch exceptions: Wrap the showOrders() call in a try-catch block to log any errors that might be preventing the list from being populated.
• Enable EL expressions: If ${order.orderId} isn’t working, add <%@ page isELIgnored="false" %> to the top of your JSP (default is false in modern containers, but worth verifying).

内容的提问来源于stack exchange，提问作者jendellkenner