Servlet異常處理：使用不同的消息轉發到同一錯誤頁面

Question

如何在Servlet中使用不同的消息轉發到同一錯誤頁面時處理異常？ 在我的servlet中，我具有以下doGet（在此簡化）方法，並帶有切換用例：

public class NewServlet extends HttpServlet {
//[...]
protected void doGet(
    //[...]
        try {
            DAO ldapdao = new DAO();
            List<Entry> entries = null;

            switch (enumPage.fromString(operation)) {                    
//  [...]
                case add:
                    String values[] = request.getParameterValues("item");
                    try {
                        ldapdao.addentry(values);
                        link = "entryadded.jsp";
                    } catch (Exception exp) {                                   // 2                            
                        throw new MyException("Entry Not Added");
                    }
                    break;


                case remove:
                    entries = ldapdao.searchEntry(request.getParameter("item"));
                    if (entries.isEmpty()) {
                        throw new MyException("Entry Not Removed");                              //3
                    } else {
                        ldapdao.remove(request.getParameter("item"));
                        link = "entryremoved.jsp";
                    }
                    break;

                case modified:
                    String values1[] = request.getParameterValues("item");
                    try {
                        ldapdao.modify(values1);
                        link = ("modified.jsp");
                    } catch (Exception exp) {                                   //4
                        throw new MyException("Entry Not Modified");
                    }
//  [...]

然后有我的自定義異常：

package com.mycompany.test_servlet;
import javax.servlet.ServletException;
public class MyException extends ServletException {
private String message = null;
public MyException() {
    super();
}
public MyException(String message) {
    super(message);
}
public MyException(Throwable cause) {
    super(cause);        
}    
}

和web.xml文件：

<error-page>
    <location>/Error.jsp</location>
</error-page>

Answer 1

嘗試指出異常類型：

<error-page>
    <exception-type>com.mycompany.test_servlet.MyException</exception-type>
    <location>/Error.jsp</location>
</error-page>

Answer 2

您可以嘗試在請求對象中設置屬性，同時捕獲異常

request.setAttribute("exception", ex.getMessage());

然后在error.jsp中獲取此屬性，例如：

                        <% if (request.getAttribute("exception") != null) {  %>
                        <tr>
                            <td colspan='2'><p align=right>Authentication Error (try again):<br>
                                <%=request.getAttribute("exception")%>
                            </p></td>
                        </tr>
                        <%}%>