java - 提交响应后无法转发 Servlet 错误

Question

当我运行此代码时，我收到 IllegalStateException: Cannotforward after respons has been commited。抱歉，我是 servlet 新手。我应该怎么做才能解决这个问题？这发生在这一行:

RequestDispatcher rd = request.getRequestDispatcher("/displayjobs.jsp");
rd.forward(request, response);

这是异常的堆栈跟踪:

java.lang.IllegalStateException: Cannot forward after response has been committed
schedule.doGet(schedule.java:71)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)

这是我的 servlet 代码的其余部分。 编辑:我还包含了我的 jsp 代码，因为我现在在第 2 行遇到了 NPE:

1 List data = (List)request.getAttribute("jobsData);
2 for(itr = data.iterator(); itr.hasNext();)

这意味着由于某种原因数据为空，而不是具有 JobsData 的信息。

/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    response.setContentType("text/html;charset=UTF-8");
    PrintWriter out = response.getWriter();
    try{
        Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver"); 
        String connectionUrl = "jdbc:sqlserver://10.11.1.246;databaseName=Test;integratedSecurity=false;user=sa;password=S0l1dConcepts";
        Connection con = DriverManager.getConnection(connectionUrl);
        String query = "select * from jobs";
        stmt = con.createStatement();
        res = stmt.executeQuery(query);
        while(res.next()){
            list.add(res.getString(1));
            list.add(res.getString(2));
            list.add(res.getString(3));
            list.add(res.getString(4));
            list.add(res.getString(5));
            list.add(res.getString(6));
            list.add(res.getString(7));
            list.add(res.getString(8));
            list.add(res.getString(9));
            list.add(res.getString(10));
        }
        res.close();
    }
    catch(Exception e){
        RequestDispatcher rd = request.getRequestDispatcher("/error.jsp");
        rd.forward(request, response);
    }
finally{
        request.setAttribute("jData", list);
        RequestDispatcher rd = request.getRequestDispatcher("/displayjobs.jsp");
        rd.forward(request, response);
        list.clear();
        out.close();
}

}


/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    response.setContentType("text/html;charset=UTF-8");
    PrintWriter out = response.getWriter();
    try{
        Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver"); 
        String connectionUrl = "jdbc:sqlserver://10.11.1.246;databaseName=Test;integratedSecurity=false;user=sa;password=S0l1dConcepts";
        Connection con = DriverManager.getConnection(connectionUrl);
        String query = "select * from jobs";
        stmt = con.createStatement();
        res = stmt.executeQuery(query);
        while(res.next()){
            list.add(res.getString(1));
            list.add(res.getString(2));
            list.add(res.getString(3));
            list.add(res.getString(4));
            list.add(res.getString(5));
            list.add(res.getString(6));
            list.add(res.getString(7));
            list.add(res.getString(8));
            list.add(res.getString(9));
            list.add(res.getString(10));
        }
        res.close();
    }
    catch(Exception e){
        RequestDispatcher rd = request.getRequestDispatcher("/error.jsp");
        rd.forward(request, response);
    }
    finally{
        request.setAttribute("jData", list);
        RequestDispatcher rd = request.getRequestDispatcher("/displayjobs.jsp");
        rd.forward(request, response);
        list.clear();
        out.close();
    }

}

}

这是 JSP 代码:

<%@page import="java.util.List" %>
<%@page import="java.util.Iterator" %>
<%@ 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>Scheduling</title>
</head>
<body>
    <table>
    <%Iterator itr;%>
    <%List data = (List)request.getAttribute("jobsData");
    for(itr = data.iterator(); itr.hasNext();){
        %>
    <tr>    
        <% String s = (String) itr.next();%>
        <td><%=s%></td>
        <td><%=itr.next()%></td>
        <td><%=itr.next()%></td>
        <td><%=itr.next()%></td>
        <td><%=itr.next()%></td>
        <td><%=itr.next()%></td>
        <td><%=itr.next()%></td>
        <td><%=itr.next()%></td>
        <td><%=itr.next()%></td>
        <td><%=itr.next()%></td>
        <td><input type="submit" value="Edit" name="edit" onclick ="editRecord(<%=s%> %>);"></td>
        <td><input type="submit" value="Delete" name="delete" onclick ="deleteRecord(<%=s%> %>);"></td>
        <%} %>
        </tr>
</table>

Answer 1

问题很简单。异常被捕获，响应从 catch block 委托(delegate)，并再次从 finally block 委托(delegate)。

您应该做的是有一个地方来委托(delegate)您的响应

请注意:- 响应只能提交一次

boolean exceptionOcured =false;    
try{
// your code
}
catch(Exception e){
  exceptionOcured  =true;
}
finally{
// your code to release resources
}

if(exceptionOcured){
 RequestDispatcher rd = request.getRequestDispatcher("/error.jsp");
        rd.forward(request, response);
}else{
RequestDispatcher rd = request.getRequestDispatcher("/displayjobs.jsp");
        rd.forward(request, response);
}