java请求。getAttribute返回null

Question

我在servlet中写过

Map<String, Integer> amounts = new HashMap<String, Integer>();
if(req.getParameter("from").equals("details")){


    employeeInformation.put("employeeName", retrievedUserInfo.getName());

    employeeInformation.put("employeeDepartment", retrievedUserInfo.getDepartment());
    employeeInformation.put("employeeDesignation", retrievedUserInfo.getDesignation());
    req.setAttribute("total", amounts.get("DayCareAmount"));

    Gson gson = new Gson();
    String jsonString = gson.toJson(employeeInformation);
    System.out.println("Servlet json from user details" + jsonString);
    PrintWriter writer = resp.getWriter();
    writer.write(jsonString);

    }

我用javascript编写了

<form action="./ssoServlet?from=amount" method="post">
<% String amount =  (String) request.getAttribute("total");%>
Total amount claimed 
 <input type="text" name="total" id="total" value = <%=amount %>  > 
</form>

但是，在总索赔金额中，显示文本字段null。如果需要。setAttribute和getAttribute不起作用我可以写两个JSONString吗？我应该如何在js中检索它

我检索数据的js函数是：

function fetchDetails(){
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function() {
        // alert("s");
        //alert(xhttp.status); 
        if (xhttp.readyState == 4 && xhttp.status == 200) {

            var JSONobj = JSON.parse(xhttp.responseText);
             document.getElementById("name").value = JSONobj.employeeName ;
             document.getElementById("department").value = JSONobj.employeeDepartment ;
             document.getElementById("designation").value = JSONobj.employeeDesignation ;

        }
      };
    xhttp.open("POST", "./ssoServlet?from=details", true);
      xhttp.send();
    }

Answer 1

您试图在通过servlet之前加载JSP文件，因此JSP页面永远不会收到该参数

记住，表单页面不会在ajax调用后重新加载，因此需要使用RequestDispatcher来传递参数

Answer 2

您的代码没有在请求属性total中放入任何内容：

Map<String, Integer> amounts = new HashMap<String, Integer>();
// amounts Map is empty, so amounts.get("DayCareAmount") will return null
req.setAttribute("total", amounts.get("DayCareAmount"));

为了确保一切正常，首先要简化代码，这样可能出错的事情就更少了：

req.setAttribute("total", 42);

现在检查42是否出现在你的网页上。如果是这样，您可以回到您的代码片段：

Map<String, Integer> amounts = new HashMap<String, Integer>();
amounts.put("DayCareAmount", 42);
req.setAttribute("total", amounts.get("DayCareAmount"));