Question

尝试从JavaScript接收参数时，我得到null。我已经发了几篇文章，但无法弄清楚我在代码中犯了哪些错误。

以下是我发送请求的代码：

function funcOnChange() {
    var index = document.detail.Class.selectedIndex;
    var valueSelected = document.detail.Class.options[index].value;

    handleRequestStateChange = function()
    {
        // Check to see if this state change was "request complete", and
        // there was no server error (404 Not Found, 500 Server Error, etc)
        if (xmlhttp.readyState==4 && xmlhttp.status==200) 
        {
            var substring=xmlHttp.responseText;
            alert("Alert Dialog! Gaurav");
        }
    }
    var xhr = new XMLHttpRequest();
    xhr.open('POST', 'http://localhost:8080/ReportFetcher/FormHandler', true);
    xhr.send(valueSelected);
}

我从以下代码中获取valueselected并且valueselected是正确的：

<select name="Class" onchange="funcOnChange()">
            <option value="None">None</option>
            <option value="FIRST">FIRST</option>
            <option value="SECOND">SECOND</option>
            <option value="THIRD">THIRD</option>
            <option value="FOURTH">FOURTH</option>
            <option value="FIFTH">FIFTH</option>
            <option value="SIXTH">SIXTH</option>
            <option value="SEVENTH">SEVENTH</option>
            <option value="EIGHTH">EIGHTH</option>
            <option value="NINTH">NINTH</option>
            <option value="TENTH">TENTH</option>
        </select><br>

我收到了关于FormHandler.java的onPost（）的回调

protected void doPost(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {
    log.info("IN form doPost");
    String selectedClass = request.getParameter("Class");
    log.info(selectedClass);

}

问题：此处selectedClass为空。

建议我犯错的地方。

Answer 1

试试这个

function funcOnChange() {
    var index = document.detail.Class.selectedIndex;
    var valueSelected = "Class="+document.detail.Class.options[index].text;

    .....
    .....

    xhr.send(valueSelected);
}

Answer 2

如果您通过任何代理/ HTTP监控工具（我使用Charles）看到您的请求和响应，您将看到您没有将请求作为键值对发送（简单来说，您不发送Class = value）但是您只发送Class属性的值作为字符串。（如果您在选择框中选择第三个选项，则为第三个）。如果要将服务器上的数据作为键值对读取，则需要发送FormData。

function funcOnChange() {
    var index = document.detail.Class.selectedIndex;
    var valueSelected = document.detail.Class.options[index].value;

    handleRequestStateChange = function()
    {
        // Check to see if this state change was "request complete", and
        // there was no server error (404 Not Found, 500 Server Error, etc)
        if (xmlhttp.readyState==4 && xmlhttp.status==200) 
        {
            var substring=xmlHttp.responseText;
            alert("Alert Dialog! Gaurav");
        }
    }
    var xhr = new XMLHttpRequest();
    xhr.open('POST', 'http://fiddle.jshell.net/', true);    
    var form = new FormData();
    form.append("Class",valueSelected)
    xhr.send(form);
}

从js传递给Servlet时，在参数中接收NULL。

2 个答案: