Question

我想更好地了解以下spring mvc 3.2应用程序的工作原理：https://github.com/rstoyanchev/spring-mvc-chat

我的问题是deferredResult Spring MVC class。我注意到，在给定时间，chatRequests地图中的条目数与用户连接到聊天应用程序的数量相同。

假设有 3个用户连接到聊天应用程序。您将看到当用户＃3发布消息时（请参阅下面的postMessage方法），然后 for循环（在postMessage方法中）迭代三次。我无法弄清楚为什么会这样。

我在下面提供示例代码。

控制器代码：

@Controller
@RequestMapping("/mvc/chat")
public class ChatController {

    private final ChatRepository chatRepository;
    private final Map<DeferredResult<List<String>>, Integer> chatRequests = new ConcurrentHashMap<DeferredResult<List<String>>, Integer>();

    @Autowired
    public ChatController(ChatRepository chatRepository) {
        this.chatRepository = chatRepository;
    }

    @RequestMapping(method = RequestMethod.GET)
    @ResponseBody
    public DeferredResult<List<String>> getMessages(@RequestParam int messageIndex) {

        final DeferredResult<List<String>> deferredResult = new DeferredResult<List<String>>(null, Collections.emptyList());
        this.chatRequests.put(deferredResult, messageIndex);

        deferredResult.onCompletion(new Runnable() {
            @Override
            public void run() {
                chatRequests.remove(deferredResult);
            }
        });

        List<String> messages = this.chatRepository.getMessages(messageIndex);
        if (!messages.isEmpty()) {
            deferredResult.setResult(messages);
        }

        return deferredResult;
    }

    @RequestMapping(method = RequestMethod.POST)
    @ResponseBody
    public void postMessage(@RequestParam String message) {

        this.chatRepository.addMessage(message);

        // Update all chat requests as part of the POST request
        // See Redis branch for a more sophisticated, non-blocking approach

        for (Entry<DeferredResult<List<String>>, Integer> entry : this.chatRequests.entrySet()) {
            List<String> messages = this.chatRepository.getMessages(entry.getValue());
            entry.getKey().setResult(messages);
        }
    }
}

Javascript代码：

$(document).ready(function() {

    function ChatViewModel() {

        var that = this;

        that.userName = ko.observable('');
        that.chatContent = ko.observable('');
        that.message = ko.observable('');
        that.messageIndex = ko.observable(0);
        that.activePollingXhr = ko.observable(null);


        var keepPolling = false;

        that.joinChat = function() {
            if (that.userName().trim() != '') {
                keepPolling = true;
                pollForMessages();
            }
        }

        function pollForMessages() {
            if (!keepPolling) {
                return;
            }
            var form = $("#joinChatForm");


            that.activePollingXhr($.ajax({url: form.attr("action"), type: "GET", data: form.serialize(), cache: false,
                success: function(messages) {
                    console.log(messages);
                    for (var i = 0; i < messages.length; i++) {
                        that.chatContent(that.chatContent() + messages[i] + "\n");
                        that.messageIndex(that.messageIndex() + 1);
                    }
                },
                error: function(xhr) {
                    if (xhr.statusText != "abort" && xhr.status != 503) {
                        resetUI();
                        console.error("Unable to retrieve chat messages. Chat ended.");
                    }
                },
                complete: pollForMessages
            }));
            $('#message').focus();
        }

        that.postMessage = function() {
            if (that.message().trim() != '') {
                var form = $("#postMessageForm");
                $.ajax({url: form.attr("action"), type: "POST",
                    data: "message=[" + that.userName() + "] " + $("#postMessageForm input[name=message]").val(),
                    error: function(xhr) {
                        console.error("Error posting chat message: status=" + xhr.status + ", statusText=" + xhr.statusText);
                    }
                });
                that.message('');
            }
        }

        that.leaveChat = function() {
            that.activePollingXhr(null);
            resetUI();
            this.userName('');
        }

        function resetUI() {
            keepPolling = false;
            that.activePollingXhr(null);
            that.message('');
            that.messageIndex(0);
            that.chatContent('');
        }

    }

    //Activate knockout.js
    ko.applyBindings(new ChatViewModel());

});

和html页面：

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:th="http://www.thymeleaf.org">
<head>
    <title>Chat</title>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body>
    <h1>Chat</h1>

    <form id="joinChatForm" th:action="@{/mvc/chat}" data-bind="visible: activePollingXhr() == null">
        <p>
            <label for="user">User: </label>
            <input id="user" name="user" type="text" data-bind="value: userName"/>
            <input name="messageIndex" type="hidden" data-bind="value: messageIndex"/>
            <button id="start" type="submit" data-bind="click: joinChat">Join Chat</button>
        </p>
    </form>

    <form id="leaveChatForm" th:action="@{/mvc/chat}" data-bind="visible: activePollingXhr() != null">
        <p>
            You're chatting as <strong data-bind="text: userName"></strong>
            <button id="leave" type="submit" data-bind="click: leaveChat">Leave Chat</button>
        </p>
    </form>

    <div data-bind="visible: activePollingXhr() != null">
        <textarea rows="15" cols="60" readonly="readonly" data-bind="text: chatContent"></textarea>
    </div>

    <form id="postMessageForm" th:action="@{/mvc/chat}" data-bind="visible: activePollingXhr() != null">
        <p>
            <input id="message" name="message" type="text" data-bind="value: message" />
            <button id="post" type="submit" data-bind="click: postMessage">Post</button>
        </p>
    </form>
</body>
<script type="text/javascript" src="../../../resources/js/jquery-1.7.2.min.js" th:src="@{/resources/js/jquery-1.7.2.min.js}"></script>
<script type="text/javascript" src="../../../resources/js/knockout-2.0.0.js" th:src="@{/resources/js/knockout-2.0.0.js}"></script>
<script type="text/javascript" src="../../../resources/js/chat.js" th:src="@{/resources/js/chat.js}"></script>

</html>

Answer 1

为了理解DeferredResult正在做什么，您需要了解Servlet 3.0异步概念。

使用Servlet 3.0，您可以从请求中获取AsyncContext，将其存储在Collection中。

AsyncContext aCtx = request.startAsync(request, response);

结果将释放您的应用程序容器线程。

在单独的线程上进行一些操作，并将结果写回Servlet响应：

aCtx.getResponse().getWriter().print(result);

从DeferredResult作品的那一点起绝对相同。

小例子：

现在考虑每5秒钟你从第三方服务获得报价。并且您拥有长期轮询服务器的客户端，以便更新内容。

你有控制器方法：

   /** put deferred result to some HashSet. This is the same logic as you 
         store async context in servlet 3.0, those are clients who are waiting for             
         response
    **/
    @RequestMapping(value="/getQuote.do", method=RequestMethod.GET)
    @ResponseBody
    public DeferredResult<String> getQuote(){
       final DeferredResult<String> deferredResult = new DeferredResult<String>();

       someMap.put(deferredResult);
       return deferredResult;
    }

现在让我们看一下控制器之外的方法，它获取引用并将响应返回给客户端。

function getQuoteAndUpdateClients(){ 

      String quote = getUpdatedQuoteFromThirdPartyService();

      for (DeferredResult<String> deferredResult: someMap){
              deferredResult.setResult(quote);
       }
 }

Answer 2

我与Spring的DeferredResult课程的作者详细讨论了这个主题，这是我们对话的相关部分：

引用Rossen Stoyanchev：

粗略地说。 DeferredResult与打开的请求相关联。请求完成后，DeferredResult将从中删除映射，然后，客户端发出一个新的长轮询请求，其中添加一个新的DeferredResult实例

Answer 3

当客户端连接时，在this.chatRequests中为该客户端存储DeferredResult。当客户端发布消息时，它会遍历所有DeferredResults（读取客户端）以设置结果。当连接3个客户端时，这种情况发生3次是合乎逻辑的。

在spring-mvc-chat github应用程序的上下文中理解Spring MVC的DeferredResult类

3 个答案: