Question

我在GWT编写Web应用程序。我有一个类来处理我的所有http请求并返回服务器的响应。这是我当前的代码:(变量messageXml被定义为常量）

try {

        Request request = builder.sendRequest(requestData, new RequestCallback() {

            @Override
            public void onResponseReceived(Request request, Response response) {
                if(200 == response.getStatusCode()) {
                    messageXml = response.getText();

                }
                else {
                    Window.alert("There was an error (1) :-(");
                    messageXml = "ERROR";

                }
            }


            @Override
            public void onError(Request request, Throwable exception) {
                Window.alert("There was an error (2) :-(");
                messageXml = "ERROR";

            }
        });
        Window.alert(messageXml);
        return messageXml;
    }
    catch (RequestException e) {
          Window.alert("Could not connect to the server :-("); 
          return "ERROR";
        }

当我调用方法httprequest时，它首先返回String，然后执行try主体中的代码。例如，当我这样做时：

request object = new request();

    String test = object.httpRequest(file, type, requestData);
    Window.alert(test);

它输出第一个null（因为String还没有内容），然后输出String的内容。有谁知道为什么这个方法表现得像这样？

Answer 1

messageXml是否定义为常量不是问题所在。 Request#sendRequest()是一个异步操作，因此您无法对messageXml何时赋值进行任何假设。有关详细信息，请参阅GWT的Getting Used to Asynchronous Calls。使用Request#sendRequest()的正确方法是使用回调：

public void getMessageXml(String requestData,
                          final AsyncCallback<String> callback) {
  try {
    // Modify for your application.
    RequestBuilder rb = new RequestBuilder(Method.GET,
      "http://www.mycompany.com/getMessageXml");

    rb.sendRequest(requestData, new RequestCallback() {
      @Override
      public void onError(Request request, Throwable caught) {
        callback.onFailure(caught);
      }

      @Override
      public void onResponseReceived(Request request, Response response) {
        if (Response.SC_OK == response.getStatusCode()) {
          callback.onSuccess(response.getText());
        } else {
          // Use a correctly typed exception here.
          callback.onFailure(...);
        }
      }
    });
  } catch (RequestException rex) {
    callback.onFailure(rex);
  }
}

并调用方法：

object.getMessageXml(someRequestString, new AsyncCallback<String>() {
  @Override
  public void onFailure(Throwable caught) {
    // Handle error.
  }

  @Override
  public void onSuccess(String messageXml) {
    // Process response
  }
});

尝试在执行正文代码之前返回String

1 个答案: