使用 Javascript 向服务器发送多个请求，直到服务器返回 200

Question

我创建了一个 html 页面，用户在等待另一个应用程序重新启动时查看该页面。我创建了一个 javascript 脚本，它向应用程序发出多个请求，直到应用程序返回 200。但是，当我尝试打开我的 html 页面时，浏览器只是崩溃了。谁能看到我的代码有什么问题：

var responseStatus = 404;
var numberOfTries = 0;
var request = new XMLHttpRequest(); 
while(responseStatus != 200){
    console.log("Checking if restart has completed. Attempt number: " + numberOfTries);
    request.open('GET', 'applicationurl', true);
    request.onreadystatechange = function(){
        if (request.readyState === 4){
            console.log("Request Status: " + request.status);
            responseStatus = request.status;
            console.log("Response Status: " + request.status);
            if (request.status != 200) {  
                numberOfTries++;
                console.log("Restart has not yet completed! NumberOfTries: " + numberOfTries);
            }  
        }
    };
    request.send();
}
console.log("Server has successfully restarted");

Answer 1

您的方法发送多个请求，但它不会等待它们响应。

这是它的工作原理：

• XMLHttpRequest 已创建
• 一个回调和一个数据分配给它
• 这个对象被发送到服务器
• 然后while 循环结束并再次运行，而不是等待回调运行

结果创建了很多XMLHttpRequest对象，同时运行了很多请求，这不是你想要的。

您可以通过两种方式做到这一点：

• 将其包装到一个函数中，并在它失败时调用它（首选）；
• 设置等待响应的时间间隔

第一种方法：

function tryRequest(numAttempts) {
  var responseStatus = 404;
  var request = new XMLHttpRequest(); 

  console.log("Checking if restart has completed. Attempt number: " + numAttempts);
  request.open('GET', 'applicationurl', true);
  request.onreadystatechange = function(){
      if (request.readyState === 4){
          console.log("Request Status: " + request.status);
          responseStatus = request.status;
          console.log("Response Status: " + request.status);
          if (request.status != 200) {  
              console.log("Restart has not yet completed! NumberOfTries: " + numAttempts);
              tryRequest(numAttempts + 1)
          }  
      }
  };
  request.send();

  console.log("Server has successfully restarted");
}
tryRequest(0);

第二种方法：

var responseStatus = 404;
var numberOfTries = 0;
var request = new XMLHttpRequest();
var interval = setInterval(tryRequest, 1000)

function tryRequest() {
  console.log("Checking if restart has completed. Attempt number: " + numberOfTries);
  request.open('GET', 'applicationurl', true);
  request.onreadystatechange = function(){
      if (request.readyState === 4){
          console.log("Request Status: " + request.status);
          responseStatus = request.status;
          console.log("Response Status: " + request.status);
          if (request.status != 200) {
              numberOfTries++;
              console.log("Restart has not yet completed! NumberOfTries: " + numberOfTries);
          } else {
              clearInterval(interval);
              console.log("Server has successfully restarted");
          }
      }
  };
  request.send();
  }

}

（免责声明：当时我没有测试两者）

Answer 2

你可以像这样使用代码

var responseStatus = 404;
var numberOfTries = 0;
function check(){
        var request = new XMLHttpRequest(); 
        console.log("Checking if restart has completed. Attempt number: " + numberOfTries);
        request.open('GET', 'applicationurl', true);
        request.onreadystatechange = function(){
            if (request.readyState === 4){
               console.log("Request Status: " + request.status);
               responseStatus = request.status;
               console.log("Response Status: " + request.status);
               if (request.status != 200) {  
                  numberOfTries++;
                  console.log("Restart has not yet completed! NumberOfTries: " + numberOfTries);
                   var tmp = setTimeout(function(){ check() },5000 ); // check after 5 second
               }  elseif (request.status == 200) {  
                    console.log("Server has successfully restarted");
               }
            }
        };
        request.send();

  }

//第一次调用

check();

Answer 3

（在我看来）最好的方法是结合@serge1peshcoff 的两个选项。将调用编写为响应失败时调用自身的函数，并在这样做时设置超时。在大多数情况下，您不想在前一个请求之后立即发送另一个请求，您希望至少等待一到五秒以防止向服务器发送垃圾邮件（您也没有说明您期望有多少客户端。什么是对您的浏览器不利对服务器来说可能是可怕的，甚至在您的 Web 应用程序完全启动并返回 200 之前，它可能正在处理请求）。

另外，setTimeout 是比 setInterval 更好的方法，因为您在每次往返时都明确地重置它。

借用@serge：

function tryRequest(numTries) {
  var responseStatus = 404;
  var request = new XMLHttpRequest(); 
  numTries = numTries || 0;

  console.log("Checking if restart has completed. Attempt number: " + numAttempts);
  request.open('GET', 'applicationurl', true);
  request.onreadystatechange = function(){
      numTries++

      if (request.readyState === 4){
          console.log("Request Status: " + request.status);
          responseStatus = request.status;
          console.log("Response Status: " + request.status);
          if (request.status != 200) {  
              console.log("Restart has not yet completed! NumberOfTries: " + this.attempts);
              setTimeout(tryRequest.bind(null, numTries), 5000); // five seconds
          }  
      }
  };
  request.send();

  console.log("Server has successfully restarted");
}
tryRequest();