如何使用AJAX来检测如果Web服务器已关闭或不接受请求如何使用、或不、服务器、Web
我一直工作在一个网站项目在我的业余时间了一段时间，最近决定我需要Web浏览器来检测Web服务器是否不下来了。很多AJAX的使用，所以我开始尝试了一些相当简单的code涉及XMLHtt prequest对象的responseText的领域。如果是空的，我会假设服务器下跌，并关闭网页。
I've been working on a web site project in my spare time for a while, and recently decided I needed the Web Browser to detect whether or not the Web Server had gone down. Lots of AJAX is used, so I initially tried some fairly simple code involving the "responseText" field of an XMLHttpRequest object. If it was empty, I would assume the Server was down, and close the web page.
想象一下，我惊讶的是，有时当responseText的领域是空的，即使我知道我的测试服务器是相当彻底的了。服务器日志显示，显然这是可能的AJAX请求失败，达到 - 但客户端，浏览器，还是设置XMLHtt prequest对象的readyState的字段为4（表示成功）服务器。
Imagine my surprise when sometimes the responseText field was empty even though I knew my test-Server was quite thoroughly "up". The Server logs indicated that apparently it is possible for the AJAX request to fail to reach the Server --but the client, the browser, still sets the XMLHttpRequest object's "readyState" field to "4" (indicating success).
这意味着我需要code足够能力去处理这两种情况，引起了我受宠若惊，我本来想测试--regarding Web服务器实际上是下跌的局面。
That means I need code robust enough to handle both this situation that caught me by surprise, and the situation I originally wanted to test --regarding the Web Server actually being down.
推荐答案
下面是出现充分的工作（不包括我可能已在提取/简化它张贴在这里的任何拼写错误）的一些code。
Here is some code that appears to work adequately (not counting any typos I might have made in extracting/simplifying it for posting here).
var AJobj, invl, tri, coun, str, rs; //some handy global variables
function initialize() //called by "onLoad" event handler for the HTML <body> tag
{ window.name="MyWindow";
if (window.XMLHttpRequest)
{ if (AJobj == null) //prevent creating AJAX object more than once
AJobj = new XMLHttpRequest();
}
else //display message about browser not being recent enough
{ window.open("Sorry.htm", "MyWindow", true);
return;
}
tri = 0; //number-of-tries counter
invl = false; //either the handle of an interval timer, or a no-timer flag
//other initialization stuff, for the web page, goes here
return;
}
function getAJAXdata1(a)
{ if(a != "a") //test for NOT "again"
{ if(invl !== false) //if interval timer already running
return; //ignore calls to this function if waiting for another AJAX request to complete
invl = setInterval("AJAXready(1);", 500); //start an interval timer for this AJAX request
coun = 0; //initialize counter associated with the interval timer
}
else
{ ; //when this function called "again", the goal is to duplicate the original AJAX request
; //If you need to do something special to ensure request is duplicated, it goes here
}
try
{ AJobj.open("GET", "datagroup1.php?info1=test&info2=thoroughly" , true); //sample AJAX request ('GET' type)
}
catch(err) //if an error happens that the client can detect
{ if(invl !== false)
clearInterval(invl); //turn off interval timer because leaving page
window.open("Sorry2.htm", "MyWindow", true); //load appropriate error-message page
}
AJobj.onreadystatechange = function(){AJAXready(1);}; //anonymous callback function
// is called when Web Server responds, and sends a parameter to a specific function below
AJobj.send(null); //complete the send-off of the AJAX request
return;
}
function useAJAXdata1()
{ ; //variable "str" holds the data received from the Web Server
; //do what you want with it
return;
}
//Below, the differences from 'getAJAXdata1(a)" are the name of this function,
// the parameter in the callback function for the interval timer,
// the name of the PHP file that gets called to provide AJAX data,
// the URL data being sent to that PHP file for processing,
// and the parameter in the anonymous callback function.
function getAJAXdata2(a)
{ if(a != "a")
{ if(invl !== false)
return;
invl = setInterval("AJAXready(2);", 500);
coun = 0;
}
else //function parameter specifies "again"
{ ; //if you need to do something special to ensure an
; // AJAX request is exactly duplicated, it goes here
}
try
{ AJobj.open("GET", "datagroup2.php?info1=thoroughly&info2=test" , true);
}
catch(err)
{ if(invl !== false)
clearInterval(invl);
window.open("Sorry2.htm", "MyWindow", true);
}
AJobj.onreadystatechange = function(){AJAXready(2);};
AJobj.send(null);
return;
}
function useAJAXdata2()
{ ; //variable "str" holds the data received from the Web Server
; //do what you want with it
return;
}
//SIMILARLY, a third, fourth, and so on, "getAJAXdata(a)" and "useAJAXdata()" function could be created if needed
//This function is called by the interval timer AND by a successful AJAX request
function AJAXready(w) //"w" refers to "which", such as getAJAXdata1() or getAJAXdata2()
{ rs = AJobj.readyState; //get status of AJAX request
if(rs < 4) //if not yet completed successfully
{ if(coun++ > 15) //15 calls from the interval timer (about 7 1/2 seconds; pick time you think best)
{ coun = 0; //reset the counter of calls originating with the interval timer
TryAgain(w); //Try sending a duplicate of the original AJAX request,
} // for which the response that took too long
return;
}
str = AJobj.responseText;//Client thinks AJAX request succeeded, so fetch the data sent by the Web Server
if(str == "") //SURPRISE! Sometimes there is no data!
//(server logs can indicate that the AJAX request never actually arrived)
{ coun = 0; //reset interval-timer counter
TryAgain(w); //try sending a duplicate of the original AJAX request
return;
}
//ACTUAL SUCCESS if JavaScript processing reaches this code
if(invl !== false)
{ clearInterval(invl); //turn off the interval timer
invl = false; //reset the flag
}
tri = 0; //reset counters
coun = 0;
eval("useAJAXdata"+w+"()"); //call the appropriate "useAJAXdata" function
return;
}
function TryAgain(w)
{ if(tri++ > 2)
{ if(invl != false)
clearInterval(invl); //turn off interval timer
alert("Three attempts to reach the Web Server have\n" +
"failed; it may be down. (The rest of this\n" +
"message is up to you.)");
window.close(); //or you could send the user to some other web site
// window.open("http://www.stackoverflow.com", "MyWindow", true);
}
else //Call the appropriate "getAJAXdata" function, with
eval("getAJAXdata"+w+'("a")'); // parameter "a" for "again", to get data from Web Server
return;
}
