Ajax synchronous asynchronous callback function

Ajax synchronization: req. open ('get', url, false );

Instance:

Js Method

Var g_success = false; // global variable

Function getUser (username ){

If ("" = username ){

Alert ("login name cannot be blank! ");

Return;

}

G_success = false;

Var url = "<% = path %>/basicdata/gyshtyCheckUsername. do? Mode = getUser & username = "+ encodeURI (username ));

If (window. XMLHttpRequest ){

Req = new XMLHttpRequest ();

} Elseif (window. ActiveXObject ){

Req = new ActiveXObject ("Microsoft. XMLHTTP ");

}

If (req ){

Req. open ('get', url, false); // synchronous

Req. setRequestHeader ("Content-Length", username. length );

Req. setRequestHeader ("CONTENT-TYPE", "application/x-www-form-urlencoded ");

Req. send ("username =" + encodeURI (username )));

If (req. readyState = 4 ){

If (req. status = 200 ){

Result = req. responseText;

If ('no. '= result ){

Alert ("the login name does not exist! ");

Document. getElementById ("username"). value = "";

Document. forms [0]. username. focus ();

} Else {

G_success = true;

}

}

}

}

}

Ii. Background Service

String username = request. getParameter ("username ");

Users users = gyshtyService. findUsersByUsername (username );

String jsonStr = "no ";

If (users! = Null ){

JsonStr = users. getUsername ();

} Else {

JsonStr = "no .";

}

Response. setContentType ("text/xml; charset = GBK ");

Response. setHeader ("Cache-control", "no-cache ");

Response. getWriter (). print (jsonStr );

Returnnull;

 

Ajax asynchronous: req. open ('get', url, true );

Instance:

I. js method

Function getNext (){

If (window. XMLHttpRequest ){

Req = new XMLHttpRequest ();

} Elseif (window. ActiveXObject ){

Req = new ActiveXObject ("Microsoft. XMLHTTP ");

}

If (req ){

Var url = "------";

Req. open ("GET", url, true );

Req. onreadystatechange = completeGetNext; // callback function

Req. send (null );

}

}

Function completeGetNext (){

If (req. readyState = 1 ){

Document. getElementById ("loadspan"). innerHTML =

" & nbsp; <font color = red> loading data. Please wait... </font> ";

}

If (req. readyState = 4 ){

If (req. status = 200 ){

// After obtaining the result, start ......

ParseMessageNext ();

}

}

}

 

Ii. Background Service

Response. setContentType ("text/xml; charset = GBK ");

Response. setHeader ("Cache-control", "no-cache ");

String xml = "<? Xml version = \ "1.0 \" encoding = \ "GBK \"?> ";

/*

Interaction with databases and business logic

*/

Response. getWriter (). write (xml + str );

 

Summary: callback functions can be defined when asynchronous (true) is used. When synchronous (false) is used, callback functions cannot define another callback function name. I do not know the reason. I hope you can tell me the reason.

Note: The above code is code that has implemented functions in development.