Jetty example source code file (timesync.js)

This example Jetty source code file (timesync.js) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

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The Jetty timesync.js source code

if(!dojo._hasResource["dojox.cometd.timesync"]){ //_hasResource checks added by build. Do not use _hasResource directly in your code.
dojo._hasResource["dojox.cometd.timesync"] = true;
dojo.provide("dojox.cometd.timesync");
dojo.require("dojox.cometd");

/**
 * this file provides the time synchronization extension to cometd.
 * Timesync allows the client and server to exchange time information on every 
 * handshake and connect message so that the client may calculate an approximate
 * offset from it's own clock epoch to that of the server.
 *
 * With each handshake or connect, the extension sends timestamps within the 
 * ext field like: {ext:{timesync:{tc:12345567890},...},...}
 * where ts is the timestamp in ms since 1970 of when the message was sent.
 *
 * A cometd server that supports timesync, should respond with and ext field
 * like: {ext:{timesync:{tc:12345567890,ts:1234567900,p:123},...},...}
 * where ts is the timestamp sent by the client, te is the timestamp on the server
 * of when the message was received and p is the poll duration in ms - ie the
 * time the server took before sending the response.
 *
 * On receipt of the response, the client is able to use current time to determine
 * the total trip time, from which p is subtracted to determine an approximate
 * two way network traversal time. The assumption is made that the network is
 * symmetric for traversal time, so the offset between the two clocks is 
 * ts-tc-(now-tc-p)/2. In practise networks (and the cometd client/server software)
 * is never perfectly symmetric, so accuracy is limited by the difference, which 
 * can be 10s of milliseconds.
 *
 * In order to smooth over any transient fluctuations, the extension keeps a sliding
 * average of the offsets received. By default this is over 10 messages, but this can
 * be changed with the dojox.cometd.timesync._window element.
 */
dojox.cometd.timesync= new function(){

	this._window=10;// The window size for the sliding average of offset samples.
	this._minWindow=4;// The window size for the sliding average of offset samples.
	this._offsets=new Array(); // The samples used to calculate the average offset.
	this.offset=0;	// The offset in ms between the clients clock and the servers clock. Add this to the local
			// time epoch to obtain server time.
	this.samples=0; // The number of samples used to calculate the offset. If 0, the offset is not valid.
	
	this.getServerTime=function(){ // return: long
		// Summary:
		//	Calculate the current time on the server
		// 
		return new Date().getTime()+this.offset;
	}
	
	this.getServerDate=function(){ // return: Date
		// Summary:
		//	Calculate the current time on the server
		// 
		return new Date(this.getServerTime());
	}
	
	this.setTimeout=function(/*function*/call,/*long|Date*/atTimeOrDate){
		// Summary:
		//	Set a timeout function relative to server time
		// call:
		//	the function to call when the timeout occurs
		// atTimeOrTime: 
		//	a long timestamp or a Date representing the server time at
		//	which the timeout should occur.
		
		var ts=(atTimeOrDate instanceof Date)?atTimeOrDate.getTime():(0+atTimeOrDate);
		var tc=ts-this.offset;
		var interval=tc-new Date().getTime();
		if (interval<=0)
			interval=1;
		return setTimeout(call,interval);
	}

	this._in=function(/*Object*/msg){
		// Summary:
		//	Handle incoming messages for the timesync extension.
		// description:
		//	Look for ext:{timesync:{}} field and calculate offset if present.
		// msg: 
		//	The incoming bayeux message
		
		var channel=msg.channel;
		if (channel &&  channel.indexOf('/meta/')==0){
			if (msg.ext && msg.ext.timesync){
				var sync=msg.ext.timesync;
				var now=new Date().getTime();
				
				this._offsets.push(sync.ts-sync.tc-(now-sync.tc-sync.p)/2);
				if (this._offsets.length>this._window)
					this._offsets.shift();		
				this.samples++;
				var total=0;
				for (var i in this._offsets)
					total+=this._offsets[i];
				this.offset=parseInt((total/this._offsets.length).toFixed());
				
				if (this.samples<this._minWindow)
					setTimeout("dojox.cometd.publish('/meta/ping',null)",100);
			}
		}
		return msg;
	}

	this._out=function(msg){
		// Summary:
		//	Handle outgoing messages for the timesync extension.
		// description:
		//	Look for handshake and connect messages and add the ext:{timesync:{}} fields
		// msg: 
		//	The outgoing bayeux message
		
		var channel=msg.channel;
		if (channel &&  channel.indexOf('/meta/')==0){
			var now=new Date().getTime();
			if (!msg.ext)
				msg.ext={};
			msg.ext.timesync={tc: now};
		}
		return msg;
	}
};

dojox.cometd._extendInList.push(dojo.hitch(dojox.cometd.timesync,"_in"));
dojox.cometd._extendOutList.push(dojo.hitch(dojox.cometd.timesync,"_out"));

}