Object Oriented JavaScript Lock

Question

I have given below simplified JavaScript problem.

var PROJ=(function(){
    var tags={}, 
    var lock=true;
    function onLoadComplete(){}
    this.Tag = function(userConfig,callBack){       
            function loadConfig(){
                    lock=false;             
                    /* Do something privately having lock with me */
                    lock=true;
            }                           
            this.load(){
                  if(lock)  loadConfig();
                  else      setTimeout(load,1000); 
              // PROBLEM is this(above) load is calling OUTER load..!
            }
       return this;
    };

    this.load(){
      var cb=onLoadComplete;
      tags[uniqueID]=new Tag(userConfig,cb);
      tags[uniqueID].load();
    }       
}).load();

I am trying to implement JavaScript locks here. Three JavaScript Tag objects are created. [ new Tag() ]These objects share and modify some public data available in PROJ.

I want to give access to public data when lock has been released by executing Tag. This load mentioned in setTimeout() is calling load() of PROJ.

You should fix your code. There are syntax errors (those {} parentheses probably get ignored by the browsers, so you don't see it), and there is no definition of the load function. — vgru
– vgru, Commented Dec 29, 2011 at 18:44
@Umesh as both Groo and FelixKing have stated, your code is not syntatically correct so we can only guess as to your problem. Even though this is cut from a much larger context please make it self consistent and syntatically correct or any answer you get will just be a shot in the dark. — chuckj
– chuckj, Commented Dec 29, 2011 at 18:52

Incognito · Accepted Answer · 2011-12-29 18:33:08Z

3

Are you aware that JavaScript runs in a single thread only? Locks (concurrency) don't do much, unless your inner loadConfig function actually recurses or call load in some way.

Note that I think that you have your syntax wrong.

this.load(){
    if(lock)  loadConfig();
    else      setTimeout(load,1000); 
}

is actually doing this:

this.load(); // invoke load

if(lock)  loadConfig();
else      setTimeout(load,1000);

edited Dec 29, 2011 at 18:33

Incognito

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answered Dec 29, 2011 at 18:16

Lucero

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9 Comments

Umesh Patil Over a year ago

The code given is very simplified and minimized. Even JS runs in single threads, Multiple Tags (Tag Object) are being created which runs simultaneously.

Felix Kling Over a year ago

@Umesh: Multiple times !== concurrent.

Umesh Patil Over a year ago

@FelixKling, Yes. I want to say, Multiple(concurrent) Tag objects are created here. Can you pls help out here?

Felix Kling Over a year ago

@Umesh: But as Lucero says, JavaScript is single threaded, so JavaScript code cannot be run concurrently, only if you use e.g. web workers and even then they cannot work on the same objects.

Lucero Over a year ago

@Umesh, unless you somehow do "manually" create new Tag instances inside of the Tag function the code will not re-enter. Fix your this.load function declarations (?) first and then try again, concurrency is not a problem here.

|

vgru · Accepted Answer · 2011-12-29 18:51:19Z

I am presuming that the code actually looks like this:

this.Tag = function(userConfig,callBack){       
        function loadConfig(){
                lock=false;             
                /* Do something privately having lock with me */
                lock=true;
        }                           
        this.load = function(){
              if(lock)  loadConfig();
              else      setTimeout(load,1000); 
        }
   return this;
};

Since JavaScript is single threaded, it is not possible for the load function to be invoked while loadConfig is still running. This means that it is not possible for the lock variable to be false inside load, unless:

There is a return statement in loadConfig which returns execution before lock is set to true.
There is an exception thrown in the middle of the function, and the last line is never executed.

For the latter case, you can use the try/finally block to make sure that lock is reset in case of an exception:

        function loadConfig(){
            try {
                lock=false;             
                /* Do something privately having lock with me */
            }
            finally {
                // this will get executed even if an exception is thrown
                lock=true;
            }
        }

The bottom line is, lock is not needed at all and can be removed completely. There is no way load could run concurrently with loadConfig.

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