关于Javascript的内存泄漏问题的整理稿

写了好长时间javascript小功能模块，从来没有关注过内存泄漏问题。记得以前写C++程序的时候，内存泄漏是个严重的问题，我想是时候关注一下了。网上找了篇文章，Mark一下。原文地址：http://www.blogjava.net/tim-wu/archive/2006/05/29/48729.html

常规循环引用内存泄漏和Closure内存泄漏

要了解javascript的内存泄漏问题，首先要了解的就是javascript的GC原理。

我记得原来在犀牛书《JavaScript: The Definitive Guide》中看到过，IE使用的GC算法是计数器，因此只碰到循环 引用就会造成memory leakage。后来一直觉得和观察到的现象很不一致，直到看到Eric的文章，才明白犀牛书的说法没有说得很明确，估计该书成文后IE升级过算法吧。 在IE 6中，对于javascript object内部，jscript使用的是mark-and-sweep算法，而对于javascript object与外部object(包括native object和vbscript object等等)的引用时，IE 6使用的才是计数器的算法。

Eric Lippert在http://blogs.msdn.com/ericlippert/archive/2003/09/17/53038.aspx一文中提到IE 6中JScript的GC算法使用的是nongeneration mark-and-sweep。对于javascript对算法的实现缺陷，文章如是说：
"The benefits of this approach are numerous, but the principle benefit is that circular references are not leaked unless the circular reference involves an object not owned by JScript. "
也就是说，IE 6对于纯粹的Script Objects间的Circular References是可以正确处理的，可惜它处理不了的是JScript与Native Object(例如Dom、ActiveX Object)之间的Circular References。
所以，当我们出现Native对象(例如Dom、ActiveX Object)与Javascript对象间的循环引用时，内存泄露的问题就出现了。当然，这个bug在IE 7中已经被修复了［http://www.quirksmode.org/blog/archives/2006/04/ie_7_and_javasc.html］。

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/IETechCol/dnwebgen/ie_leak_patterns.asp 中有个示意图和简单的例子体现了这个问题：

< html > 
     < head > 
         < script language = " JScript " > 

         var  myGlobalObject;

         function  SetupLeak()  // 产生循环引用，因此会造成内存泄露 
        {
             //  First set up the script scope to element reference 
            myGlobalObject  = 
                document.getElementById( " LeakedDiv " );

             //  Next set up the element to script scope reference 
            document.getElementById( " LeakedDiv " ).expandoProperty  = 
                myGlobalObject;
        }


         function  BreakLeak()  // 解开循环引用，解决内存泄露问题 
        {
            document.getElementById( " LeakedDiv " ).expandoProperty  = 
                 null ;
        }
         </ script > 
     </ head > 

     < body onload = " SetupLeak() "  onunload = " BreakLeak() " > 
         < div id = " LeakedDiv " ></ div > 
     </ body > 
</ html >

   上面这个例子，看似很简单就能够解决内存泄露的问题。可惜的是，当我们的代码中的结构复杂了以后，造成循环引用的原因开始变得多样，我们就没法那么容易观察到了，这时候，我们必须对代码进行仔细的检查。
尤其是当碰到Closure，当我们往Native对象(例如Dom对象、ActiveX Object)上绑定事件响应代码时，一个不小心，我们就会制造出Closure Memory Leak。其关键原因，其实和前者是一样的，也是一个跨javascript object和native object的循环引用。只是代码更为隐蔽，这个隐蔽性，是由于javascript的语言特性造成的。但在使用类似内嵌函数的时候，内嵌的函数有拥有一 个reference指向外部函数的scope，包括外部函数的参数，因此也就很容易造成一个很隐蔽的循环引用，例如：
DOM_Node.onevent ->function_object.[ [ scope ] ] ->scope_chain ->Activation_object.nodeRef ->DOM_Node。

［http://msdn.microsoft.com/library/default.asp?url=/library/en-us/IETechCol/dnwebgen/ie_leak_patterns.asp］有个例子极深刻地显示了该隐蔽性：

< html > 
     < head > 
         < script language = " JScript " > 

         function  AttachEvents(element)
        {
             //  This structure causes element to ref ClickEventHandler  //element有个引用指向函数ClickEventHandler() 
            element.attachEvent( " onclick " , ClickEventHandler);

             function  ClickEventHandler()
            {
                 //  This closure refs element  //该函数有个引用指向AttachEvents(element)调用Scope，也就是执行了参数element。 
                
            }
        }

         function  SetupLeak()
        {
             //  The leak happens all at once 
            AttachEvents(document.getElementById( " LeakedDiv " ));
        }

         </ script > 
     </ head > 

     < body onload = " SetupLeak() "  onunload = " BreakLeak() " > 
         < div id = " LeakedDiv " ></ div > 
     </ body > 
</ html >

还有这个例子在IE 6中同样原因会引起泄露

function  leakmaybe() {
var  elm  =  document.createElement( " DIV " );
  elm.onclick  =   function () {
return   2   +   2 ;
  }
}

for  ( var  i  =   0 ; i   10000 ; i ++ ) {
  leakmaybe();
}

btw:
关于Closure的知识，大家可以看看http://jibbering.com/faq/faq_notes/closures.html这篇文章，习惯中文也可以看看zkjbeyond的blog，他对Closure这篇文章进行了简要的翻译：http://www.blogjava.net/zkjbeyond/archive/2006/05/19/47025.html。 之所以会有这一系列的问题，关键就在于javascript是种函数式脚本解析语言，因此javascript中“函数中的变量的作用域是定义作用域，而 不是动态作用域”，这点在犀牛书《JavaScript: The Definitive Guide》中的“Funtion”一章中有所讨论。
http://support.microsoft.com/default.aspx?scid=KB;EN-US;830555中也对这个问题举了很详细的例子。


一些 简单的解决方案


目前大多数ajax前端的javascript framework都利用对事件的管理，解决了该问题。

如果你需要自己解决这个问题，可以参考以下的一些方法：

http://outofhanwell.com/ieleak/index.php?title=Main_Page:有个不错的检测工具
http://youngpup.net/2005/0221010713 中提到：可以利用递归Dom树，解除event绑定，从而解除循环引用：

														
      if (window.attachEvent) {
          var clearElementProps = [
              'data',
              'onmouseover',
              'onmouseout',
              'onmousedown',
              'onmouseup',
              'ondblclick',
              'onclick',
              'onselectstart',
              'oncontextmenu'
          ];

          window.attachEvent("onunload", function() {
              var el;
              for(var d = document.all.length;d--;){
                  el = document.all[d];
                  for(var c = clearElementProps.length;c--;){
                      el[clearElementProps[c]] = null;
                  }
              }
          });
      }												

而http://novemberborn.net/javascript/event-cache一文中则通过增加EventCache，从而给出一个相对结构化的解决方案

/*     EventCache Version 1.0
    Copyright 2005 Mark Wubben

    Provides a way for automagically removing events from nodes and thus preventing memory leakage.
    See <http://novemberborn.net/javascript/event-cache> for more information.
    
    This software is licensed under the CC-GNU LGPL <http://creativecommons.org/licenses/LGPL/2.1/>
*/ 

/*     Implement array.push for browsers which don't support it natively.
    Please remove this if it's already in other code  */ 
if (Array.prototype.push  ==   null ){
    Array.prototype.push  =   function (){
         for ( var  i  =   0 ; i  <  arguments.length; i ++ ){
             this [ this .length]  =  arguments[i];
        };
         return   this .length;
    };
};

/*     Event Cache uses an anonymous function to create a hidden scope chain.
    This is to prevent scoping issues.  */ 
var  EventCache  =   function (){
     var  listEvents  =  [];
    
     return  {
        listEvents : listEvents,
    
        add :  function (node, sEventName, fHandler, bCapture){
            listEvents.push(arguments);
        },
    
        flush :  function (){
             var  i, item;
             for (i  =  listEvents.length  -   1 ; i  >=   0 ; i  =  i  -   1 ){
                item  =  listEvents[i];
                
                 if (item[ 0 ].removeEventListener){
                    item[ 0 ].removeEventListener(item[ 1 ], item[ 2 ], item[ 3 ]);
                };
                
                 /*  From this point on we need the event names to be prefixed with 'on"  */ 
                 if (item[ 1 ].substring( 0 ,  2 )  !=   " on " ){
                    item[ 1 ]  =   " on "   +  item[ 1 ];
                };
                
                 if (item[ 0 ].detachEvent){
                    item[ 0 ].detachEvent(item[ 1 ], item[ 2 ]);
                };
                
                item[ 0 ][item[ 1 ]]  =   null ;
            };
        }
    };
}();

使用方法也很简单：

												
<script type="text/javascript">
function addEvent(oEventTarget, sEventType, fDest){
        if(oEventTarget.attachEvent){
		oEventTarget.attachEvent("on" + sEventType, fDest);
	} elseif(oEventTarget.addEventListener){
		oEventTarget.addEventListener(sEventType, fDest, true); 
	} elseif(typeof oEventTarget[sEventType] == "function"){
                var fOld = oEventTarget[sEventType];
		oEventTarget[sEventType] = function(e){ fOld(e); fDest(e); };
	} else {
		oEventTarget[sEventType] = fDest;
	};

	/* Implementing EventCache for all event systems */
	EventCache.add(oEventTarget, sEventType, fDest, true);
};


function createLeak(){
         var body = document.body;

	function someHandler(){
               return body;
	};

	addEvent(body, "click", someHandler);
};

window.onload = function(){
       var i = 500;
       while(i > 0){
		createLeak();
		i = i - 1;
	}
};

window.onunload = EventCache.flush;
</script>													

http://talideon.com/weblog/2005/03/js-memory-leaks.cfm 一文中的方法类似：

/* 
 * EventManager.js
 * by Keith Gaughan
 *
 * This allows event handlers to be registered unobtrusively, and cleans
 * them up on unload to prevent memory leaks.
 *
 * Copyright (c) Keith Gaughan, 2005.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Common Public License v1.0
 * (CPL) which accompanies this distribution, and is available at
 * http://www.opensource.org/licenses/cpl.php
 *
 * This software is covered by a modified version of the Common Public License
 * (CPL), where Keith Gaughan is the Agreement Steward, and the licensing
 * agreement is covered by the laws of the Republic of Ireland.
  */ 

//  For implementations that don't include the push() methods for arrays. 
if  ( ! Array.prototype.push) {
    Array.prototype.push  =   function (elem) {
         this [ this .length]  =  elem;
    }
}

var  EventManager  =  {
    _registry:  null ,

    Initialise:  function () {
         if  ( this ._registry  ==   null ) {
             this ._registry  =  [];

             //  Register the cleanup handler on page unload. 
            EventManager.Add(window,  " unload " ,  this .CleanUp);
        }
    },

     /* *
     * Registers an event and handler with the manager.
     *
     * @param  obj         Object handler will be attached to.
     * @param  type        Name of event handler responds to.
     * @param  fn          Handler function.
     * @param  useCapture  Use event capture. False by default.
     *                     If you don't understand this, ignore it.
     *
     * @return True if handler registered, else false.
      */ 
    Add:  function (obj, type, fn, useCapture) {
         this .Initialise();

         //  If a string was passed in, it's an id. 
         if  ( typeof  obj  ==   " string " ) {
            obj  =  document.getElementById(obj);
        }
         if  (obj  ==   null   ||  fn  ==   null ) {
             return   false ;
        }

         //  Mozilla/W3C listeners? 
         if  (obj.addEventListener) {
            obj.addEventListener(type, fn, useCapture);
             this ._registry.push({obj: obj, type: type, fn: fn, useCapture: useCapture});
             return   true ;
        }

         //  IE-style listeners? 
         if  (obj.attachEvent  &&  obj.attachEvent( " on "   +  type, fn)) {
             this ._registry.push({obj: obj, type: type, fn: fn, useCapture:  false });
             return   true ;
        }

         return   false ;
    },

     /* *
     * Cleans up all the registered event handlers.
      */ 
    CleanUp:  function () {
         for  ( var  i  =   0 ; i  <  EventManager._registry.length; i ++ ) {
             with  (EventManager._registry[i]) {
                 //  Mozilla/W3C listeners? 
                 if  (obj.removeEventListener) {
                    obj.removeEventListener(type, fn, useCapture);
                }
                 //  IE-style listeners? 
                 else   if  (obj.detachEvent) {
                    obj.detachEvent( " on "   +  type, fn);
                }
            }
        }

         //  Kill off the registry itself to get rid of the last remaining 
         //  references. 
        EventManager._registry  =   null ;
    }
};

使用起来也很简单

<html>
<head>
<script type=text/javascript src=EventManager.js></script>
<script type=text/javascript>
    function onLoad() {

    EventManager.Add(document.getElementById(testCase),click,hit );
returntrue;
    }

    function hit(evt) {
        alert(click);
    }
</script>
</head>

<body onload='javascript: onLoad();'>

<div id='testCase' style='width:100%; height: 100%; background-color: yellow;'>
  <h1>Click me!</h1>
</div>

</body>
</html>

google map api同样提供了一个类似的函数用在页面的unload事件中，解决Closure带来的内存泄露问题。
当然，如果你不嫌麻烦，你也可以为每个和native object有关的就阿vascript object编写一个destoryMemory函数，用来手动调用，从而手动解除Dom对象的事件绑定。

还有一种就是不要那么OO,抛弃Dom的一些特性，用innerHTML代替appendChild，避开循环引用。详细见http://birdshome.cnblogs.com/archive/2005/02/16/104967.html中的讨论贴。

Cross-Page Leaks

    Cross-Page Leaks和下一节提到的Pseudo-Leaks在我看来，就是IE的bug， 虽然MS死皮赖脸不承认:)

     大家可以看看这段例子代码:

< html > 
     < head > 
         < script language = " JScript " > 

         function  LeakMemory()  // 这个函数会引发Cross-Page Leaks 
        {
             var  hostElement  =  document.getElementById( " hostElement " );

             //  Do it a lot, look at Task Manager for memory response 

             for (i  =   0 ; i  <   5000 ; i ++ )
            {
                 var  parentDiv  = 
                    document.createElement( " <div onClick='foo()'> " );
                 var  childDiv  = 
                    document.createElement( " <div onClick='foo()'> " );

                 //  This will leak a temporary object 
                parentDiv.appendChild(childDiv);
                hostElement.appendChild(parentDiv);
                hostElement.removeChild(parentDiv);
                parentDiv.removeChild(childDiv);
                parentDiv  =   null ;
                childDiv  =   null ;
            }
            hostElement  =   null ;
        }


         function  CleanMemory()  // 而这个函数不会引发Cross-Page Leaks 
        {
             var  hostElement  =  document.getElementById( " hostElement " );

             //  Do it a lot, look at Task Manager for memory response 

             for (i  =   0 ; i  <   5000 ; i ++ )
            {
                 var  parentDiv  =   document.createElement( " <div onClick='foo()'> " );
                 var  childDiv  =   document.createElement( " <div onClick='foo()'> " );

                 //  Changing the order is important, this won't leak 
                hostElement.appendChild(parentDiv);
                parentDiv.appendChild(childDiv);
                hostElement.removeChild(parentDiv);
                parentDiv.removeChild(childDiv);
                parentDiv  =   null ;
                childDiv  =   null ;
            }
            hostElement  =   null ;
        }
         </ script > 
     </ head > 

     < body > 
         < button onclick = " LeakMemory() " > Memory Leaking Insert </ button > 
         < button onclick = " CleanMemory() " > Clean Insert </ button > 
         < div id = " hostElement " ></ div > 
     </ body > 
</ html >

LeakMemory和CleanMemory这两段函数的唯一区别就在于他们的代码的循序，从代码上看，两段代码的逻辑都没有错。

但LeakMemory却会造成泄露。原因是LeakMemory()会先建立起parentDiv和childDiv之间的连接，这时候，为了让 childDiv能够获知parentDiv的信息，因此IE需要先建立一个临时的scope对象。而后parentDiv建立了和 hostElement对象的联系，parentDiv和childDiv直接使用页面document的scope。可惜的是，IE不会释放刚才那个临 时的scope对象的内存空间，直到我们跳转页面，这块空间才能被释放。而CleanMemory函数不同，他先把parentDiv和 hostElement建立联系，而后再把childDiv和parentDiv建立联系，这个过程不需要单独建立临时的scope，只要直接使用页面 document的scope就可以了， 所以也就不会造成内存泄露了

详细原因，大家可以看看http://msdn.microsoft.com/library/default.asp?url=/library/en-us/IETechCol/dnwebgen/ie_leak_patterns.asp这篇文章。

btw:
IE 6中垃圾回收算法，就是从那些直接"in scope"的对象开始进行mark清除的：
Every variable which is "in scope" is called a "scavenger". A scavenger may refer to a number, an object, a string, whatever. We maintain a list of scavengers – variables are moved on to the scav list when they come into scope and off the scav list when they go out of scope.

Pseudo-Leaks

这个被称为“秀逗泄露”真是恰当啊：）
看看这个例子:

< html > 
     < head > 
         < script language = " JScript " > 

         function  LeakMemory()
        {
             //  Do it a lot, look at Task Manager for memory response 

             for (i  =   0 ; i  <   5000 ; i ++ )
            {
                hostElement.text  =   " function foo() { } " ;／／看内存会不断增加
            }
        }
         </ script > 
     </ head > 

     < body > 
         < button onclick = " LeakMemory() " > Memory Leaking Insert </ button > 
         < script id = " hostElement " > function  foo() { } </ script > 
     </ body > 
</ html >

MS是这么解释的，这不是内存泄漏。如果您创建了许多无法获得也无法释放的对象，那才是内存泄漏。在这里，您将创建许多元素，Internet Explorer 需要保存它们以正确呈现页面。Internet Explorer 并不知道您以后不会运行操纵您刚刚创建的所有这些对象的脚本。当页面消失时（当您浏览完，离开浏览器时）会释放内存。它不会泄漏。当销毁页面时，会中断循 环引用。

唉~~~

详细原因，大家可以看看http://msdn.microsoft.com/library/default.asp?url=/library/en-us/IETechCol/dnwebgen/ie_leak_patterns.asp这篇文章。

其它一些琐碎的注意点

变量定义一定要用var，否则隐式声明出来的变量都是全局变量，不是局部变量；
全局变量没用时记得要置null；
注意正确使用delete，删除没用的一些函数属性；
注意正确使用try...cache，确保去处无效引用的代码能被正确执行；
open出来的窗口即使close了，它的window对象还是存在的，要记得删除引用；
frame和iframe的情况和窗口的情况类似。

参考资料

http://jibbering.com/faq/faq_notes/closures.html
http://javascript.weblogsinc.com/2005/03/07/javascript-memory-leaks/
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/IETechCol/dnwebgen/ie_leak_patterns.asp
http://72.14.203.104/search?q=cache:V9Bt4_HBzQ8J:jgwebber.blogspot.com/2005/01/dhtml-leaks-like-sieve.html+DHTML+Leaks+Like+a+Sieve+&hl=zh-CN&ct=clnk&cd=9 (这是DHTML Leaks Like a Sieve)一文在google上的cache，原文已经连不上了）
http://spaces.msn.com/siteexperts/Blog/cns!1pNcL8JwTfkkjv4gg6LkVCpw!338.entry
http://support.microsoft.com/default.aspx?scid=KB;EN-US;830555
http://www.ajaxtopics.com/leakpatterns.html
http://blogs.msdn.com/ericlippert/archive/2003/09/17/53028.aspx
http://www.quirksmode.org/blog/archives/2005/02/javascript_memo.html
http://youngpup.net/2005/0221010713
http://blogs.msdn.com/ericlippert/archive/2003/09/17/53038.aspx =
http://support.microsoft.com/kb/266071/EN-US ==>IE 5.0至5.5一些版本中的GC bug
http://www.quirksmode.org/blog/archives/2006/04/ie_7_and_javasc.html ==>ie 7的改进
http://erik.eae.net/archives/2006/04/26/23.23.02/ ==>ie 7的改进
http://www.feedbackarchive.com/spamvampire/today.html ==> Try this script for memory leaks - it leaked 50 megabytes in 15 minutes with firefox on linux:
http://birdshome.cnblogs.com/archive/2005/02/15/104599.html
http://www.quirksmode.org/dom/innerhtml.html
http://www.crockford.com/javascript/memory/leak.html
《JavaScript: The Definitive Guide》4th Edition
http://outofhanwell.com/ieleak/index.php?title=Main_Page