关于Javascript的内存泄漏问题
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();
}
关于Closure的知识,大家可以看看 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 。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);
}
else if(obj.detachEvent){// IE-style listeners?
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);
return true;
}
function hit(evt) {
alert(click);
}
</script>
</head>
<body >
<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">
// 这个函数会引发Cross-Page Leaks
function LeakMemory()
{
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 >");
var childDiv = document.createElement("<div >");
// This will leak a temporary object
parentDiv.appendChild(childDiv);
hostElement.appendChild(parentDiv);
hostElement.removeChild(parentDiv);
parentDiv.removeChild(childDiv);
parentDiv = null ;
childDiv = null ;
}
hostElement = null ;
}
// 而这个函数不会引发Cross-Page Leaks
function CleanMemory()
{
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 >");
var childDiv = document.createElement("<div >");
// 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 > Memory Leaking Insert </button>
<button > 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。
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 > 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。
考资料参:
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