JavaScript 解析Json字符串的性能比较分析代码

解析时用到的方法一般是eval或者new function，而目前IE8和Firefox3.1又内置了原生的JSON对象（据说会有一定的性能提升）。那我们在实际使用的时候怎样从这三种方法（因为性能问题，不考虑用javascript实现的解析）里面来选择呢？面对众多的浏览器，哪种方式的性能是最好的呢？

一、测试方法

1、首先指定测试次数及JSON字符串

var count = 10000, o = null, i = 0, jsonString = '{"value":{"items": [{"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}';

2、循环解析并记录时间

eval

var beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = eval( "(" + jsonString + ")" ); 
} 
Console.output( "eval:" + ( new Date() - beginTime ) );

new Function

var beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = new Function( "return " + jsonString )(); 
} 
Console.output( "new Function:" + ( new Date() - beginTime ) );

native

if ( typeof JSON !== "undefined" ) { 
var beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = JSON.parse( jsonString ); } 
Console.output( "native:" + ( new Date() - beginTime ) ); 
} else { 
Console.output( "native:not support!" ); 
}

二、测试对象

选择目前主流的浏览器（不考虑Maxthon一类的外壳），包括IE6、7、8，Firefox2、3、3.1，Chrome，Opera及Safari3、4。

三、测试环境

T9300 CPU + 4G RAM + Windows2003，其中IE8使用的是Vista的环境，IE7在另外一台工作机（2G CPU + 2G RAM + Windows2003），考虑到主要是测试浏览器客户端的性能，结果的误差应该能够接受。

四、测试结果
 
*数值越小越好

*在当前列中绿色背景的表示性能最好，红色性能最差
1、Firefox2、3全部垫底，IE6的性能优于IE7（可能和机器不一致有关），Chrome和Safari4的性能远远超出其它浏览器。

2、不同的浏览器下eval和new Function的性能不一致，总的来说eval更好，但Firefox下new Function的性能是eval的一倍，为了更好的兼容各个浏览器，我们把对JSON的解析单独封装成一个对象来处理：
wrapper

var __json = null; 
if ( typeof JSON !== "undefined" ) { 
__json = JSON; 
} 
var browser = Browser; 
var JSON = { 
parse: function( text ) { 
if ( __json !== null ) { 
return __json.parse( text ); 
} 
if ( browser.gecko ) { 
return new Function( "return " + text )(); 
} 
return eval( "(" + text + ")" ) 
} 
}; 
var beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = JSON.parse( jsonString ); } 
Console.output( "wrapper:" + ( new Date() - beginTime ) );

加入Wrapper后的结果：
 
由于涉及到调用对象的开销，封装后JSON对象会比单独调用更慢，但它能保证在各个浏览器下使用最适合的方法。

五、结论

解析Json字符串时，不同的浏览器选择不同的方法：

IE6、7使用eval
IE8使用原生的JSON对象
Firefox2、3使用new Function
Safari4使用eval
其它浏览器下eval和new Function的性能基本一致

如果有不同意见欢迎拍砖：）

Update:

2009.03.23：屏蔽所有Firefox的Add-Ons再进行测试
由于Known在Firefox下运行代码得到了完全不一致的结果，怀疑是Firefox的插件导致，于是禁掉所有插件后（后来表明几乎由Firebug导致），重新在Firefox2、3下测试了一下，结果如下：
 
这表明Firefox本身的性能并不是象我们先前测试的那样低，在去掉插件后性能还是很不错。但是没有Firebug一类的插件支持，Firefox对我们的吸引力也大大降低了。

2009.03.31：循环中每次使用新的json字符串
根据Oliver的描述，他猜测是由于Safari4和Chrome缓存了eval的结果从而导致它们的测试成绩“虚”高，测试结果证明了他的推测:
 
从这个结果中我们可以看到，Opera的性能最好，Ie8其次。

主要修改的代码：

//eval 2: var beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = eval("(" + '{"value":{"items": [{"x":' + i + ',"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}' + ")"); 
} 
Console.output( "eval:" + ( new Date() - beginTime ) ); 
//new Function 
beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = new Function("return " + '{"value":{"items": [{"x":' + i + ',"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}')(); 
} 
Console.output( "new Function:" + ( new Date() - beginTime ) ); 
//native 
if ( typeof JSON !== "undefined" ) { 
beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = JSON.parse('{"value":{"items": [{"x":' + i + ',"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}'); 
} 
Console.output( "native:" + ( new Date() - beginTime ) ); 
} else { 
Console.output( "native:not support!" ); 
} 
//wrapper 
var __json = null; 
if ( typeof JSON !== "undefined" ) { 
__json = JSON; 
} 
var browser = Browser; 
var JSON = { 
parse: function( text ) { 
if ( __json !== null ) { 
return __json.parse( text ); 
} 
if ( browser.gecko ) { 
return new Function( "return " + text )(); 
} 
return eval( "(" + text + ")" ) 
} 
}; 
beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = JSON.parse('{"value":{"items": [{"x":' + i + ',"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}'); 
} 
Console.output( "wrapper:" + ( new Date() - beginTime ) );

附：全部代码

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> 
<html> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 
<title>Parse JsonString</title> 
</head> 
<body> 
<div id="consoleRegion"></div> 
<script type="text/javascript"> 
//yui 
var Browser = function() { 
var o = { 
ie: 0, 
opera: 0, 
gecko: 0, 
webkit: 0 
}; 
var ua = navigator.userAgent, m; 
if ( ( /KHTML/ ).test( ua ) ) { 
o.webkit = 1; 
} 
// Modern WebKit browsers are at least X-Grade 
m = ua.match(/AppleWebKit\/([^\s]*)/); 
if (m&&m[1]) { 
o.webkit=parseFloat(m[1]); 
} 
if (!o.webkit) { // not webkit 
// @todo check Opera/8.01 (J2ME/MIDP; Opera Mini/2.0.4509/1316; fi; U; ssr) 
m=ua.match(/Opera[\s\/]([^\s]*)/); 
if (m&&m[1]) { 
o.opera=parseFloat(m[1]); 
} else { // not opera or webkit 
m=ua.match(/MSIE\s([^;]*)/); 
if (m&&m[1]) { 
o.ie=parseFloat(m[1]); 
} else { // not opera, webkit, or ie 
m=ua.match(/Gecko\/([^\s]*)/); 
if (m) { 
o.gecko=1; // Gecko detected, look for revision 
m=ua.match(/rv:([^\s\)]*)/); 
if (m&&m[1]) { 
o.gecko=parseFloat(m[1]); 
} 
} 
} 
} 
} 
return o; 
}(); 
var Console = { 
consoleRegion: null, 
getRegion: function() { 
if ( this.consoleRegion === null ) { 
this.consoleRegion = document.getElementById( "consoleRegion" ); 
} 
return this.consoleRegion; 
}, 
output: function( text ) { 
this.getRegion().innerHTML += "<br/>" + text; 
} 
}; 
//test code 
var count = 10000, o = null, i = 0, jsonString = '{"value":{"items": [{"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}'; 
//eval 
var beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = eval( "(" + jsonString + ")" ); 
} 
Console.output( "eval:" + ( new Date() - beginTime ) ); 
//new Function 
beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = new Function( "return " + jsonString )(); 
} 
Console.output( "new Function:" + ( new Date() - beginTime ) ); 
//native 
if ( typeof JSON !== "undefined" ) { 
beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = JSON.parse( jsonString ); 
} 
Console.output( "native:" + ( new Date() - beginTime ) ); 
} else { 
Console.output( "native:not support!" ); 
} 
//wrapper 
var __json = null; 
if ( typeof JSON !== "undefined" ) { 
__json = JSON; 
} 
var browser = Browser; 
var JSON = { 
parse: function( text ) { 
if ( __json !== null ) { 
return __json.parse( text ); 
} 
if ( browser.gecko ) { 
return new Function( "return " + text )(); 
} 
return eval( "(" + text + ")" ) 
} 
}; 
beginTime = new Date(); 
for ( i = 0; i < count; i++ ) { 
o = JSON.parse( jsonString ); 
} 
Console.output( "wrapper:" + ( new Date() - beginTime ) ); 
//alert( o.value.items[0].z ); 
</script> 
</body> 
</html>