JavaScript解析Json字符串的性能比较

我们在使用AJAX来做服务器端和客户端交互的时候，一般的做法是让服务器端返回一段JSON字符串，然后在客户端把它解析成JavaScript对象。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
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>