判断jquery是否已经加载，如果没有动态加载

方法一：

// Only do anything if jQuery isn't defined
if (typeof jQuery == 'undefined') {

    if (typeof $ == 'function') {
        // warning, global var
        thisPageUsingOtherJSLibrary = true;
    }

    function getScript(url, success) {

        var script     = document.createElement('script');
             script.src = url;

        var head = document.getElementsByTagName('head')[0],
        done = false;

        // Attach handlers for all browsers
        script.onload = script.onreadystatechange = function() {

            if (!done && (!this.readyState || this.readyState == 'loaded' || this.readyState == 'complete')) {

            done = true;

                // callback function provided as param
                success();

                script.onload = script.onreadystatechange = null;
                head.removeChild(script);

            };

        };

        head.appendChild(script);

    };

    getScript('http://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js', function() {

        if (typeof jQuery=='undefined') {

            // Super failsafe - still somehow failed...

        } else {

            // jQuery loaded! Make sure to use .noConflict just in case
            fancyCode();

            if (thisPageUsingOtherJSLibrary) {

                // Run your jQuery Code

            } else {

                // Use .noConflict(), then run your jQuery Code

            }

        }

    });

} else { // jQuery was already loaded

    // Run your jQuery Code

};

//该代码片段来自于: http://www.sharejs.com/codes/javascript/4281

方法二：document.write

var jQueryScriptOutputted = false;
function initJQuery() {

    //if the jQuery object isn't available
    if (typeof(jQuery) == 'undefined') {

        if (! jQueryScriptOutputted) {
            //only output the script once..
            jQueryScriptOutputted = true;

            //output the script (load it from google api)
            document.write("<scr" + "ipt type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></scr" + "ipt>");
        }
        setTimeout("initJQuery()", 50);
    } else {

        $(function() {
            // do anything that needs to be done on document.ready
            // don't really need this dom ready thing if used in footer
        });
    }

}
initJQuery();
//该代码片段来自于: http://www.sharejs.com/codes/javascript/4281