ajax getJSON请求

// http GET请求(json类型)
function httpGetJSON(url, callback, async) {
	var xmlhttp;
	xmlhttp = window.XMLHttpRequest ? new XMLHttpRequest : new ActiveXObject("Microsoft.XMLHTTP");
	xmlhttp.onreadystatechange = function() {
		if(4 == xmlhttp.readyState && 200 == xmlhttp.status) {
			try {
				callback(JSON.parse(xmlhttp.responseText));
			} catch(e) {
				callback(eval("(" + xmlhttp.responseText + ")"));
			}
		}
	};
	async = false == async ? false : true;
	xmlhttp.open("GET", url, true);
	xmlhttp.send();
};