执行ajax请求成功后的代码会延迟执行怎么办

function takeScreenshot() {
    var canvas = document.getElementById("canvasEdit");
              var imgUrl = convertCanvasToImage(canvas); //截取图片路径,该路径为服务器参数
              var filedata = encodeURIComponent(imgUrl);
              var name = new Date().getTime();
            var filename = name+".png";
            var data = { 
                filename: filename, 
                filedata: filedata,
            };
              $.ajax({ 
                url : ‘${pageContext.request.contextPath}/fileUpload!upBase64.do‘, 
                data : data, 
                dataType : ‘json‘,
                async: false,//去查api看的async 默认为异步加载、为false是为同步加载
                success : function(result) {
                    if(result.success){
                        $("#sign").val(result.obj);
                    }else{
                        msg_obj.innerHTML="由于网络原因图片没有正常上传成功,请重新拍照、签名上传!";  
                    }
                } 
            });
  };