ajax异步请求loading

1、找到一张loading图片

2、添加样式

<style>
.loadingWrap{
    position:fixed;
    top:;
    left:;
    width:100%;
    height:100%;
    z-index:;
    background-image:url(${ctx}/webapp/img/loading.gif);
    background-repeat:no-repeat;
    background-position:center center;
    background-color:#000;
    background-color:rgba(0,0,0,0.5);
    filter:alpha(opacity=50);
}
</style> 

3、ajax调用

function downloadTaskPackageRel(projectId){
    $.ajax({
        url : ctx + '/exportTaskPackageRel.do',
        type : 'POST',
        dataType : 'json',
        data : {
            projectId:projectId,
        },
        success : function(data) {
            $("#msg").val(data.msg);
            downLoadExportFile(data.fileName);
        },
        error : function() {
        },
        beforeSend: function(){
            $('<div class="loadingWrap"></div>').appendTo("body");
        },
        complete: function(){
           $(".loadingWrap").remove();
        }
    });
}  

参考：https://blog.csdn.net/jwiang/article/details/52850703  感谢作者