效果就是这个图
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title></title>
<style>
.box {
width: 100px;height:100px;
border: 2px solid green;
position: relative;
border-radius:50%;
}
.box:before{
content:'';
border:2px solid red;
position: absolute;
display: block;
top: 0px;
bottom:0px;
right: 0px;
left: 0px;
border-radius:50%;
}
</style>
</head>
<body>
<div class="box"></div>
</body>
![Spring MVC 自学杂记（五） -- SpringMVC与前台的json数据交互[图]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)