如何使div设position:relative属性的高度自适应？

div+css中 父容器用position:relative定义，子容器用position:absolute定义 。父容器不能高度自适应
eg:

<div style="position:relative" class="div1">
    <div style="position:absolute" class="div2">
        <img src="http://a.hiphotos.baidu.com/image/w%3D1366%3Bcrop%3D0%2C0%2C1366%2C768/sign=f30ddb3e8601a18bf0eb164ca8193c6b/0824ab18972bd4074d238eef7a899e510fb3093f.jpg" style="max-width:100%">
    </div>
    <div style="position:absolute" class="div2">
    </div>
</div>