How to handle multiple submit buttons in the same form

//前端代码
<form method="POST" onsubmit="return ad.checkform(this);return false;">
       ......
       ......
　　　　 <a href="javascript:void(0);" onclick="this.parentNode.submit();">
       <input type="submit" onclick="this.form.target='_blank'" name="preview" value=" 预 览 " >
       <input type="submit" onclick="this.form.target='_self'" name="save" value=" 保 存 " >
</form>