post parameters in doTag() are null

[Lord Yoda]

I have a tag with a doTag() function. The parameters are passed using
POST but are all null. They're fine using get. How do I get parameters
using POST? My code is:

public void doTag() throws JspException {

JspWriter out=getJspContext().getOut();
PageContext pageContext = (PageContext) this.getJspContext();
ServletRequest request = pageContext.getRequest();
String imageid = request.getParameter("imageid");
String caption = request.getParameter("caption");
String imagePath = request.getParameter("imagepath");

.....
}

Thanks. Any help gratefully received.