在JSP页中中增加 onChange="check_sum()" 
/**数量*单价=金额*/
function check_sum(){
	var num,item_price,item_money;
    num  = trim(myform.num.value);
	item_price =trim(myform.item_price.value);
	if(num == "")
	    num=0.0;
		alert("dd="+num);
	if(item_price == "")
	    item_price=0.0;
	item_money=num*item_price;
    window.parent.record_handler.myform.item_money.value=item_money.toFixed(2);
}