Question

jsp StringBuffer 的问题

String user = request.getParameter("txt_name");

System.out.print(user) ;

StringBuffer allusersb = new StringBuffer(session.getAttribute("alluser").toString()) ;

if(allusersb == null){
session.setAttribute("alluser",user) ;
} else {
StringBuffer adduser = allusersb.append(user+"\r\n") ;
session.setAttribute("alluser",user) ;
}

Answer 1

StringBuffer allusersb = new StringBuffer(session.getAttribute("alluser").toString()) ;

这一句不能直接这样用。。。当session里没有alluser时，session.getAttribute("alluser")会返回null，而null调用toString方法会报空指针异常的。所以得先判断session里这个属性是不是空，然后才能获取里面的值。即
// session为空
if(session.getAttribute("alluser") == null){
session.setAttribute("alluser",user) ;
}else{//session里有东西
StringBuffer allusersb = new StringBuffer(session.getAttribute("alluser").toString()) ;
StringBuffer adduser = allusersb.append(user+"\r\n") ;
session.setAttribute("alluser",user) ;
}

Answer 2

你这个出了什么问题舍