如何用snmp4j实现对mib信息的收发的例子，大家给看看哪里出了问题

package   snmp; import   java.io.IOException; 
import   java.net.InetAddress; import   org.snmp4j.CommunityTarget; 
import   org.snmp4j.PDU; 
import   org.snmp4j.Snmp; 
import   org.snmp4j.event.ResponseEvent; 
import   org.snmp4j.mp.SnmpConstants; 
import   org.snmp4j.smi.OID; 
import   org.snmp4j.smi.OctetString; 
import   org.snmp4j.smi.UdpAddress; 
import   org.snmp4j.smi.VariableBinding; 
import   org.snmp4j.transport.DefaultUdpTransportMapping; public   class   test3{         public   static   void   main(String[]   args) 
        { 
                try   { 
                        Snmp   snmp   =   new   Snmp(new   DefaultUdpTransportMapping()); 
                        CommunityTarget   target   =   new   CommunityTarget();//agent对象 
                        target.setCommunity(new   OctetString("public"));//设置共同体名,没发现设置RWCommnity的方 
                                                                                                                                                                                                            //法,大概只能设一个. 
                        target.setVersion(SnmpConstants.version2c);//设置版本 
                        target.setAddress(new   UdpAddress("127.0.0.1/161"));//设置IP地址和端口号,这里竟然用'/'来分       
                                                                                                                                                                                                      //隔,当初确实没有料到,JDOC的说明等于没有. 
                        target.setRetries(1);                                             //设置重试次数 
                        target.setTimeout(5000);                           //设置超时                         snmp.listen();                                                                   //监听 
                        PDU   request   =   new   PDU();                   //new   request   PDU包 
                        //set   pud   type   and   set   oid 
                        request.setType(PDU.GET);                 //设置PDU类型, 
                        request.add(new   VariableBinding(new   OID(".1.3.6.1.2.1.1.1.0")));   //OID添加 
                      //   request.add(new   VariableBinding(new   OID(".1.3.6.1.2.1.1.2.0"))); 
                        System.out.println("request   UDP:"   +   request);//请求包内内容输出, 
                        PDU   response   =   null;//定义response包 
                        ResponseEvent   responseEvent   =   snmp.send(request,   target);   //发出request   PDU 
                        //接收response   PDU 
                        response   =   responseEvent.getResponse();                                 //response   PDU包解析 
                        if(response!=null){ 
                                if(response.getErrorIndex()==response.noError&&response.getErrorStatus()==response.noError){ 
                                        System.out.println("no   error."); 
                                        String   pause=responseEvent.getResponse().getVariableBindings().toString(); 
                                        String   getvalue=pause.substring(   pause.indexOf("=   ")+2,pause.indexOf(']')); 
                                        String   oid=pause.substring(pause.indexOf("VBS[")+2,pause.indexOf("=")-1); 
                                        System.out.println(oid); 
                                        System.out.println(response);                                 }else{ 
                                        System.out.println("get   error:"+response.getErrorStatusText()); 
                                } 
                        }else{ 
                                System.out.println("get   response   error"); 
                        } 
                }   catch   (IOException   e)   { 
                        //   TODO   Auto-generated   catch   block 
                        e.printStackTrace(); 
                }         } 
} 运行结果是：
request   UDP:GET[requestID=0, errorStatus=Success(0), errorIndex=0, VBS[1.3.6.1.2.1.1.1 = Null]]
get   response   error为什么得到的response是null呢？是不是还需要安装什么agent? 请大家帮忙指点指点