java - SNMP 协议(protocol)中的 NullPointerException

Question

我正在尝试将客户端添加到我的 snmp 协议(protocol)程序中。我为我的主要方法添加了这段代码:

public static void main(String[] args) throws IOException{
        SimpleSnmpClient client = new SimpleSnmpClient("udp:10.0.0.50/161");
        String sysDescr = client.getAsString(new OID(".1.3.6.1.2.1.1.1.0"));
        System.out.println(sysDescr);
    }

我从网站上了解到，输出应该是关于运行该程序的设备的某种信息。

我的 SimpleSnmpClient 代码:

public class SimpleSnmpClient {

    private String address;

    private Snmp snmp;


    public static void main(String[] args) throws IOException{
        SimpleSnmpClient client = new SimpleSnmpClient("udp:10.0.0.50/161");
        String sysDescr = client.getAsString(new OID(".1.3.6.1.2.1.1.1.0"));
        System.out.println(sysDescr);
    }

    public SimpleSnmpClient(String address) {
        super();
        this.address = address;
        try {
            start();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    // Since snmp4j relies on asynch req/resp we need a listener
    // for responses which should be closed
    public void stop() throws IOException {
        snmp.close();
    }

    private void start() throws IOException {
        TransportMapping transport = new DefaultUdpTransportMapping();
        snmp = new Snmp(transport);
        // Do not forget this line!
        transport.listen();
    }

    public String getAsString(OID oid) throws IOException {
        ResponseEvent event = get(new OID[]{oid});
        return event.getResponse().get(0).getVariable().toString();
    }


    public void getAsString(OID oids,ResponseListener listener) {
        try {
            snmp.send(getPDU(new OID[]{oids}), getTarget(),null, listener);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }


    private PDU getPDU(OID oids[]) {
        PDU pdu = new PDU();
        for (OID oid : oids) {
            pdu.add(new VariableBinding(oid));
        }

        pdu.setType(PDU.GET);
        return pdu;
    }

    public ResponseEvent get(OID oids[]) throws IOException {
       ResponseEvent event = snmp.send(getPDU(oids), getTarget(), null);
       if(event != null) {
           return event;
       }
       throw new RuntimeException("GET timed out");   
    }

    private Target getTarget() {
        Address targetAddress = GenericAddress.parse(address);
        CommunityTarget target = new CommunityTarget();
        target.setCommunity(new OctetString("public"));
        target.setAddress(targetAddress);
        target.setRetries(2);
        target.setTimeout(1500);
        target.setVersion(SnmpConstants.version2c);
        return target;
    }

    /**
     * Normally this would return domain objects or something else than this...
     */
    public List<List<String>> getTableAsStrings(OID[] oids) {
        TableUtils tUtils = new TableUtils(snmp, new DefaultPDUFactory());

        @SuppressWarnings("unchecked") 
            List<TableEvent> events = tUtils.getTable(getTarget(), oids, null, null);

        List<List<String>> list = new ArrayList<List<String>>();
        for (TableEvent event : events) {
            if(event.isError()) {
                throw new RuntimeException(event.getErrorMessage());
            }
            List<String> strList = new ArrayList<String>();
            list.add(strList);
            for(VariableBinding vb: event.getColumns()) {
                strList.add(vb.getVariable().toString());
            }
        }
        return list;
    }

    public static String extractSingleString(ResponseEvent event) {
        return event.getResponse().get(0).getVariable().toString();
    }
}

但是我收到了 NULLPointerException:

Exception in thread "main" java.lang.NullPointerException
    at org.bihe.SimpleSnmpClient.getAsString(SimpleSnmpClient.java:70)
    at org.bihe.SimpleSnmpClient.main(SimpleSnmpClient.java:41)

它指的是行:

return event.getResponse().get(0).getVariable().toString();

我不知道为什么会这样？谁能帮我解决这个问题？

Answer 1

如果您超时，您会得到一个空响应。来自 ResponseEvent 文档:“a PDU instance if a response has been received. If the request timed out then null will be returned.”

也许您的服务器未启动或不可用。尝试使用一些已知的应用程序访问它，例如 snmpwalk只是为了将您的代码与某些网络问题区分开来。