NullPointerException in SNMP protocol

Question

I am trying to add a client to my snmp protocol program. I added this code for my main method:

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);
    }

and the site I learned from said the output should be about some kind of information about the device run this program.

My SimpleSnmpClient code:

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();
    }
}

But I received NULLPointerException:

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

It refer to the line:

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

and I have no idea why it is happening? Can anyone help me to solve this problem?

Answer 1

If you time-out you can get a null response. From the ResponseEvent docs: "a PDU instance if a response has been received. If the request timed out then null will be returned."

Perhaps you server isn't up or isn't available. Try accessing it with some known app like snmpwalk just to distinguish your code from some network issue.

Answer 2

Does the device you are trying to get a response from have a community string set? A community is like a password, and is set on a device to prevent people from retrieving information from this device without authorization. If the community string is incorrect, you won't receive a response.

Answer 3

make sure you added same community string for SNMP service agent.

for windows... open services -> right click in SNMP - > under security tab ->add community String

Answer 4

I faced the same issue today. And, I resolved it by doing the following:

• Type "services.msc" in Windows Run dialog box and hit enter. This will open the Services window.
• Double click on the "SNMP Service" and select "Security" tab. In "Accepted Community Names" section, add the community name "public" with Security level "READ ONLY" or "READ WRITE" (based on your need). Then click OK button to finish this process.
• Now, try to run the program. It will return the result.

I've got the below while running it from NetBeans on my Windows 10 Operating System. Output: Hardware: Intel64 Family 6 Model 60 Stepping 3 AT/AT COMPATIBLE - Software: Windows Version 6.3 (Build 16299 Multiprocessor Free)