NullPointerException when trying to communicate between Windows and Linux

[Ahmid]

Hello,

Simply, I am running Modbuspal.java on a windows machine, where I have created a slave and filled some coils and registers.

Then from the same windows machine, I used j2mod in order to read coils/registers, the operation wen successfully.

The problem is that when I ran the same code on a linux (ubuntu) virtual machine, a null pointer exception is being released everytime, the connection phase is succeeding, but the reading is not.

The code is:

import java.net.InetAddress; import java.util.logging.Level; import java.util.logging.Logger;

import com.ghgande.j2mod.modbus.Modbus; import com.ghgande.j2mod.modbus.ModbusException; import com.ghgande.j2mod.modbus.io.ModbusTCPTransaction; import com.ghgande.j2mod.modbus.msg.ReadCoilsRequest; import com.ghgande.j2mod.modbus.msg.ReadCoilsResponse; import com.ghgande.j2mod.modbus.msg.ReadInputDiscretesRequest; import com.ghgande.j2mod.modbus.msg.ReadInputDiscretesResponse; import com.ghgande.j2mod.modbus.msg.ReadMultipleRegistersRequest; import com.ghgande.j2mod.modbus.msg.ReadMultipleRegistersResponse; import com.ghgande.j2mod.modbus.net.TCPMasterConnection;

//Process public class ModbusTest {

/* The important instances of the classes mentioned before */
protected static TCPMasterConnection con = null; //the conection
/* Variables for storing the parameters */
protected static InetAddress addr = null; //the slave's address
protected static final int port = 502;
protected static final int ref = Integer.parseInt("1", 16); //the reference; offset where to start reading from
protected static final int count = 100; //the number of DI's to read
protected static final int repeat = 1; //a loop for repeating the transaction
static int SlaveAddr=1;

public static void main(String[] args) {

    try {
        addr = InetAddress.getByName("130.190.97.101");
        //Coils
        ReadCoilsRequest RCreq = new ReadCoilsRequest (3,1);
        ReadCoilsResponse RCres = new ReadCoilsResponse();

        //Registers
        ReadMultipleRegistersRequest RRreq = new ReadMultipleRegistersRequest (4,1);
        ReadMultipleRegistersResponse RRres = new ReadMultipleRegistersResponse();

        RCreq.setUnitID(SlaveAddr); //set Slave Address  
        RCres.setUnitID(SlaveAddr); //set Slave Address

        RRreq.setUnitID(SlaveAddr);
        RRres.setUnitID(SlaveAddr);

        //2. Open the connection
        con = new TCPMasterConnection(addr);
        con.setPort(5555);
        con.connect();

        ModbusTCPTransaction trans = null;
        trans = new ModbusTCPTransaction(con);

        trans.setRequest(RCreq);
        trans.execute();
        RCres = (ReadCoilsResponse) trans.getResponse();
        System.out.println("Connected to = "+ addr + " " + con.isConnected() + " /Start Register " + Integer.toHexString(ref));
        System.out.println("The value READ (coils): " + RCres.getCoils().toString());

        trans.setRequest(RRreq);
        trans.execute();
        RRres = (ReadMultipleRegistersResponse) trans.getResponse();
        for (int n = 0; n < RRres.getWordCount(); n++) {
            System.out.println("Word = " + RRres.getRegisterValue(n));
        }

        //6. Close the connection
        con.close();
    } catch (Exception ex) {
        Logger.getLogger(ModbusTest.class.getName()).log(Level.SEVERE, null, ex);
    }
}

}

The error is in the line: System.out.println("The value READ (coils): " + RCres.getCoils().toString());

Connected to = /130.190.97.101 true /Start Register 1 Apr 19, 2017 2:55:16 AM ModbusTest main SEVERE: null java.lang.NullPointerException at ModbusTest.main(ModbusTest.java:62)

[steveohara]

It's not clear from your code if the exception is because RCres is null or the result of RCres.getCoils(). Could you tell me which it is? Also, can you turn on debug logging to see why an exception is not being thrown for an empty response.

[steveohara]

It has been 2 months and nobody else has reported an issue so I'm closing this ticket