Null Pointer Exception when calling ModbusTCPTransaction.execute()

[Mark Walton]

Hello:
I have an active TCP connection that has been verfied. When I run the code below I get a NullPointerException when calling trans.execute. Is there anyone that can tell me what I am doing wrong. Thanks in advance.

public void readWriteTest(){

    int refRead = Integer.parseInt("30001", 16);

    ReadMultipleRegistersRequest request = new ReadMultipleRegistersRequest(refRead, 1);
    ModbusTCPTransaction trans = new ModbusTCPTransaction(tcpMasterConnection);
    trans.setRequest(request);

   try{
        trans.execute();
    }
    catch(ModbusSlaveException modbusSlaveException){}
    catch(ModbusIOException modbusIOException){}
    catch(ModbusException modbusException){}

   ReadMultipleRegistersResponse response = (ReadMultipleRegistersResponse)trans.getResponse();

   for(int iIndex = 0; iIndex < response.getWordCount(); iIndex++){
       System.out.println("Value = " + response.getRegisterValue(0));
   }
}

[R. J. Mathar]

I propose you put the modbusSlaveException.printStackTrace() into the block of the ModbusSlaveException and the corresponding stack traces into the other two blocks to get more information on why the execute() fails.

[Andrey Lavrukhin]

I have the same issue. I connected my TCPMasterConnection to the modbus TCP emulator (modsim32). If m_Transaction.setReconnecting(false); then m_Transaction.execute() throw java.lang.NullPointerException at com.ghgande.j2mod.modbus.io.ModbusTCPTransport.writeMessage(ModbusTCPTransport.java:151)
at com.ghgande.j2mod.modbus.io.ModbusTCPTransaction.execute(ModbusTCPTransaction.java:218)

The higher frequency of requests (lower delays in Thread.sleep() inside loop) - the fasters NullPointerException appears. But always in 1-5 seconds from start.

If m_Transaction.setReconnecting(true), then application create too many (over hundred) TCP-connetions in TIME_WAIT state. It's unacceptable in production, cause many of modbus-tcp slaves have limit of simultaneously incoming connection (max 16 connection in my case).

I need to request modbus-slaves several times in second.
How can I work around NullPointerException without hundreds of unclosed connection?

[Andrey Lavrukhin]

This issue was on j2mod-1.06.
Found it solved in j2mod-2.3.3