Java SNMP4J Trap application freezes GUI

I have a Java SNMP trap application that is designed to listen to an SNMP agent and print the received SNMP messages to the JTextArea in a JFrame window.

The part I below is my source code showing the contents of the TrapReceiver class. In this class, the listener is the place that does most of the work. The intantiated class in the JFrame class on which I intend to display messages on the JTeaxtArea mentioned above. I send the JTextArea object reference, SNMP agent URL and port to the constructor of the TrapReceiver class, and then calls the start method of the TrapReceiver object to start execution on a separate thread other than the JFrame instance. Part II below shows how I instantiate the TrapReeceiver class in a JFrame instance.

When I launch the application, I noticed that the JFrame instance (i.e. the GUI) is freezing and no message is being printed on the said JTeaxtArea in the JFrame instance that instantiates the TrapReeceiver class shown in part I below.

My question is, why does the JFrame instance (i.e. the GUI) freeze even though the TRapReceiver itself runs as a separate thread? Also, I'm wondering what a possible solution to this freezing problem might be. Thank you in advance.

PS: I have confirmed that TrapReceiver works fine and can print messages to stdout when launched as a standalone application without a GUI, but this particular GUI is frozen somehow due to some thread synchronization issue. I tried to start TrapReceiver without inserting the thread and even so, the GUI was still hanging.


 package com.[Intenionally removed].snmp;

 import javax.swing.JTextArea;
 import org.snmp4j.*;
 import org.snmp4j.smi.OctetString;
 import org.snmp4j.smi.TcpAddress;
 import org.snmp4j.smi.TransportIpAddress;
 import org.snmp4j.smi.UdpAddress;
 import org.snmp4j.transport.AbstractTransportMapping;
 import org.snmp4j.transport.DefaultTcpTransportMapping;
 import org.snmp4j.transport.DefaultUdpTransportMapping;
 import org.snmp4j.util.MultiThreadedMessageDispatcher;
 import org.snmp4j.util.ThreadPool;

 public class TrapReceiver implements CommandResponder, Runnable {

   private String targetSnmpAgentURL;
   private int targetSnmpAgentPort;
   private JTextArea outConsole;

   public TrapReceiver() {

   public TrapReceiver(JTextArea outConsole) {
       this.outConsole = outConsole;

   public TrapReceiver(JTextArea outConsole, String targetSnmpAgentURL, int  targetSnmpAgentPort) {
       this.targetSnmpAgentURL = targetSnmpAgentURL;
       this.targetSnmpAgentPort = targetSnmpAgentPort;
       this.outConsole = outConsole;

       try {
           listen(new UdpAddress(targetSnmpAgentURL + "/" + targetSnmpAgentPort));
       } catch (IOException e) {

    public final synchronized void listen(TransportIpAddress address) throws IOException {

        AbstractTransportMapping transport;
        if (address instanceof TcpAddress) {
            transport = new DefaultTcpTransportMapping((TcpAddress) address);
        } else {
            transport = new DefaultUdpTransportMapping((UdpAddress) address);

        ThreadPool threadPool = ThreadPool.create("DispatcherPool", 10);
        MessageDispatcher mDispathcher = new MultiThreadedMessageDispatcher(
            threadPool, new MessageDispatcherImpl());

        // add message processing models
        mDispathcher.addMessageProcessingModel(new MPv1());
       mDispathcher.addMessageProcessingModel(new MPv2c());

        // add all security protocols
        SecurityProtocols.getInstance().addPrivacyProtocol(new Priv3DES());

        // Create Target
        CommunityTarget target = new CommunityTarget();
        target.setCommunity(new OctetString("public"));

        Snmp snmp = new Snmp(mDispathcher, transport);

        System.out.println("Listening on " + address);

        try {
        } catch (InterruptedException ex) {

    * This method will be called whenever a pdu is received on the given port
    * specified in the listen() method
    public synchronized void processPdu(CommandResponderEvent cmdRespEvent) {
        //System.out.println("Received PDU...");
        outConsole.append("Received PDU...\n");
        PDU pdu = cmdRespEvent.getPDU();

        if (pdu != null) {            
        outConsole.append("Trap Type = " + pdu.getType() + "\n");
        outConsole.append("Alarm Type: " + pdu.getVariableBindings().get(4) + "\n");
        outConsole.append("Alarm Message:  " + pdu.getVariableBindings().get(9) +  "\n\n");


     public void run() {
        try {           
            listen(new UdpAddress(targetSnmpAgentURL + "/" + targetSnmpAgentPort));
        } catch (IOException e) {
            outConsole.append("\nError occured while listening to SNMP messages: \n" + e.getMessage() + "\n\n");

} //end of class TrapReceiver



In the following example, I am launching an instance of the TrapReceiver class on a thread.

   private void jButtonStartListeningSNMPActionPerformed(java.awt.event.ActionEvent evt)   {                                                          

    Thread snmpThread = 
           new Thread(new TrapReceiver(jTextAreaSNMPAlarmOutput, jTextFieldSnmpAgentUrl.getText().trim(), Integer.parseInt(jTextFieldSnmpAgentPort.getText().trim())));




source to share

1 answer

the problem is what you are calling listen()

in the constructor TrapReceiver

which happens on the gui thread. you just want to call listen()

in a method run()

, like the part that happens on a new thread.



All Articles