java空指针异常-发生在while循环中的不同点

Question

I have the following Java while loop:

while(true){
    byte buffer[] = new byte[MAX_PDU_SIZE];
    packet = new DatagramPacket(buffer, buffer.length);

    socket.receive(packet);

    Pdu pdu = pduFactory.createPdu(packet.getData());

    System.out.print("Got PDU of type: " + pdu.getClass().getName());
    if(pdu instanceof EntityStatePdu){
        EntityID eid = ((EntityStatePdu)pdu).getEntityID();
        Vector3Double position = ((EntityStatePdu)pdu).getEntityLocation();
        System.out.print(" EID:[" + eid.getSite() + ", " + eid.getApplication() + ", " + eid.getEntity() + "] ");
        System.out.print(" Location in DIS coordinates: [" + position.getX() + ", " + position.getY() + ", " + position.getZ() + "]");
        }
    System.out.println();
    }
}

The intended function of the while loop is to capture any PDUs that are being sent across the network, and display information about them.

When I run the code, I go get the output that I had intended in the console- at least initially... But after it has returned information about a number of the PDUs, I got an error displayed in the console (can't remember what it said now- but I thought it may be because it was trying to capture a PDU when there wasn't one being sent).

I have tried amending my code to account for the occasion that a PDU may not be being received over the network at the time that it is trying to capture the information about the PDU by surrounding the code with the following try- catch loop:

try{
    socket = new MulticastSocket(EspduSender.PORT);
    address = InetAddress.getByName(EspduSender.DEFAULT_MULTICAST_GROUP);
    socket.joinGroup(address);

    while(true){
        byte buffer[] = new byte[MAX_PDU_SIZE];
        packet = new DatagramPacket(buffer, buffer.length);

        socket.receive(packet);

        Pdu pdu = pduFactory.createPdu(packet.getData());

        System.out.print("Got PDU of type: " + pdu.getClass().getName());
        if(pdu instanceof EntityStatePdu){
            EntityID eid = ((EntityStatePdu)pdu).getEntityID();
            Vector3Double position = ((EntityStatePdu)pdu).getEntityLocation();
            System.out.print(" EID:[" + eid.getSite() + ", " + eid.getApplication() + ", " + eid.getEntity() + "] ");
            System.out.print(" Location in DIS coordinates: [" + position.getX() + ", " + position.getY() + ", " + position.getZ() + "]");
            }
        System.out.println();
        }
    }
catch(Exception e){
    System.out.println(e);
    System.out.println("This is where the error is being generated");
}

However, when I now run the code- it still displays the first x number of DIS packets it captures (x varies every time I run the code), but then gives me a java.lang.NullPointerException . As I understand, this will occur either because the PDU that the code has captured does not contain any information, (ie an 'empty' PDU), or because it is attempting to receive a PDU when there isn't one being sent over the network.

How can I make the code 'skip' the occasion that it doesn't receive a PDU, and just keep running? Or is there something else I should do to get rid of this error?

Answer 1

This may not fix your problem (won't know until you put a stack trace up), but you could check to see if pdu is null before using it.

while(true){
    byte buffer[] = new byte[MAX_PDU_SIZE];
    packet = new DatagramPacket(buffer, buffer.length);

    socket.receive(packet);

    Pdu pdu = pduFactory.createPdu(packet.getData());

    if (pdu != null) {
        System.out.print("Got PDU of type: " + pdu.getClass().getName());
        if(pdu instanceof EntityStatePdu){
            EntityID eid = ((EntityStatePdu)pdu).getEntityID();
            Vector3Double position = ((EntityStatePdu)pdu).getEntityLocation();
            System.out.print(" EID:[" + eid.getSite() + ", " + eid.getApplication() + ", " + eid.getEntity() + "] ");
            System.out.print(" Location in DIS coordinates: [" + position.getX() + ", " + position.getY() + ", " + position.getZ() + "]");
            }
        System.out.println();
    }
}

If you get a stack trace out, I can change this for your actual problem.

Answer 2

You code looks perfectly alright , except the below missing line.

if (pdu != null) {

//execute this
}