Java Multithread UDP Server Error

Question

I write UDP server and client in Java to send message from client to server. In my case, every message from client will be stored in database. I tried to make a multithread UDP server using fixed thread pool. But now I encountered a problem, I found out that one message from client can be processed by more than one thread. My server look like this :

    DatagramSocket serverSocket = new DatagramSocket(9876);
    Map<String, Integer> retryMap = new ConcurrentHashMap<>();
    byte[] receiveData = new byte[1024];

    ExecutorService executor = Executors.newFixedThreadPool(2);

    while(true){
        DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
        serverSocket.receive(receivePacket);
        executor.execute(new ThreadServer(receivePacket, retryMap));
    }

ThreadServer class :

public void run(){
  String temp = new String(packet.getData(), 0, packet.getLength()); 
  long threadId = Thread.currentThread().getId();
  System.out.println("Thread Id = "+threadId+" Message = "+temp);
  // insert message to db
}

And my client look like this :

public class UDPClient1{
String message;
public UDPClient1(String message){
    this.message = message;
    try{
        byte[] sendData;

        DatagramSocket clientSocket = new DatagramSocket();
        InetAddress IPAddress = InetAddress.getByName("localhost");

        sendData = message.getBytes("UTF-8");

        DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
        clientSocket.send(sendPacket);

        clientSocket.close();
    }catch(Exception e){
        e.printStackTrace();
    }
}

public static void main(String args[]) throws InterruptedException{
    int i = 0;
    while(i < 1000){
        new UDPClient1("T000"+i+"_"+i);
        System.out.println(i);
        i++;
        Thread.sleep(2);
    }
}}

The output from the server show that sometimes one message from client will be processed by 2 thread or more.

Thread Id = 18 Routing Id = T000975 09:54:11,759 ERROR LogRoutingFileDaoImpl:59 - AMS - Error insert to database org.springframework.dao.DuplicateKeyException: PreparedStatementCallback; SQL [INSERT INTO LOG_ROUTING_FILE (ID_ROUTING_FILE, FILE_NM , PATH, TYPES, PROTOCOL, SOURCE_NM, TARGET_NM) VALUES (?,?,?,?,?,?,?)]; ERROR: duplicate key value violates unique constraint "log_routing_file_id_routing_file_key"; nested exception is org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "log_routing_file_id_routing_file_key" 
Thread Id = 9 Routing Id = T000975 09:54:11,759 ERROR LogRoutingFileDaoImpl:59 - AMS - Error insert to database org.springframework.dao.DuplicateKeyException: PreparedStatementCallback; SQL [INSERT INTO LOG_ROUTING_FILE (ID_ROUTING_FILE, FILE_NM , PATH, TYPES, PROTOCOL, SOURCE_NM, TARGET_NM) VALUES (?,?,?,?,?,?,?)]; ERROR: duplicate key value violates unique constraint "log_routing_file_id_routing_file_key"; nested exception is org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "log_routing_file_id_routing_file_key"

The message T000975 is processed by thread 9 and 18.. Can anyone help me to solve this? :)

user207421 · Accepted Answer · 2013-10-21 03:04:28Z

2

byte[] receiveData = new byte[1024];

You're sharing the same byte array among all the DatagramPackets. Move this line inside the receive loop.

answered Oct 21, 2013 at 3:04

user207421

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2 Comments

Mari_Yaguchi Over a year ago

Thanks, it worked :) can you please kindly explain what is the difference between placing the recevieData declaration outside the loop and inside the loop?

user207421 Over a year ago

I've answered that. 'You're sharing the same byte array among all the DatagramPackets.' If you move it inside the loop, there is a new byte array per packet.

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