Java - Synchronized

Question

Hi i have made a something that extends thread that adds adds an object that has a IP in it. then i made two instances of this thread and started them. they use the same list.

I now want to use Synchronized to stop the concurrent update problem. But its not working and i cant work out why.

My main class:

import java.util.*;
import java.io.*;
import java.net.*;

class ListTest2 {
    public static LinkedList<Peer>  myList = new LinkedList<Peer>();

    public static void main(String [] args) {   
        try {
            AddIp test1 = new AddIp(myList);
            AddIp test2 = new AddIp(myList);

            test1.start();
            test2.start();      
        } catch(Exception e) {
            System.out.println("not working");
        }
    }
}

My thread class:

 class AddIp extends Thread {
     public static int startIp = 0;

     List<Peer> myList;

     public  AddIp(List<Peer> l) {
         myList = l;
     }


     public synchronized void run() {      
        try {
            startIp = startIp+50;
            int ip = startIp;
            InetAddress address = InetAddress.getByName("127.0.0.0");
            Peer peer = new Peer(address);

            while(ip <startIp+50) { 
                ip++;
                address = InetAddress.getByName("127.0.0."+ip);

                peer = new Peer(address);

                myList.add(peer);

                if(myList.indexOf(peer)== (myList.size() -1)) {
                } else {
                    System.out.println("Lost"+peer.peerIp);
                }
            }     
        } catch(Exception e) {
        }
    }
}

Can anyone help me out here im lost for ideas thanks.

Community · Accepted Answer · 2017-05-23 12:05:12Z

5

 public synchronized void run()

Synchronizes on calling instance: this.

So, 1st thread synchronizes on test1 and 2nd thread synchronizes on test2, which doesn't help at all.

You want to synchronize on the shared resource, in this case: myList

public void run() {
  synchronize(myList){
   //your Logic
  }
}

As a side note: Implement runnable instead of extending a Thread. Read more here.

edited May 23, 2017 at 12:05

CommunityBot

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answered Sep 15, 2013 at 7:58

rocketboy

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2 Comments

1ftw1 Over a year ago

i have tried this and im getting compiler error ";" expected. any ideas why?

1ftw1 Over a year ago

all good worked it out synchronized not synchronize and it worked. thanks for the help.

mcr619619 · Accepted Answer · 2013-09-15 08:20:28Z

1

You'd be better off implementing Runnable oppose to extending thread

also

public void run() {
  synchronize(list){
   //stuffs
  }
}

answered Sep 15, 2013 at 8:20

mcr619619

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Comments

Maxim Shoustin · Accepted Answer · 2013-09-15 08:01:47Z

0

they use the same list.

You can try to use Vector instead List. Vector is synchronized

or set your List to be synchronized:

List myList = Collections.synchronizedList(myList);

instead to use:

synchronize(myList){

 }

answered Sep 15, 2013 at 8:01

Maxim Shoustin

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Comments

Bex · Accepted Answer · 2013-09-15 08:06:07Z

0

The easiest way is to use a List implementation that can handle multiple threads. Try CopyOnWriteArrayList.

answered Sep 15, 2013 at 8:06

Bex

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