How to get tcp dump & snoop

Snoop to file
# snoop -xv -o snoopfilename port portNo


Read from file with Snoop
# snoop -xv -i snoopfilename port portNo

TcpDump
# tcpdump -i eth0 -s 0 -w snoopfilename port portNo

Vector vs ArrayList vs HashSet

List syncList = Collections.synchronizedList(new ArrayList());

Set syncSet = Collections.synchronizedSet(new HashSet());

Vector is naturally synchronized.

Performance Test Result
#1 :ArrayList
#2 :Vector
#3 : HashSet

Read more: Java Code Geeks: Java Best Practices – Vector vs ArrayList vs HashSet

http://www.javacodegeeks.com/2010/08/java-best-practices-vector-arraylist.html#ixzz0wxSNUeuS

Scalability hints

Avoid lock on static variables or static methods:
It locks your class. Even worse than locking your object!

private static Object myVar = null;
private foo{
    synchronized (myVar){// do not do that
       ....
   }
}
private static synchronized myMethod(){
     .....
}

Use lock free data structures
Use java.util.concurrent.atomic package.It uses lock-free algorithms under the help with hardware synchronization primitives without using native code.

Reduce lock granularity:

Try to prefer  "block locks" instead of "method locks" if possible.synchronized methods locks the Object instance as you know.
public class SchemaManager {
private HashMap schema;
private HashMap treeNodes;
//Bad way!
public boolean synchronized updateSchema(HashMap nodeTree) {
          String nodeName = (String)nodeTree.get("nodeName");
          String nodeAttributes = (List)nodeTree.get("attributes");
          if (nodeName == null)
                  return false;
          else
                  return schema.update(nodeName,nodeAttributes);
}
//Good Way
public boolean updateSchema(HashMap nodeTree) {
String nodeName = (String)nodeTree.get("nodeName");
String nodeAttributes = (List)nodeTree.get("attributes");
synchronized (schema) {
          if (nodeName == null)
                   return false;
          else
                   return schema.update(nodeName,nodeAttributes); }
}
public boolean synchronized updateTreeNodes() { ...... } }


Make synchronized blocks as short as possible
Move the thread safe code outside of the synchronized block.
 
public boolean updateSchema(HashMap nodeTree) {
               synchronized (schema) {
                          String nodeName = (String)nodeTree.get("nodeName");
                          String nodeAttributes = (List)nodeTree.get("attributes");
                          if (nodeName == null)
                                 return false;
                          else
                                 return schema.update(nodeName,nodeAttributes);
                }
}
 
public boolean updateSchema(HashMap nodeTree) {
           String nodeName = (String)nodeTree.get("nodeName");
           String nodeAttributes = (List)nodeTree.get("attributes");
           synchronized (schema) {
                     if (nodeName == null)
                            return false;
                     else
                            return schema.update(nodeName,nodeAttributes);
          }
}




Try to use ThreadLocal variables if possible.

Java 6 allows classpath wildcards

All we hate to complete our $CLASSPATH  !

We always try to setup a CLASSPATH enviroment variable while dealing with all & everytime & anywhere.

Fortunately Sun noticed our tears at the end :)

Now we can specify lib/*, and all of the JAR files listed in that directory (not recursively), in the classpath.

We collect all dependencies under a directory and it is enough to say everthing is out there...

It is my blog :) It is not forbidden for me to share off topic pictures huh ?

Me

&

Marc Dacascos

&

Tolga Karadeniz

Reliable Multicasting with JGroups

Append the jgroups jar to your classpath.

Then execute the following code twice in different consoles like this.These are our chat clients.

java -Djava.net.preferIPv4Stack=true SimpleChat


import java.io.*;
import java.net.InetAddress;
import org.jgroups.*;

public class SimpleChat implements Receiver {
    JChannel channel;

    public static void main (String[] args) throws Exception {
        new SimpleChat ().start ();
    }

    private void start () throws Exception {
        channel = new JChannel ();
        channel.connect ("ChatCluster");
        channel.setReceiver (this);
        eventLoop ();
        channel.close ();
    }

    private void eventLoop () {
        BufferedReader in = new BufferedReader (new InputStreamReader (System.in));
        while ( true ) {
            try {
                System.out.print ("> ");
                System.out.flush ();
                String line = in.readLine ().toLowerCase ();
                if (line.startsWith ("quit") || line.startsWith ("exit")) {
                    break;
                }
                line = "[" + this + "] " + line;
                Message msg = new Message (null, null, line);
                channel.send (msg);
            } catch (Exception e) {
                e.printStackTrace ();
            }
        }
    }

    public void receive (Message message) {
        System.out.println ("message = " + new String (message.getBuffer ()));
    }

    public byte[] getState () {
        return new byte[0]; 
    }

    public void setState (byte[] bytes) {
        //To change body of implemented methods use File | Settings | File Templates.
    }

    public void viewAccepted (View view) {
        System.out.println ("view = " + view.toString ());
    }

    public void suspect (Address address) {
        System.out.println ("address = " + address);
    }

    public void block () {

    }
}

There are more fancy examples in there. Like org.jgroups.demos.Draw !