Sharing data in the JRE

Question

I Have a class called syncList that basically has a static List and is also a singleton. I have 2 other classes that use syncList called syncListTest1 and syncListTest2.
syncListTest1 puts an entry into the static list and then displays its contents then waits for thirty seconds(just for testing purposes).
syncListTest2 simply reads the static Lists contents.
When I execute syncListTest1 from a command line and then syncListTest2 from another command line syncListTest2 shows the list as being empty.
My question is: Is there any way of sharing the same syncList between separate applications so that each application can share the same List data?
Any help would be much appreciated before I tear out the last of my receding hairline!!!
Code for syncList.
import java.util.*;
public class syncList
   private static List<String> sl = new ArrayList<String>();
   private static syncList ref;
   private syncList()
   public static synchronized syncList getSyncList()
      if (ref == null)
         System.out.println( "ref is null, creating syncList" );
         ref = new syncList();
      else
         System.out.println( "ref is NOT null, creting syncList" );
      return ref;
   public void add( String Val )
      sl.add( Val );
   public void remove( String Val )
      sl.remove( sl.indexOf( Val ) );
   public void removeAll()
      sl.clear();
   public void showMe()
      for( int i = 0; i < sl.size(); i++ )
         System.out.println( i + " " + sl.get(i) );
}Code for syncListTest1.
public class syncListTest1
   public static void main( String[] args )
      syncList.getSyncList().add( "1 Added" );
      syncList.getSyncList().showMe();
      try
         Thread.sleep(30000);
      catch( InterruptedException e )
}Code for syncListTest2.
public class syncListTest2
   public static void main( String[] args )
      syncList.getSyncList().showMe();
}

Answer 1

Hi
When you say that a class is a Singleton, you basically want the JVM to ensure that there is only one instance of the object [This you do by declaring the costructor private and an instance static member]... Now, look at it from JVM's point of view, there is one class with one static member that is created within the class [getInstance() method]... The JVM has no way to tell anyone or even know the existence of anyone else outside itself.... in your case another JVM. You obviously have to make the two JVMs talk... now the problem remains, how to get them talk.....
There are many solutions to your problem... You are trying to do a IPC [Inter Process Communication] which can be done using socket programming/shared memory... etc...
There is another set of solution which deals with J2EE application server clustering....
But looking at your problem, looks like you want a clean and simple way to do this....
well... one of the very easy ways I would have done it was to use JGroups [Assuming I am not in a J2EE container which provides better options]. This is a very simple clustering framework which helps you replicate some datastructures [in your case a hashmap] over different JVMs and maybe on different machines.... In a very simple High Level overview, it broadcasts the shared datastructure content over the network and all the peers listen to the message thereby updating the shared data.
Infact JGroups provides a DistributedHashtable [http://www.jgroups.org/javagroupsnew/docs/newuser/node55.html]
Hope this helps
Fasih