How to implement a singleton class across apps in a managed server}

Question

Hi ,
I tried implementing a singleton class , and then invoking the same in a filter class.
Both are then deployed as a web app (war file) in a managed server.
I created a similar app , deployed the same as another app in the same managed server .
I have a logger running which logs the singleton instances as well.
But am getting two instances of the singleton class in the two apps - not the same .
I was under the impression that , a singleton is loaded in the class loader level , and since all apps under the same managed server used the same JVM , singleton will only get initialized once.
Am i missing something here ? or did i implement it wrong..?
public class Test
   private static Test ref ;
   private DataSource X; 
   static int Y;
   long Z ;  
   private Test ()
      // Singleton
       Z= 100 ;
   public static synchronized Test getinstance()  throws NamingException, SQLException
      if(ref == null)
         ref = new Test() ;        
         InitialContext ic = new InitialContext();
         ref.X = (DataSource)ic.lookup ("jdbc/Views");
      return ref ;       
   public Object clone()throws CloneNotSupportedException
       throw new CloneNotSupportedException();
   public int sampleMethod (int X) throws SQLException
public final class Filter implements Filter
     public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException
          try
               Test ref = Test.getinstance();
               log.logNow(ref.toString());
}Edited by: Tom on Dec 8, 2010 2:45 PM
Edited by: Tom on Dec 8, 2010 2:46 PM

Answer 1

Tom wrote:
Hi ,
I tried implementing a singleton class , and then invoking the same in a filter class.
Both are then deployed as a web app (war file) in a managed server.
I created a similar app , deployed the same as another app in the same managed server .
I have a logger running which logs the singleton instances as well.
But am getting two instances of the singleton class in the two apps - not the same .Two apps = two instances.
Basically by definition.
>
I was under the impression that , a singleton is loaded in the class loader level , and since all apps under the same managed server used the same JVM , singleton will only get initialized once. A class is loaded by a class loader.
Any class loader that loads a class, by definition loads the class.
A VM can have many class loaders. And far as I know every JEE server in existance that anyone uses, uses class loaders.
And finally there might be a problem with the architecture/design of a JEE system which has two applications but which is trying to solve it with a singleton. That suggests a there might be concept problem with understanding what an "app" is in the first place.