Interesting Session Exception...

Question

Dear sirs,
I am getting some interesting Session scope Exception..
I am using the technology Struts, EJB with JBoss
I am using Session Facade and Service Locator as desin pattern
Now the scenario is given below,
I am requesting for a collection of Entity Bean by passing the request as given below,
Action Class -> Session Bean (Stateless) -> Service Locator -> Entity Bean(CMP)
I am extending my action class with org.apache.struts.actions.DispatchAction
When two users of different sessions are logging in to the application and accessing the same collection; only one user is getting the Collection.
I tried to print the session ID ob each users at the console. Before the request is made the session id of each user is different as given below,
User 1 : 257B112049648471974498E34F9B1EDC
User 2 : A674D5852CB31EA975929E833E5C90B4
But after getting the collection from the bean; when i printedthe session ID along with the Collection size; the console is printing the same session for both request as given below,
1. showMessage positionMap.size()==> 124 <=> Session Id A674D5852CB31EA975929E833E5C90B4
2. showMessage positionMap.size()==> 124 <=> Session Id A674D5852CB31EA975929E833E5C90B4
This means that the user with session "257B112049648471974498E34F9B1EDC" is not getting the collection. This problem is occuring when both users are requesting for the same collection simultaneously, I checked the beam part and service locator part. I think they are working fine.
The ServiceLocator Code is given below...
public class ServiceLocator implements ServiceNames{
     private static ServiceLocator serviceLocator = null;
     private static Hashtable<String,Object> ejbHomeCache = null;     
     private static Hashtable<String,DataSource> dataBaseCache = null;
     static{
          serviceLocator = new ServiceLocator();
     private ServiceLocator(){
          ejbHomeCache = new Hashtable<String,Object>();
          dataBaseCache = new Hashtable<String,DataSource>();
     public static ServiceLocator getInstance(){
          return serviceLocator;
     private static EJBHome getEJBHome(String serviceName, Class ejbHomeClass){
          EJBHome ejbHome = null;
          try{
               if(ejbHomeCache.containsKey(serviceName)){
                    System.out.println("%%%%%%%%%%%%%%%%%%%%%% from ejbHomeCache Remote HashTable");
                    return((EJBHome)ejbHomeCache.get(serviceName));
               }else{
               Properties properties = new Properties();
               properties.put(Context.INITIAL_CONTEXT_FACTORY, "org.jnp.interfaces.NamingContextFactory");
               properties.put(Context.URL_PKG_PREFIXES, "org.jnp.interfaces");
               properties.put(Context.PROVIDER_URL, "jnp://localhost:1099");
               InitialContext ctx = new InitialContext(properties);
                    Context ctx = new InitialContext();
                    Object jndiRef = ctx.lookup(serviceName);
                    Object objRef = PortableRemoteObject.narrow(jndiRef,ejbHomeClass);
                    ejbHomeCache.put(serviceName, objRef);
                    return ((EJBHome)objRef);
          }catch(Exception e){
               System.out.println("Error in getRemoteEJBHome()-Remote in finding Remote EJBHome ");
               e.printStackTrace();
          return null;
     private static EJBLocalHome getEJBHome(String serviceName){
          EJBLocalHome ejbHome = null;
          try{
               if(ejbHomeCache.containsKey(serviceName)){
                    System.out.println("%%%%%%%%%%%%%%%%%%%%%% from ejbHomeCache HashTable");
                    return((EJBLocalHome)ejbHomeCache.get(serviceName));
               }else{
                    Context ctx = new InitialContext();
                    Object objRef = (EJBLocalHome)ctx.lookup(serviceName);
                    ejbHomeCache.put(serviceName,objRef);
                    return ((EJBLocalHome)objRef);
          }catch(Exception e){
               System.out.println("Error in getEJBHome()-Local in finding Remote EJBHome ");
               e.printStackTrace();
          return null;
     public static ImportsVzaHeaderLocalHome getVzaHeaderLocalHome(){
          ImportsVzaHeaderLocalHome vzaHeaderHome = null;
          try{
               vzaHeaderHome = (ImportsVzaHeaderLocalHome)getEJBHome(VZAHeaderLocal);
          }catch(Exception e){
               System.out.println("From ServiceLocator's getVzaHeaderHome method Exception in finding the VzaHeaderHome ");               
               e.printStackTrace();
          return vzaHeaderHome;
/************************************END Of ServiceLocator *************************************************************/
Your help and suggestions solicited..

Answer 1

Hi,
In that case, wont there be conflicting if OS wants to
save the portion of the memory which the app server
wants to retain in the memory. Or even before that,
will OS give app server a chance to let it know, what
it (app server ) wants to send to harddisk ?The OS hands the JVM a fixed-size heap of memory to use when the JVM starts. Only the JVM can tell when it is running low on memory because the OS doesn't pay attention to how much of that pool is filled with data.
There is the possibility that an OS will swap some of the pages from that heap out to disk; however, a decent OS will only swap out memory on a least-recently-used basis.
Maybe a JVM could hint to the OS through a system call that some of its heap could be paged, but I doubt that they do that. The app server is in a good position to know when and what to page out because it is aware of how much heap space is available. It could do this paging on its own via -- you guessed it -- serialization.