Initial design review

[Fred Barnes]

Hi Guys,
Here is my plan of attack! Please comment and advise...
suncertify.db

public interface DataInterface
Includes all the public methods of Data, plus criteriaFind method.
All methods throws IOExceptions.
getRecord, find, add, modify, delete also throws DatabaseExceptions.

public class Data
Data is changed to implement DataInterface
citeriaFind is implemented while lock and unlock remain empty.
Datainfo, FieldInfo and DatabaseException remain unchanged.

suncertify.server

public interface RemoteDataInterface
Extends DataInterface and implements Remote.
public class RemoteData
Extends UnicastRemoteObject implements RemoteDataInterface and Unreferenced.
Has a static reference to Data and LockManager.
Implement lock and unlock methods.
public interface ConnectionFactory
Implements Remote
Has a getConnection method that returns a DataInterface. For local connection it will return Data and remote an instance of RemoteData.
public class ConnectionFactoryImpl
Extends UnicastRemoteObject and implements ConnectionFactory.
public class LockManager
Uses a HashMap to store locked records.
suncertify.client
public class DataModel extends AbstractTableModel
Implemented as a facade.
Contains the book, search and getConnection methods.
public class DataController
The controller class in my client side MVC.
Here I will implement all needed listeners.
public class DataView
Class containing all my GUI components.
Kind Regards
Fred

[Omar Abdel Moniem]

Originally posted by Fred Barnes:
Hi Guys,
Here is my plan of attack! Please comment and advise...
suncertify.db

public interface DataInterface
Includes all the public methods of Data, plus criteriaFind method.
All methods throws IOExceptions.
getRecord, find, add, modify, delete also throws DatabaseExceptions.

public class Data
Data is changed to implement DataInterface
citeriaFind is implemented while lock and unlock remain empty.
Datainfo, FieldInfo and DatabaseException remain unchanged.

suncertify.server

public interface RemoteDataInterface
Extends DataInterface and implements Remote.
public class RemoteData
Extends UnicastRemoteObject implements RemoteDataInterface and Unreferenced.
Has a static reference to Data and LockManager.
Implement lock and unlock methods.
public interface ConnectionFactory
Implements Remote
Has a getConnection method that returns a DataInterface. For local connection it will return Data and remote an instance of RemoteData.
public class ConnectionFactoryImpl
Extends UnicastRemoteObject and implements ConnectionFactory.
public class LockManager
Uses a HashMap to store locked records.
suncertify.client
public class DataModel extends AbstractTableModel
Implemented as a facade.
Contains the book, search and getConnection methods.
public class DataController
The controller class in my client side MVC.
Here I will implement all needed listeners.
public class DataView
Class containing all my GUI components.
Kind Regards
Fred

Hello I just have one remark. Why RemoteData class and ConnectionFactoryImpl both extend UnicastRemoteObject.
Regards
Omar

[Peter den Haan]

Hi Fred

Originally posted by Fred Barnes:
public class RemoteData [...]
Has a static reference to Data and LockManager.

Static? As in "private static FooBar foobar"? Reconsider that -- this would limit you to a single database table, ruining most prospects of reusability for the server.
One fix would be to turn them into final instance variables, supplied via the constructor by the ConnectionFactory. Each factory instance would have one Data and LockManager. You could have as many factories as you'd want.
Looks fine to me otherwise.
- Peter

[Peter den Haan]

Originally posted by Omar Abdel Moniem:
[...] Why RemoteData class and ConnectionFactoryImpl both extend UnicastRemoteObject.

The factory gives each client its own connection object to talk to -- that way, clients can be identified and unlock() implemented as required (see the javadoc). Both the factory and the connections it creates need to be remote objects, but only the factory is bound in the RMI registry; connections are private to each client.
If you want to read more, search this forum for the word "connection".
- Peter

[Jawad Kakar]

To give each client its own connection, I do the following.

 
public class RemotedataServer extends java.rmi.server.UnicastRemoteObject implements RemoteServer{
  private static Data dataWrapper = null;
  private static LockManager lockManager = null;
private String dbFilePath = null;
private  String hostName = null;
private  int portNumber = 0;

  public RemotedataServer(String dbFilePath, String hostName, String port) throws RemoteException{
    this.dbFilePath = dbFilePath;
    File databaseFile = new File(this.dbFilePath);
    if(!databaseFile.exists()) {
      System.out.println("Could not find the database file.\n Exiting the server");
      System.exit(1);
    }
    this.hostName = hostName;
    portNumber = new Integer(portNumber).intValue();
    lockManager = LockManager.getLockManagerInstance();
    try{
      if(dataWrapper == null){
        dataWrapper = new Data(dbFilePath);
      }
    }
    catch(IOException io){
      System.out.println(io.getMessage());
    }
    try{
       int portNumber = new Integer(port).intValue();
       Naming.rebind("//"+hostName.trim()+":"+portNumber+"/RemoteServer", this);
      }
    catch(MalformedURLException url){
      System.out.println(url.getMessage());
      url.printStackTrace();
    }
    System.out.println("\n\nServer is HOT and RUNNING\n\n");
  }
  public RemotedataServer()throws RemoteException{
    if(dataWrapper == null){
      try{
      dataWrapper = new Data(dbFilePath);
      }
      catch(IOException ioe){
      }
    }
  }
  public DataClient getConnection() throws RemoteException{
    return new RemotedataServer();
  } 
...
...

To give each client its own connection object to talk to Server I call the following code in a factory method.

 public  static DataClient getRemoteConnection()throws RemoteException, NotBoundException, ConnectException{
    RemoteServer connection = null;
    try{
      String hostName =StartupDialog.getHostName().trim();
      String port = StartupDialog.getPortNumber().trim();
      int portNumber = new Integer(port).intValue();
      rs = (RemoteServer) Naming.lookup("//"+hostName+":"+portNumber+"/"+"RemoteServer");
    }
    catch(MalformedURLException url){
      System.out.println(url.getMessage());
      url.printStackTrace();
    }
    catch(NotBoundException nbe){
      System.out.println(nbe.getMessage());
      nbe.printStackTrace();
      throw new NotBoundException();
    }
    catch(RemoteException re){
      System.out.println(re.getMessage());
      throw new RemoteException();
    }
    try{
      rs = (RemoteServer)rs.getConnection();
    }
    catch(ConnectException ce){
      throw new ConnectException();
   }
    catch(IOException ioe){
      ioe.printStackTrace();
    }
    return rs;
  }
 

By doing it this way I extend UnicastRemoteObject only once.
Jawad

[Sri Addanki]

return new RemotedataServer();

But by doing this, each client will have its own instance of Data class and its own instance of lockManager class. is this ok?
What i feel is that each client should have its own instance of only lockManager and not Data.
Any comments Peter?
thanks,
sri

[Jawad Kakar]

There are 2 constructor, by calling

 public DataClient getConnection() throws RemoteException{    return new RemotedataServer();  } 

you get an instance of remote object only.
Jawad

[Sri Addanki]

Originally posted by Jawad Kakar:
There are 2 constructor, by calling

 public DataClient getConnection() throws RemoteException{    return new RemotedataServer();  } 

you get an instance of remote object only.
Jawad

Yes, thats right.
But when you instantiate a new RemoteDataServer object, then you are also creating a new instance of Data class for each remote object you've created. I am not sure if this is fine.
But here's your code:

public class RemotedataServer extends java.rmi.server.UnicastRemoteObject implements RemoteServer{  
private static Data dataWrapper = null;  private static LockManager lockManager = null;
.....
}

thanks,
sri

[Jawad Kakar]

Sri,
Thank you for your reply, you are right that was a mistake and I fixed it.
Thank you
Jawad

[Sri Addanki]

Okay...I'll come again. I think i'm confused too.
Hope this should be clear.

return new RemotedataServer();  

Each time you create a new instance of RemoteDataServer, you also are binding this.

 
Naming.rebind("//"+hostName.trim()+":"+portNumber+"/RemoteServer", this);
 

So for 'n' clients there would be 'n' objects bound to registry. This shouldn't be like this.
Instead at any point intime, theres only one remote object bound to registry.
Each client should get a new connection, also each client should be bind to only one remote object in registry.
Hence you have to define a different class, like DataAccessRemote, which creates a factory of remote connection objects.
Thanks,
sri

[Jawad Kakar]

Constructor which binds the remote object to the registry is called only once( when I start the server from command line) which is the following constructor

 
public RemotedataServer(String dbFilePath, String hostName, String port) throws RemoteException{...}
public static void main(String [] agrc) throws RemoteException{
   new RemotedataServer(agrc[0], agrc[1], agrc[2]);
 }

To bind the remote object to the registry.
The second constructor which no parameter the one you are talking about is called inside getConnection() method.
Therefor remote object is bind to the registry only once.
Jawad

[Sri Addanki]

The second constructor which no parameter the one you are talking about is called inside getConnection() method.

so, what exactly happens in this no args constructor?
thanks,
sri

[Jawad Kakar]

This is a default constructor which is for getting different client id.
Jawad

[Sri Addanki]

Originally posted by Jawad Kakar:
This is a default constructor which is for getting different client id.
Jawad

Does this mean that the constructor will be empty
thanks,
sri

[Jawad Kakar]

Yes

[Sri Addanki]

Am i missing something here.
1. Server creates factory of connection Objects. Right.
2. What i feel is that each client should have one connectionObject through which it accesses the database.
3. So, what i feel is that the empty RemoteDataServer constructor should take the variable dataWrapper as parameter. So each client can handle its lock/unlock of records, with that variable.
thanks,
sri
[ December 26, 2002: Message edited by: Sri Addanki ]

[Jawad Kakar]

have a look at my getRemoteConnection() method, that is how I do it and it works fine for me.
I am still not done with the assignment, I might change it in future if I find a better reason to do so.
Jawad