Chiji Nwankwo

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since Jun 21, 2002
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Recent posts by Chiji Nwankwo

I was taking a brief pause from my certification path when I realised that a new certification had been released ....
I have been working in an environment where EJB has been the core back-end technology of choice and in many instances have had some experience of designing and implementing EJB's and deploying J2EE applications on WSAD 4 and 5. Most of the work I have done has been on the front-end, using Struts primarily, and have decided to move more to the back end.
My question is how much work is involved in terms of preparation. I am currently reading the EJB book by Richard Monson-Haefel and just wondered what other resources members of the ranch found useful.
By the way keep up the good work, I have used to ranch to prepare for 3 of my java certifications and you guys haven't failed me yet so I guess you must be doing something right .
Although, I didn't comment much on the scwcd forum I used some of the resource links provided by JavaRanch. The following links proved to be invaluable resources in my preparation; (excellent summary, I used it a couple of days before the exam) (went over some of the patterns that were relevant to the scwcd exam)
I have worked with J2EE technologies for about 2years so just needed to recap on certain things.
Thanks and see you soon,
21 years ago
I just got my results and I am elated to say the least. Here is a break down of my score;
Maximum=58 Deductions=6 Actual=52
Maximum=20 Deductions=0 Actual=20
Maximum=24 Deductions=6 Actual=18
Maximum=53 Deductions=0 Actual=53
Total: Maximum=155 Deductions=12
Certification Score=143
I want to thank everyone who contributed and took the time out to answer my questions, Thank you very much. I especially would like to thank Mark Spritzler, Max Habibi, Peter de Haan and Eugene Kononov, you guys pushed me that much harder and helped me understand the full implications of some of my design decisions. To you all I say THANK YOU VERY MUCH.
21 years ago
Thanks for the tip. Could you please tell me if my line of thinking, from the quote below, is correct.

As far as I understand it, the user specifies the name and location of the datafile in 'local' mode and specifies only the hostname/ip and port number in 'remote' mode.

Am I correct in thinking that the name and location of the database file is not supposed to be supplied by the user in 'remote' mode.
I guess my major concern is with the fact that it was specified in your last post that the ConnectionFactory's connect() method, specifies a database file name to connect to.
Can you please clarify this.

Ok, not if you see your Data instance as a database being available 24-7.
But if you are connecting to a table/view , you may want to think about lazy initialising your Data instances , also.

I see what you mean. Thanks for the clarification and perseverance .

Similarly, if you are handling multiple databases , the database schemas, will also be encapsulated in the Connection object as parameters. A standard solution repeated in many database packages.....

I didn't quite understand the statements you made above. Please clarify. I will use lazy initialiation as you have suggested. Supposing 2 datafile names were specified on the command-line, when starting the server, will it be wrong to give a seperate datafile name to different DataServiceImpl (your Connection object) instances when they are being created?
Thanks for your input.

That's wrong. How else will the user know why his application failed?
How are you handling the error message, i.e. how does creating data and lock manager instances in advance help in this case ?

I view the starting of the server and client as two seperate operations. What I have tried to do is create a distinction between the error messages that are displayed on the server side when the server is started and the error messages that are displayed on the client side when the client is started. I feel the server should be started up and made available for the clients. I think the user should be shown error messages that relate directly to the operation they are trying to achieve. When the server is started up any errors that pertain to the creating and registring of the the factory to the registry and creating the Data instances are displayed on the console. That way the server side of the application is fully available and awaiting client connections. All the factory does when the client calls the getDataService method is create and serve out remote DataService instances. I guess I could have delayed the creation of the LockManager instances until the later on, but I still think the creation of the Data instances is not entirely wrong.
Let me know what you think.

I am confused why you are creating Data and LockManager instances in advance. Is it for Connection pooling , which I think I am right in saying is outside the requirements ?

Did you implement multiple databases in your solution? How did you go about it?

Thanks for the feel-good support on leaning towards handling of dead client locks being a system requirement. There is now a third option to be considered , do nothing about dead client locks .

NO worries.

This is a new slant on the use of the LockManager class.
If you search for threads on connection on this forum , you'll find lots of discussions supporting the following :
Each client creates it's own Connection object, with a connect(table name) method and it's own LockManager object with a reference to a shared HashMap/WeakHashMap for storing the unique client identifiers.
I suspect it's semantics again, and what you call LockManager , I call HashMap/WeakHashMap i.e the Collection class for storing the identifiers.
Not quite

When you say each client what are you referring to? The factory responsible for creating DataServiceImpl instances creates the Data and LockManager instances and passes references of these objects to the newly created DataServiceImpl class. The reason I create the LockManager and Data instances in to keep track of the number of created Data and LockManager instances. The Data and LockManager instances are created when the DataServiceFactoryImpl is created, before it is registered with the rmiregistry. This is the sequence of events from the client-side of the application.
  • Client connects to remote server by specifying a URL (ip/host name and / or port number)
  • Client performs a lookup to obtain a remote reference to the DataServiceFactory
  • Client calls DataServiceFactory's getDataService() method to retrieve a DataService implementation

  • When the getDataService() method is called a new DataServiceImpl instance is created and the previously created Data and LockManager instances are passed into the constructor of the DataServiceImpl class. The Data and LockManager instances are created when the server is started up, that way if the administrator, who starts the server, provides an invalid database name and location an exception will be thrown and a message displayed on the console to this effect. By creating the Data instances in advance I am cutting down the amount of things that can go wrong when the client requests a remote DataService instance. I also create the Data instances when the server is starting up due to the fact that I don't think the user should be concerned with the fact that database name and location specified on the server side, when the server was started, was/is incorrect. Each LockManager created, contains its own HashMap of locks that corresponds to the locks stored for clients accessing the same Data instance. So in effect the DataServiceImpl class indirectly ties a Data instance to a LockManager instance. Since the server is capable of receiving 1 or more database names and locations, I create an amount of LockManager instances that correspond to the number created Data instances.

    Assigning responsibilities in the right place?
    Starting the server, should just start the server registry. The Client Connection class is responsible for connecting to the data , the LockManager is responsible for acquiring and releasing locks on the data and uses a HashMap/WeakHashMap to achieve this.

    My Server class, is responsible for creating the registry, creating the DataServiceFactory instance, binding the instance to the registry and stopping the server. The main responsibility of DataServiceImpl class, which I think is the same as you Client Connection class, is to delegate database calls to the Data instance and lock/unlock calls to the LockManager class, among other things. I think the main difference between your implementation and mine is where and when the Data and LockManager instances are created. But as I explained, I don't see why the client of the application should be concerned with the fact the back-end system was unable to connect to the database when the client has not specified the name of a database to connect to. As far as I understand it, the user specifies the name and location of the datafile in 'local' mode and specifies only the hostname/ip and port number in 'remote' mode.

    Looking at your code snippet , I don't think you should have data.getInstances and LockManager.getInstances in your code.

    All the piece of code, from the previous message, is doing is retrieving a Data instance and a LockManager instance, which is passed to the newly created DataServiceImpl instance. dataServerInstances and lockManagerInstances are two arraylists used to store the previously created Data and LockManager Instances, see below

    The arraylists shown in the code snippet above, are created and populated when the server is started up.
    Thanks for your feedback.

    This thread shows you are OK with having both a daemon thread and using unreferenced to handle dead client locks as long as you have given a good reason (i.e. understand the mechanics behind it).

    Thanks for the link.

    I hope they'll accept my reasoning that this is a system requirement and not an application requirement. By system I mean the RMI distributed garbage collection sub-system.

    I am sure your reasoning will be fine. From what I have seen so far on the forum, there have been so many mixed views and all the different views seems to have passed. I think the most important thing as you pointed out is being able to defend the solution that have implemented and the line of reasoning behind it.

    Okay, I'll come back to what I think are two design options after doing some reading up, or if someone jumps in and kindly volunteers to agree/disagree.

    Ok, I will await your response on the subject.

    It could be the wording again.
    All DataService implementations (client connections) WILL share the same Data but each client connection has it's own LockManager instance but shares the HashSet or whatever it is you store your unique client identifier in.
    Just semantics, but the two could be implemented very differently.

    The wording is probably wrong again. I will try to explain with words and a code snippet. I am getting my syntax all mixed up it seems.
    The DataServiceFactoryImpl, which is exported to the rmiregistry as a DataServiceFactory interface, class is responsible for creating DataServiceImpl, Data and LockManager instances. Each time a new client connects to the system, in remote mode, the getDataService() method of the DataServiceFactory interface creates a new DataServiceImpl instance and passes it a previously created Data and LockManager instance. The getDataService() method returns a DataService, remote, interface to the client of the system. When the server is started up, if 3 database file names are specified on the command-line then 3 LockManagers will be created to correspond with the 3 Data instances. Please see a snippet of the getDataService() method below;


    That is 1: use Unreferenced and let the "system" decide when to release the lock for the dead client.
    2: Don't specify a lease period and the "application" decides when to release the lock.

    I don't quite understand the 2 options that you have specified. Please elaborate. Is that the only part of the system that you have a query with so far. Please your comments are very welcome and thanks for the feed back.

    Ah , a dead lock rather than deadlock (LOL).
    That makes more sense.

    Sorry about the confusion .

    But if you are using Unreferenced, do you need this daemon thread?

    As far as I understand, correct me if I am wrong, Unreferenced is called after the lease time on the remote reference expires. This value may vary from machine to machine and since I am not explicitly providing a value for the lease period, I didn't want a scenario where a client tries to access a record, that has been locked by a dead client and will have to wait for the lease period to expire before it can access the record. At the moment the thread within the LockManager will periodically check for locks that have exceeded a specified amount of time and release those locks, so that other waiting clients can access the record. I must also mention that the thread only checks for DEAD client's locks, when there are locks present.
    I think I have sparked some confusion in my wording. My use of the word dead locks here is wrong. What I meant to say is the thread contained within the LockManager class releases locks that have been acquired by clients, who for one reason or the other died / disconnected before the lock->read->modify->unlock sequence could be completed. I hope this makes more sense.
    Hi all,
    Please go over my review and let me know any comments or critisms you might have.
    public class BookingDetail
    Class used to encapsulate all the details of a booking, such as flight number, seats, booking date and status (Confirmed, Unconfirmed ...).
    public class ClientLauncher
    Sets the look and feel of the application and starts off the client application.
    public class FlightDetail
    Class used to encapsulate all the details of a flight, such as flight number, origin airport, destination airport, etc..
    public class TravelAgency
    Facade that implements all the business methods, such as searchFlight() and bookFlight().

    Most of the buttons and menu items contained within the GUI application are represented by seperate actions, whose names give an indication of the role they play. For example the 'book' button's interaction is handled by a 'BookAction' instance. All the Action instances are declared external to the GUI. Most of the action instances are passed a reference to a controller (see controller definition below)object.
    public class MainController
    This class serves as the controller (in the context of MVC) for the GUI application. Its primary purpose is to create a connection between the view and data parts of the GUI application. The presence of this class abstracts the view from the data parts of the application. This class contains methods which can be used to set data contained within the model.

    public interface ClientModel
    Defines all the operations that can be used to access the state of the main GUI application. This interface is 'accessed' by the view of the GUI application. Only accessors are provided in this interface. This is the interface that is used by the GUI application, which means that the GUI application (view) cannot modify the state of the model directly.
    public class ClientModelEvent extends EventObject
    ClientModelEvent is used to notify listeners that a client model's state has changed.
    public interface ClientModelListener extends EventListener
    Defines the interface which is implemented by objects that are interested in receiving notification changes that occur in the ClientModel object.
    public class DefaultClientModel implements ClientModel
    This class serves as a default implementation of the ClientModel interface, which is used by the main GUI application. This class also provides mutators that can be used modify the state of the model. This class is only accessed from with the MainController class (defined above).
    public class ResultsTableColumnModel extends DefaultTableColumnModel
    A custom DefaultTableColumnModel implementation used by the main GUI application.
    public class ResultsTableModel extends AbstractTableModel
    A custom AbstractTableModel implementation used by the results table which is displayed as part of the main GUI application.

    public class BookingDialog extends JDialog
    A dialog box that is used to display all the details of a flight that has been selected by the user. This dialog box gives the user the option to
    cancel or proceed with a booking.
    public class ConnectionDialog extends JDialog
    This the first dialog box presented to the user when the client side of the application starts up. The dialog has options that allow the user to specify the name of a database, when in local mode and the host name/ip and rmi port,
    when in remote mode.
    public class GUIMediator
    A mediator, that coordinates the interaction between most of the components contained within the main GUI application.
    public class MainView extends JFrame implements ClientModelListener
    Main GUI, which is notified when a change in the model's state occurs.
    public class UserGuideDialog extends JDialog
    Dialog in which the user guide is displayed. This dialog is invoked by the user guide menu item in the MainView GUI.

    public class Data implements DataInterface
    Modified data class to provide criteriaFind() method and fixed deprecated methods. CriteriaFind method is capable of searching for any number of
    fields provided to it.
    public class DatabaseException extends Exception
    Unmodified except for javadoc comments.
    public class DataInfo implements Serializable
    Unmodified except for javadoc comments.
    public interface DataInterface
    Exposes methods that can be used by both local and remote applications. The methods in this interface throw both DatabaseException and IOException.
    public final class DataInterfaceFactory
    This class creates and returns remote or local DataInterface implementations. Exceptions are rethrown by this class as a custom application exception, which is caught in the in the MainController class. The MainController class
    is responsible for displaying the correct dialog box with the relevant error message.
    public class FieldInfo implements Serializable
    Unmodified except for javadoc comments.

    public interface DataService extends Remote, DataInterface
    Interface that is exported to clients of the remote system.
    public interface DataServiceFactory extends Remote
    Remote object registered with the RMI registry. Contains a single method, getDataService(), which is responsible for creating and returning DataService instances to the client. Each client will get a unique DataService implementation.
    class DataServiceFactoryImpl extends UnicastRemoteObject implements DataServiceFactory
    A DataServiceFactory implementation responsible for creating and returning DataService objects. This class is also responsible for creating Data and LockManager instances and allocating them to the new created DataService implementation. Several DataService implementations can share the same Data and LockManager instances. Each newly created Data instance has a corresponding LockManager instance tied to it within the DataService implementation.
    class DataServiceImpl extends UnicastRemoteObject implements DataService, Unreferenced
    A DataService implementation, exported to clients of the remote system which is used to wrap calls made to the database file. Instances of this class, which are created by the DataServiceFactoryImpl, contain a reference to the
    LockManager and Data instances passed in from the DataServiceFactoryImpl class. All the database file calls are delegated to the Data instance and
    the lock and unlock calls are delegated to the LockManager instance.
    class LockManager implements Runnable
    Deals with concurrent database file access issues. Provides implementations for the lock and unlock methods. Contains an internal, daemon thread which deals with deadlocks.
    public class Server
    Responsible for intialializing the server's environment, binding the remote DataServiceFactory, object to the registry and starting the server. This class contains a key listener, which listens out for a 'q' or 'Q' keystroke. Either of these keystrokes can be used to stop the server application at any time. Stopping the server will unbind any remote objects bound to the registry and gracefully exit the application. The key listener runs in its own, low priority, thread.
    public class ServerLauncher
    Starts the server side of the application. Command-line parameters are passed to the server side of the application.
    public interface Stoppable
    Contains a single call back method, stop() and is implemented by classes that will need to be stopped before the application exits. I have not included which classes implement the stoppable interface in this review.

    Thank you very much for taking the time out to go over my review.
    Does anyone have any other issues with my design. Please comment as your input is very important.