[B&S]: Using a Singleton class to represent the file resource

[Vincent Hernandez]

Hello fellow ranchers

I was hoping for some feedback on part of my design for the B&S project.

I'm aiming to create a singleton class to represent the file itself. I understand I must create a class sunceritfy.db.Data which implements suncertify.db.DBAccess. However, I want to have some sort of object representation of the file itself. In addition, I don't want to reopen and close the file for every Data instance. The name for this singleton is class ContractorDB.

Here's the main jist of what I am currently implementing:

public class ContractorDB{
    private static ContractorDB DBInstance = null;
 
    private FileChannel fileChannel = null;
 
    private RandomAccessFile randomAccessFile = null;
 
    private ContractorDB(){
    ...
    }    
  
    //create connection to database file
    private void StartUp(){
    ...
    }
 
    //shut down database 
    private void ShutDown()
    {
    ...
    }
 
    public synchronized ContractorDB getInstance(){
    ...
    }
  
    public Contractor getContractor(){
    ... 
    }
 
    public synchronized void putContractor(int recNo, 
        Contractor contractor){
    ... 
    }
}

Multiple instances of Data will be invoked, so I figured having a singleton resource will best suit my needs. As for record locking, I will have a static Vector used to keeps track of the locked Contractor records (no, I wouldn't lock the entire DB!).

Again, any feedback or questions would be appreciated.

Thank you!
[ December 26, 2004: Message edited by: Vincent Hernandez ]

[peter wooster]

Originally posted by Vincent Hernandez:
Hello fellow ranchers

I was hoping for some feedback on part of my design for the B&S project.

I'm aiming to create a singleton class to represent the file itself. I understand I must create a class sunceritfy.db.Data which implements suncertify.db.DBAccess. However, I want to have some sort of object representation of the file itself. In addition, I don't want to reopen and close the file for every Data instance. The name for this singleton is class ContractorDB.

Here's the main jist of what I am currently implementing:

public class ContractorDB{
    private static ContractorDB DBInstance = null;
 
    private FileChannel fileChannel = null;
 
    private RandomAccessFile randomAccessFile = null;
 
    private ContractorDB(){
    ...
    }    
  
    //create connection to database file
    private void StartUp(){
    ...
    }
 
    //shut down database 
    private void ShutDown()
    {
    ...
    }
 
    public synchronized ContractorDB getInstance(){
    ...
    }
  
    public Contractor getContractor(){
    ... 
    }
 
    public synchronized void putContractor(int recNo, 
        Contractor contractor){
    ... 
    }
}

Multiple instances of Data will be invoked, so I figured having a singleton resource will best suit my needs. As for record locking, I will have a static Vector used to keeps track of the locked Contractor records (no, I wouldn't lock the entire DB!).

Again, any feedback or questions would be appreciated.

Thank you!

[ December 26, 2004: Message edited by: Vincent Hernandez ]

You're on the right track. The database table needs to be represented as a single object and will need to be thread safe. For this purpose you will need to synchronize on the file for short periods when doing a read or write. You will also need to provide the longer lived record locking that's specified in the requirements. The record locking is probably easier to implement if you use a HashMap rather than a Vector. The HashMap isn't synchronized, so you can control the synchronization yourself and the Map interface allows you to use the record number as a key.

I use a singleton for the database and multiple instances of the Data class to represent client access. There has been discussion of using a multiton for the database, but I believe this is unnecessarily complicated as there is only a single table required by this project.