How to use a factory pattern to get the instance of my Database Client?

Question

Below is my Interface -

public interface IDBClient {

    public String read(String input);

    public String write(String input);

}

This is my Implementation of the Interface -

public class DatabaseClient implements IDBClient {

    @Override
    public String read(String input) {

    }

    @Override
    public String write(String input) {

    }
}

Now I am thinking to write Thread Safe Singleton Factory to get the instance of DatabaseClient so that I can call read and write method accordingly.. So I wrote like this by following the Initialization On Demand Holder idiom, it is still incomplete -

public class DatabaseClientFactory {

    public static DatabaseClientFactory getInstance() {
        return ClientHolder.s_instance;
    }

    private static class ClientHolder {

    }
}

And I am not sure how to get the instance of DatabaseClient correctly in my above Factory? Do I need to add another method getClient() to get the instance of DatabaseClient and then call like this -

IDBClient client = DatabaseClientFactory.getInstance().getClient();
client.read(input); // or client.write(input)

Answer 1

You shold use Initialization-on-demand holder idiom, implementing your factory:

public class DatabaseClientFactory {
    private DatabaseClientFactory() {}

    private static class LazyHolder {
        private static final DatabaseClient INSTANCE = new DatabaseClient();
    }

    public static DatabaseClient getInstance() {
        return LazyHolder.INSTANCE;
    }
}

This code doesn't need synchronization because of the contract of the class loader:

• the class loader loads classes when they are first accessed
• all static initialization is executed before anyone can use class
• class loader has its own synchronization that make previous two points guaranteed to be thread safe