Two applications sharing database

Question

Hi,
I have one database with information and two different applications who both can read and write in that database.
But how can i implement that if one application is writing in the database that the other one can't access the database?

Answer 1

It can be done with Lock and Conditions
An example of Producer/Consumer problem , Producer and Consumer are 2 different threads Producer uses set and Consumer uses get methods. Both of those methods are implemented in the class below, with locks you make sure that each time ONLY one of these threads has exclusive acess to the class methods.
In the example Conditions are used so that Producer cant write when the buffer is full (buffer is an int variable here, capable of holding one int at a time) so the thread enters in await state.Similar the Consumer cant read before the Producer writes the new number (avoiding the consumer to read any duplicates). When the condition is satisfied a signal occurs so that if a thread was in the waiting state for that condition it can now continue.
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.Condition;
Lock accessLock = new ReentrantLock();
Condition canWrite = accessLock.newCondition();
Condition canRead = accessLock.newCondition();
public void set(int value)
        accessLock.lock();   
        try
            while(occupied)
                displayState("Buffer full. Producer waits.\n");
                canWrite.await();
            buffer = value;
            occupied = true;
            displayState("Producer writes "+value);
            canRead.signal();
        catch(InterruptedException exception)
            exception.printStackTrace();
        finally
            accessLock.unlock();
public int get()
        int readValue = 0;
        accessLock.lock();
        try
            while(!occupied)
                displayState("Buffer empty.Consumer waits\n");
                canRead.await();
            occupied = false;
            readValue = buffer;
            displayState("Consumer reads "+readValue);
            canWrite.signal();
        catch(InterruptedException exception)
            exception.printStackTrace();
        finally
            accessLock.unlock();
        return readValue;
    }If you understand the concept of locks and conditions you can easily apply them to control the access over your database.
Hope i ve been of some assistance!