Implementing the back end functions

OperationsQueue class

The operations queue is a simple utility class that runs in a separate thread executing operations serially. The OperationsQueue class takes objects implementing the Operation interface, making it independent of the kind of operations executed. Operation implementers are expected to handle their exceptions locally since OperationsQueue will discard any exceptions thrown.

To create the OperationsQueue class:

1. Create the OperationsQueue class file.

2. Import the required classes.

   import java.util.*;

3. Define the interface.

   // Simple Operations Queue
   // It runs in an independent thread and executes Operations serially
   class OperationsQueue implements Runnable
   {
   
       private volatile boolean running = true;
       private final Vector operations = new Vector();
   
   
       OperationsQueue()
       {
           // Notice that all operations will be done in another
           // thread to avoid deadlocks with GUI thread
           new Thread(this).start();
       }
   
   
       void enqueueOperation(Operation nextOperation)
       {
           operations.addElement(nextOperation);
           synchronized (this)
           {
               notify();
           }
       }
   
   
       // stop the thread
       void abort()
       {
           running = false;
           synchronized (this)
           {
               notify();
           }
       }

4. Use the run() method to execute the operations on the queue.

       public void run()
       {
           while (running)
           {
               while (operations.size() > 0)
               {
                   try
                   {
                       // execute the first operation on the queue
                       ((Operation) operations.firstElement()).execute();
                   }
                   catch (Exception e)
                   {
                       // Nothing to do. It is expected that each operations handle
                       // their own locally exception but this block is to ensure
                       // that the queue continues to operate
                   }
                   operations.removeElementAt(0);
               }
               synchronized (this)
               {
                   try
                   {
                       wait();
                   }
                   catch (InterruptedException e)
                   {
                       // it doesn't matter
                   }
               }
           }
       }
   
   }
   

Operation interface

To create the Operation interface:

1. Create the Operation class file.

2. Import the required classes and implement the interface.

   // Defines the interface for a single operation executed by
   // the commands queue
   interface Operation
   {
     // Implement here the operation to be executed
     void execute();
   }
   import java.util.*;