Null Object mgmt with Lifecycle Callbacks

lacou · **Joined:** Fri Feb 20, 2004 12:15 pm **Posts:** 8

Hello,

I am using the NullObject Pattern which I find very convenient for not testing for nulls everywhere in my programs. I am using the Lifecycle callbacks to replace my NullObject by null when Hibernate saves / updates my classes and I instantiate a new NullObject when I load from Hibernate.

Here is the code for that.

Code:

// For Operation Null
public boolean onSave(Session s) throws CallbackException {
  if (this.getOperation() instanceof OperationNullImpl)
     this.setOperation(null);
  return false;
}

public void onLoad(Session s, Serializable id) {
  if (this.getOperation() == null)
     this.setOperation(new OperationNullImpl());
}

The problem I have is that when I try to load, in this case, an operation which is null, Hibernate complains that I don't have a Persister for class 'OperationNullImpl'.

Do I really need a mapping for the OperationNullImpl class ? And if yes, how do I map it since I will never have a table in my database for null objects.

lacou

emmanuel · **Posted:** Mon Mar 01, 2004 12:06 pm

I don't think there is a orkaround for that except a private unmapped setter which do the job.

lacou · **Joined:** Fri Feb 20, 2004 12:15 pm **Posts:** 8

The simple things are sometimes not so obvious.

Thanks alot.

So now my instance variables always represents what's on the database. Its on my public getter and setter that I deal with my NullObject and a new private getter/setter has been defined and mapped in the hbm.

Here's the code for those interested

Code:

   public void setOperation(Operation operation) {
      if (operation instanceof OperationNullImpl)
         setPrivateOperation(null);
      else
         setPrivateOperation(operation);
   }

   public Operation getOperation() {
      if (this.operation == null){
         return new OperationNullImpl();
      }
      return operation;
   }
   
   /**
    * for Hibernate.
    * @param operation
    */
   private void setPrivateOperation(Operation operation){
      this.operation = operation;
   }
   
   private Operation getPrivateOperation(){
      return operation;
   }

And now the hbm needs to point to set/getPrivateOperation.

Code:

<many-to-one class="...bo.OperationImpl" name="privateOperation" column="operation" />