Two step transactions?

This is more or less just me thinking out loud, but how would you people feel about a transaction that validates fields prior to adding it to the journal and executing it? 

Why? I'm not quite sure. I just sort of enjoy the idea of separating the code that loads objects and throws possible exception from the part that actually execute the change on the prevalence.  And I suppose it opens up for catching some bad use of transactions and fail faster in business logic.  

I was thinking something along the lines of this:


interface Validating<P, V> {
  V prepare(P prevalence)  throws Exception;
}

interface ValidatingTransaction<P, V> extends Validating<P, V> {
  void executeOn(P prevalence, Date executionTime, V validation) throws Exception;
}

interface ValidatingTransactionWithQuery<P, R, V>, Validating<P, V> {
  R executeAndQuery(P prevalence, Date executionTime, V validation) throws Exception;
}

class AddBarToFoo implements ValidatingTransaction<Root, Preparation> {

  static class Preparation {
    Bar bar;
    Foo foo;
  }

  long barIdentity;
  long fooIdentity;

  Preparation prepare(Root root) throws Exception {
    Preparation p = new Preparation();
    p.bar = root.bars.get(barIdentity); 
    if (p.bar == null) throw new NullPointerException("No bar with identity " + barIdentity);
    p.foo = root.foos.get(foodentity); 
    if (p.foo == null) throw new NullPointerException("No foo with identity " + barIdentity);
    return preparation;
  }

  void executeOn(P prevalence, Date executionTime, Preparation p) throws Exception {
    p.foo.bars.add(bar);
  }
}


class BusinessLogic {
  void logic() throws Exception {
     …
     try {
       Service.getInstance().getPrevayler().execute(new AddBarToFoo(foo.identity, bar.identity);
     } catch (PreparationException pe) {
       // Preparation failed, transaction not appended to the journal
     } catch (Exception e) {
       // Execution failed, transaction was appended to the journal
     }
  }
}



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