Hi,
Stanislav showed me a really cool pattern that can be used with
visitors. Basically, if a visitor is visiting a structure and it needs
to throw an exception, for whatever, reason, it is possible to do so in
a type-safe way by declaring the exception through generics.
The visitor interface might look like this:
interface EntityStateVisitor<ThrowableType extends Exception>
{
void visitEntityState( EntityState entityState )
throws ThrowableType;
}
--
and the visit methods that take such a visitor would be defined as:
<ThrowableType extends Exception> EntityStoreUnitOfWork
visitEntityStates( EntityStateVisitor<ThrowableType> visitor, ModuleSPI
module )
throws ThrowableType;
---
If a visitor implementation needs to be able to throw for example
SQLException you would specify that in the visitor implementation, and
then do try/catch on it around the visitEntityStates call.
This makes it easy to abort a visitor traversal, and also allows the
traversal code to do any necessary cleanup.
I have changed *ALL* visitor interfaces and methods (quite a bit) in
Qi4j to use this pattern.
You will probably want to make an update to get this, as it affects
quite a bit of code.
regards, Rickard
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