Hello all,
I have been researching relationships between entity beans. Especially into
the area of applying constraints between properties on different beans.
I found the container managed relationships in the EJB2.0 spec interesting.
But I am not convinced that they will solve some important problems.
+ Lets look at this hypothetical example
----------- Bob-Jim (1..1) ------------
|BobBean | <----------------------------------> | JimBean |
| getJim()| constraint: | getBob()|
| +value | bob.value > jim.value | +value |
----------- ------------
There is a Bean called Bob and a bean called Jim. both have a
single property called value.
There is a bi-directional, one to one relationship between bob beans and jim
beans.
+ How can we make sure the constraint will always hold?
If the client modifies jim.value then we must validate it against bob.value,
and vise-versa.
+ How should we implement the constraint?
>From an encapsulation point of view The BobBean should not know about the
JimBean (vise versa). Therefore relationship should be modelled as a
seperate object which uses the BobBean and the JimBean.
+ How does the relationship object intercept the calls to set property
methods?
Use the Observable pattern. If we look at the JavaBeans API(not EJB) there
is some interesting functionality defined. A bean may add a
VetoableChangeListener. Using this change listener you can add Vetoable
change listeners to specific properties. This is exactly the behaviour we
require.
A VetoableChangeListener allows a property change event to intercepted. And
if the change causes an invalid state the change can be cancelled.
+ How can we implement this Pattern inside a Container?
Is this pattern something that we must implement from scratch? Or are there
features of the container which we can use?
Now this is the tough bit, which I am working on at the moment.
Any ideas would be appreciated
Greg.
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