Question

Just finished a very long debug session. Please help me understand what root-cause/bad-practice caused this bug:

Beans A,B both had a property with an inner bean that looked exactly the same. Happily, I created bean C and reused it by doing <bean ref="C">.

However, bean C had a method called "setX" and a method called "getLongComputationBasedOnX". Turns out that upon first call to this method, it lazily cached with a member variable the computation. Further calls to setX do not reset the cache.

So, after I tried to make the world a better place and reuse, the 2 uses of C in A and B each set a different value to X, but now it was a single instance and the cache gave the value appropriate to A when called in B… 🙁

Is this a spring issue? Should I use parent instead of ref? Or is it a bad design of my “setX” and it should reset the cache?

Answer 1

Just use prototype scope:

<bean id="C" scope="prototype"/>

This way you have a single C declaration but two instances, one created when injecting to A and another one for B.

In general it is hard to tell what the right design should be. Based on your explanation it seems like C was supposed to be used with only one x value. It should have been passed via constructor and made final to make this explicit though.

Otherwise you will get nasty concurrent errors: one thread calls setX and getLongComputationBasedOnX subsequently but in the meantime other thread called setX. That being said I believe my prototype solution is the right one.