Query plan and execution time of a query

Hello,

I am trying to get the execution time of a query by using JDBC (Java).
To do that I make the difference between the time (in milliseconds)
after the query execution and the time before the query execution.

The first time I do that, I get the real execution time, but the query
plan seems to be cached. And the next time I do the same operation,
for a simple query I do not get the real time (in fact my 'simple'
query seems to execute in 0 millisecond).

So I need help: I would like to solve this problem but I do not know
how. I have tried some researches on Internet but I did not find. Is
there a possibility to disable the query plan cache temporarily? It is
also possible I am totally wrong and the problem is not where I see
it... Anyway I need help.

Thank you in advance,
Mickael