Non-reentrant locking

Question

Hi,
I am trying to prevent someone from submitting the same information twice in quick succession on my Tomcat web application before the first submit action has completed.
Here is basically what my code looks like now:
String email = getEmailAddressOfSubmitter();
synchronized (email) {
// Process data that has been submitted
// This processing needs to finish completely before
// this block is executed again for this same email
This is preventing other threads from entering this synchronized block until it has completed, but it looks to me like the same thread entering this method again with another request is allowed to go through, because the thread already has the lock on this email address object.
This is causing data to be saved twice because in the short time between when I check if a record already exists in the database and when a new one has been saved another request sneeks in and also saves a record.
How can I prevent the same thread from entering this block again with another request until processing has finished for this email?
I suspect it has something to do with non-reentrant locking, but I don't understand a word of what I've read about it so far.
Thanks in advance,
Evert

Answer 1

I don't know if this is the problem with your
program, but what you describe is possible: in Java
(differing from POSIX threads) a thread can access a
single lock (a synchronized method) more than once.
It's called reentrant locking.I assume that in practice this can really just happen if I have a method that calls itself. In my case with a web application, I assume the thread would complete one request completely before starting with the next http request. Other threads might interject, but the same thread should finish one request before starting the next.
I've done what BinaryDigit suggested and synchronized on email.intern() instead of just on email, and it looks like that did the trick. After 6 years of Java programming, I've forgotten that two strings are not necessarily == when they are equal. It's time for me to grab my beginner Java books again!