Activiti's StrongUuidGenerator uses fasterxml's TimeBasedGenerator to generate unique ids. Today, during performance testing, I encountered a lot of collisions. Diving in the sources I found that TimeBasedGenerator is, contrary to what the documentation suggests, not thread safe.This can be easily demonstrated using the following test, which will find a collision even with just 2 threads:
final TimeBasedGenerator timeBasedGenerator = Generators.timeBasedGenerator(EthernetAddress.fromInterface());
final Set<UUID> ids = new HashSet<UUID>();
Runnable r = new Runnable() {
public void run() {
for (int i = 0; i < 10000; i++) {
UUID id = timeBasedGenerator.generate();
synchronized (ids) {
if (!ids.add(id)) {
throw new RuntimeException("Id not unique: " + id);
}
}
}
}
};
ExecutorService executorService = Executors.newFixedThreadPool(5);
for (int i = 0; i < 2; i++) {
executorService.execute(r);
}
executorService.shutdown();
Synchronizing on timeBasedGenerator resolves this.The root problem is in TimeBasedGenerator, where _uuidBytes is shared between calls to generate().Edit: I may be using an old version of JUG…
