Help With strategy for id generation

Question

I have a J2SE application that conducts simple testing on people and tracks data, and takes pictures of them while the test is going on. Data and images are constantly streaming to the database as entities are being created and persisted this all works great.
I want multiple people to be able to undergo testing at the same time and all store data in one database. When I try this I get errors for duplicate keys.
I was wondering if anyone had ideas on what would be the best thing to do in this case?
Ben

Answer 1

First Thanks alot for your help! It is much appreciated.
Ah ok i see a table called "sequence" does get put into the database.
I am using mysql db 5.0.
All my entity classes use
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
with over rides.
@Override
public int hashCode() {
int hash = 0;
hash += (id != null ? id.hashCode() : 0);
return hash;
@Override
public boolean equals(Object object) {
// TODO: Warning - this method won't work in the case the id fields are not set
if (!(object instanceof TempStat)) {
return false;
TempStat other = (TempStat) object;
if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
return false;
return true;
The application has an entity class(encapsulating class) that in turn has collections of other entity classes which are all cascade ALL. The application does various things I will simplified a use-case for you.
User sits down at a computer, sensors which are also attached to the computer monitor things like temperature and take pictures of the user. These statistics and data's are added to the various "Statistics" collections in the encapsulating class constantly. Depending on the activity the encapsulating class gets persisted or merged to the DB.
So
Encapsulating class
|
L--> temperature collections
| |
| L--> temp objects (Sample gets taken and added to collection every second)
|
L--> Images Collection
| |
| L--> image objects (Sample gets taken and added to collection every second)
|
L--> more collections
So there are various threads monitoring sensors and sticking things into the Encapsulating class's statistics collections at various intervals. At some point the monitoring threads get paused and the main thread persists the Encapsulating class which cascades the collections.
Question:
How would duplicate keys get made? Is there some time between setting the ID's for the entity class and persisting it where another instance of the software on a different computer could attain the same key and then throw a duplicate key exception when it was persisted?
Thanks in advance