Surprisingly Hibernate 3 does not support UUID’s. I think the main reason for this is that UUID is not yet an ANSI database type. Each database manufacturer has implemented it in a different way, stored it as a slightly different type and has a driver (or set of drivers) that handle it differently!

For example, MySQL has a UUID() function that returns a 36 character string in the form aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee and expects you to store it in a VARCHAR(36) where as the .NET driver for MySQL will spot that a database column is a BINARY(16) and automatically convert it for you.

There follows a post explaining how to plug a java.util.UUID key into Hibernate with a couple of the pitfalls that I fell into.

There are several supporting classes that you need to write, and a bit of configuration:

1. Serialiser and de-serialiser for the java.util.UUID class
2. java.util.UUID generator
3. Hibernate type

The UUIDUserType class below was mostly lifted from the Springframework forum site. Thanks bessette for the starting point. I have added the corrections he discovered.

This class takes care of converting a java.util.UUID into something that the database column can coup with and converting the contents of a database column into a normal java.util.UUID. More information can be found in the JavaDoc for UserType.

Code:

We are probably going to be using this UUID type for a lot of things, for example if it is a key field, there will be a corresponding column in another table.

I like one hibernate definition per file (which has so many advantages I could get another blog out of it!), so I need a new file to hold UUIDUserType’s indirection name so that we can reuse it in our other hibernate mapping files.

It would be sensible to call it uuid or guid but this will clash with existing (built-in) indirection names so we must pick another.

XML:

The main reason for using UUID’s is that they are unique! So now we need a generator that generates them and returns them in the form we want. Hibernate has 2 uid generators; none of which fit the bill.

The first, uuid, returns a uid in the form “aaaaaaaabbbbccccddddeeeeeeeeeeee” which is generated by hibernate and the second, guid, returns it in the form “aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee” which is generated from a call to the database.

The problem with both of these is that they return the object as a String, which means that your POJO must have the id as a String with the method signature of setId(String id).
Hibernate infers that because the setter for id is a String then id must be a String. We want our id to be a UUID. We don’t want to duplicate getters and setters depending on what situation we are in.

The UuidGenerator below generates a UUID which implements the IdentifierGenerator hibernate interface and allows us to return a real UUID which travels through hibernate as a Serializable object, until it passes through UUIDUserType (above) and ends up in our POJO’s setter as a real UUID.

Code:

Below is an example instance of a POJO class with a UUID for an id:

Code:

Finally we need to bring it all together,

XML:

This structure will work with any type of class not just UUID, the only caveat is that is must implement the Serializable interface.