How to create custom methods for use in spring security expression language annotations

I would like to create a class that adds custom methods for use in spring security expression language for method-based authorization via annotations.

For example, I would like to create a custom method like 'customMethodReturningBoolean' to be used somehow like this:

  @PreAuthorize("customMethodReturningBoolean()")
  public void myMethodToSecure() { 
    // whatever
  }

My question is this. If it is possible, what class should I subclass to create my custom methods, how would I go about configuring it in the spring xml configuration files and come someone give me an example of a custom method used in this way?