T unwrap(
  Class<T> cls
)
 Return an object of the specified type to allow access to the provider-specific API. If the provider's implementation of the 
Cache interface does not support the specified class, the PersistenceException is thrown. - Parameters:
- cls- the class of the object to be returned. This is usually either the underlying class implementing {@code Cache} , or an interface it implements.
- Return:
- an instance of the specified type
- Throws:
- PersistenceException - if the provider does not support the given type
- Since:
- JPA 2.1