When should I implement Spring Security ACL in my

Spring Security ACL looks very powerful, and easy to implement when you can stick to their database implementation. However it appears to become much more complicated when you have to implement your own Acl and AclService (see for example this (old) very basic tutorial of only ~26 pages) and it seems difficult to find references and examples for it (that tutorial was from 2008).

In our application for example, users have roles and belong to departments. Most of the time, they are allowed to perform some operations on objects that belong to their department based on their roles. In all cases, department + role is sufficient to decide whether a user should be granted a specific operation on a specific object.

Users, roles and departments are managed by an external application from which we retrieve them when the user connects (we are using REST services but it could as well be an LDAP server).

We would like to rely on @PreAuthorize('hasPermission(…)') for implementing domain object security. 2 solutions are thus in sight:

Implement a custom PermissionEvaluator that does the whole checks; or
Implement ACL with a custom AclService that builds the object structure necessary for ACL's to work properly.

It seems that implementing the whole AclService would be more difficult and more complex than implementing a PermissionEvaluator, but ACL's seem to be more standard.

Based on which criteria should you implement one or the other?

The PermissionEvaluator is responsible for expression evaluation to determine whether a user has a permission for a given domain object. On the other hand the AclService provides an interface for retrieval of Acl instances. In the spirit of Separation of concerns each component addresses a separate concern.

If any PermissionEvaluator implementation needs to perform evaluation based on Acl instances, it should delegate to AclService to retrieve them. Actually AclPermissionEvaluator does exactly that.

I would suggest you to go this way. Separate evaluation from ACL retrieval. If the concept of Spring AclService and Acl is too complicated or complex for your use case, you can introduce your own service to retrieve custom ACL. Then implement PermissionEvaluator that will delegate to your ACL service.

Actually, I had to do something similar because I needed to store ACLs in NoSQL database and what Spring provides did not work for me.

I would say that it is all about the effort needed to adjust Spring ACL to meet your requirements and the effort to implement a custom solution. If your requirements can be satisfied with the default Spring ACL implementation, go for it. It will definitely save you time to implement your custom solution. However, if it is not possible to adapt Spring ACL to your requirements or it would be too difficult, then it can be easier to implement your custom solution.