AclCache addAcl(Uuid id, StandardAcl acl) {
        StandardAcl prevAcl = this.aclsById.get(id);
        if (prevAcl != null) {
            throw new RuntimeException("An ACL with ID " + id + " already exists.");
        }

        ImmutableMap<Uuid, StandardAcl> aclsById = this.aclsById.updated(id, acl);

        if (this.aclsByResource.contains(acl)) {
            throw new RuntimeException("Unable to add the ACL with ID " + id +
                    " to aclsByResource");
        }

        ImmutableNavigableSet<StandardAcl> aclsByResource = this.aclsByResource.added(acl);
        return new AclCache(aclsByResource, aclsById);
    }
Contains all of the current ACLs indexed by UUID.
metadata/src/main/java/org/apache/kafka/metadata/authorizer/AclCache.java