https://github.com/JonnyD/greetapp-api-java

Small example of the Specification pattern:

/**
     * GET  /greets-by-user/:groupId : get all the greets by group.
     *
     * @return the ResponseEntity with status 200 (OK) and the list of greets in body
     */
    @GetMapping("/greets-by-group/{groupId}")
    @Timed
    public Object getAllGreetsByGroup(@PathVariable Long groupId) {
        log.debug("REST request to get all Greets by user");
        List<Greet> greets = greetService.getByGang(groupId);

        Optional<Gang> optionalGang = this.gangService.getById(groupId);
        Gang gang = optionalGang.get();

        CanViewGang canViewGang = new CanViewGang(this.userService);
        if (!canViewGang.isSatisfiedBy(gang)) {
            return new ResponseEntity<>(HttpStatus.FORBIDDEN);
        }

        CanViewGreetsByGang canViewGreetsByGang = new CanViewGreetsByGang(this.userRepository);

        List<Greet> greetsToShow = new ArrayList<Greet>();
        for (Greet greet : greets) {
            if (canViewGreetsByGang.isSatisfiedBy(greet)) {
                greetsToShow.add(greet);
            }
        }

        return greetsToShow;
    }

public class CanViewGreetsByGang extends AbstractSpecification<Greet> {
    private User loggedInUser;

    public CanViewGreetsByGang(UserRepository userRepository) {
        Optional<String> login = SecurityUtils.getCurrentUserLogin();
        this.loggedInUser = userRepository.findOneByLogin(login.get()).get();
    }

    public boolean isSatisfiedBy(Greet greet) {
        IsGangPublic isGangPublic = new IsGangPublic();
        IsHostOfGreet isHostOfGreet = new IsHostOfGreet(this.loggedInUser);
        IsMemberOfGang isMemberOfGang = new IsMemberOfGang(this.loggedInUser);

        boolean isGangPublicOrIsMemberOfGangBoolean = isGangPublic
            .or(isMemberOfGang)
            .isSatisfiedBy(greet.getGang());

        boolean isHostOfGreetBoolean = isHostOfGreet
            .isSatisfiedBy(greet);

        return isGangPublicOrIsMemberOfGangBoolean || isHostOfGreetBoolean;
    }
}