So46193254Application.java

package com.stackoverflow.so46193254;

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.data.jpa.domain.AbstractPersistable;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.persistence.*;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Configuration:
 * <p>
 * spring.jpa.open-in-view=false
 * spring.jpa.show-sql=true
 * spring.jpa.hibernate.ddl-auto=create
 * <p>
 * spring.datasource.url=jdbc:h2:mem:test
 * spring.datasource.driver-class-name=org.h2.Driver
 * spring.datasource.username=sa
 * spring.datasource.password=
 * <p>
 * server.error.whitelabel.enabled=false
 */
@SpringBootApplication
public class So46193254Application {

    public static void main(String[] args) {
        SpringApplication.run(So46193254Application.class, args);
    }

    @Entity(name = "role")
    static class Role extends AbstractPersistable<String> {
        private String name;

        private Role() {
        }

        public Role(String name) {
            this.name = name;
        }

        public String getName() {
            return this.name;
        }
    }

    @Entity(name = "user")
    static class User extends AbstractPersistable<String> {
        private String username;

        @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
        @JoinColumn(name = "user_id", foreignKey = @ForeignKey(name = "fk_user_roles"))
        private Set<Role> roles = new HashSet<>();

        public Set<Role> getRoles() {
            return this.roles;
        }
    }

    @Service
    static class UserService {
        private final UserDao userDao;

        UserService(UserDao userDao) {
            this.userDao = userDao;
        }

        @Transactional(readOnly = true)
        public Set<String> findUserRoles() {
            final List<User> users = this.userDao.findAll();
            return users.stream()
                    .flatMap(user -> user.roles.stream())
                    .map(Role::getName)
                    .collect(Collectors.toSet());
        }
    }

    // insert some test data
    @Bean
    CommandLineRunner startup(UserDao userDao) {
        return args -> {
            final User user = new User();
            user.username = "admin";
            user.roles.add(new Role("ROLE_ADMIN"));
            userDao.save(user);
        };
    }

    // curl -XGET 'http://localhost:8080'
    @RestController
    static class UserRolesApi {
        private final UserService userService;

        UserRolesApi(UserService userService) {
            this.userService = userService;
        }

        @GetMapping
        public String index() {
            return this.userService.findUserRoles().toString();
        }
    }
}

interface UserDao extends JpaRepository<So46193254Application.User, String> {}