@Table(name = "users")
@Entity
public class UserEntity {
	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "id")
	Integer id;

	@NotNull
	@Column(name = "username")
	String username;

	@ElementCollection
	@CollectionTable(name = "users_roles_mapping", joinColumns = @JoinColumn(name = "user_id"))
	@Column(name = "role_name")
	Set<String> roles;
	
	// ...
}

CREATE TABLE users (
    id INT NOT NULL IDENTITY,
    username VARCHAR(50) NOT NULL,
    CONSTRAINT pk_users PRIMARY KEY (id)
);

CREATE TABLE roles (
    name VARCHAR(50) NOT NULL,
    CONSTRAINT pk_roles PRIMARY KEY (name)
);

CREATE TABLE users_roles_mapping (
    user_id INT NOT NULL,
    role_name VARCHAR(50) NOT NULL,
    CONSTRAINT pk_users_roles_mapping PRIMARY KEY (user_id, role_name),
    CONSTRAINT fk_users_roles_mapping_user FOREIGN KEY (user_id) REFERENCES users (id),
    CONSTRAINT fk_users_roles_mapping_role FOREIGN KEY (role_name) REFERENCES roles (name)
);

INSERT INTO users (username) VALUES ('user1');
INSERT INTO roles (name) VALUES ('role1'), ('role2');
INSERT INTO users_roles_mapping (user_id, role_name) VALUES (1, 'role1'), (1, 'role2');

public Set<String> getUserRoles() {
	EntityGraph<UserEntity> entityGraph = entityManager.createEntityGraph(UserEntity.class);
	entityGraph.addAttributeNodes("roles");

	Query query = entityManager.createQuery("SELECT u FROM UserEntity u WHERE u.id = 1");
	query.setMaxResults(1);
	query.setHint(EntityGraphType.FETCH.getKey(), entityGraph);

	UserEntity userEntity = (UserEntity) query.getSingleResult();
	return userEntity.getRoles();
}