import java.util.Set;

import javax.persistence.*;

@Entity
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    private String email;

    private String password;

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

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

    public void setRoles(Set<Role> roles) {
        this.roles = roles;
    }

    @ManyToMany(fetch = FetchType.EAGER,cascade = CascadeType.ALL)
    @JoinTable(joinColumns = @JoinColumn(name = "id"),inverseJoinColumns = @JoinColumn(name = "roleId"))
    private Set<Role> roles;

    public User(String email, String password, Set<Role> roles) {
        super();
        this.email = email;
        this.password = password;
        this.roles = roles;
    }

}

import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entity
public class Role {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    private String role;

    @ManyToMany(mappedBy = "roles",fetch = FetchType.LAZY)
    private Set<User> users;

    public Role(String role) {
        super();
        this.role = role;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getRole() {
        return role;
    }

    public void setRole(String role) {
        this.role = role;
    }

    public Set<User> getUsers() {
        return users;
    }

    public void setUser(Set<User> users) {
        this.users = users;
    }



}

try {
    List<String> roleNames = Lists.newArrayList("user, admin, superuser");
    User user = new User("Peter", "Pan");
    List<Role> roles = new ArrayList<>();
    entityManager.getTransaction().begin();
    for (String roleName : roleNames) {
        List<Role> found = entityManager.createQuery("select r from Roles r where r.name = :roleName", Role.class)
                .setParameter("roleName", roleName).getResultList();
        if (found.isEmpty()) {
            Role role = new Role(roleName);
            entityManager.persist(role);
            roles.add(role);
        } else {
            roles.addAll(found);
        }
    }
    user.setRoles(roles);
    entityManager.persist(user);
    entityManager.getTransaction().commit();
} finally {
    entityManager.close();
}