How to read table with hibernate and ajax request

Question

I have the following code to retrieve data from my db. Im running on wildfly and MySQL servers.

The below snippet should return all the user groups available.

@WebServlet(urlPatterns = {"/groups_list"})
public class UserGroupList extends HttpServlet {
    /**
     * Get all users groups available
     * @param request
     * @param response
     * @throws IOException
     */
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        Session session = HibernateHelper.getSessionFactory().openSession();
        List<UserGroup> userGroups= session.createQuery("FROM UserGroup UG").getResultList();
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            String json = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(userGroups);
            response.getWriter().println(json);
        } catch(Exception e) {
            response.getWriter().println(e.getMessage());
        }
    }
}

The below snippet should return all the users available.

@WebServlet(urlPatterns = {"/users_list"})
public class UsersList extends HttpServlet {
    /**
     * Get all users available
     * @param request
     * @param response
     * @throws IOException
     */
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        Session session = HibernateHelper.getSessionFactory().openSession();
        List<User> users = session.createQuery("FROM User s").getResultList();
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            String json = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(users);
            response.getWriter().println(json);
        } catch(Exception e) {
            response.getWriter().println(e.getMessage());
        }
    }
}

The ajax requests are sent as below:

 $(function() {
        // Fetch all user groups
        $.ajax({
            type: 'GET',
            url: 'http://localhost:8080/shopping_cart/groups_list',
            dataType:'json',
            success: function(data){
     
            },
            error: function(data){
                toastr.error('Error. Couldn\'t fetch user groups.')
            }
        });

        // Fetch all users
        $.ajax({  
            type: "GET",  
            url: "http://localhost:8080/shopping_cart/users_list",  
            dataType: "json",  
            contentType: "application/json",  
            success: function(response) {  
              console.log(response); 
            },  
            error: function(data){
                toastr.error('Error fetching user goups.')
            }
        });  
    });

My User entity is shown:

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name="user_id")
    private int id;

    @Embedded
    private Person person;

    @Column(columnDefinition = "VARCHAR(255)")
    private String image;

    @ManyToOne
    @JoinColumn(name="user_group_id", nullable = false, referencedColumnName = "user_group_id")
    private UserGroup userGroup;

My User Group Entity:

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name="user_group_id")
    private int id;

    @NotNull
    @Column(nullable = false, columnDefinition = "VARCHAR(20)")
    private String name;

    @NotNull
    @Column(nullable = false, columnDefinition = "TEXT")
    private String permission;

    @OneToMany(mappedBy="userGroup",
    cascade={CascadeType.PERSIST, CascadeType.MERGE},
    fetch = FetchType.LAZY)
    private List<User> users = new ArrayList<User>();

However, I might use the exact code above and return all the data available in the tables, and if the data is not there I get an empty array as the response in the browser console window. The next time I send the request, I get the following:

Infinite recursion (StackOverflowError) (through reference chain: models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]->org.hibernate.collection.internal.PersistentBag[0]->models.User["userGroup"]->models.UserGroup["users"]-

Answer 1

The reason can be that you don't really return any error to the client, cause of this

catch(Exception e) {
  e.printStackTrace();
}

Also, if you opened Hibernate session you have to close it in the final block.

The reason that Jackson during converting entities to JSON infinitely processes User -> UserGroup -> User

You can fix the issue using additional Jackson annotations

Infinite Recursion with Jackson JSON and Hibernate JPA issue

I would like to advice you to change your model:

UserGroup is tabular data, so you don't have to store users there. Just remove this from UserGroup

private List<User> users = new ArrayList<User>();