如何使用休眠和Ajax请求读取表

我有以下代码从我的数据库中检索数据。我在wildfly和MySQL服务器上运行。

以下代码段应返回所有可用的用户组。

@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());
        }
    }
}

以下代码段应返回所有可用用户。

@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());
        }
    }
}

ajax请求的发送方式如下：

 $(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.')
            }
        });  
    });

显示我的用户实体：

    @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;

我的用户组实体：

    @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>();

但是，我可能使用上面的确切代码，并返回表中所有可用的数据，如果数据不存在，我会在浏览器控制台窗口中得到一个空数组作为响应。下次发送请求时，我得到以下信息：

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"]-