使用jpa的实体对象转json符串时懒加载的问题及解决

使用JPA的实体对象转JSON字符串时，懒加载的问题是指如果在实体中存在多个关联关系，当使用jpa将实体转为JSON字符串时，如果关联关系采用懒加载方式，则在序列化关联对象时，可能发生延迟加载异常，导致程序抛出错误。下面是解决该问题的攻略：

1.解决方法

1.1 使用Jackson的ObjectMapper来序列化关联对象

在JPA的实体类上使用@JsonManagedReference对关系进行标注，然后在需要序列化关联对象的实体类上使用@JsonBackReference标注。

示例代码如下：

@Entity
public class User {
    @Id
    private Long id;
    private String username;
    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
    @JsonManagedReference
    private List<Order> orders = new ArrayList<>();
    // getter and setter
}

@Entity
public class Order {
    @Id
    private Long id;
    private String name;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    @JsonBackReference
    private User user;
    // getter and setter
}

public class Main {
    public static void main(String[] args) {
        User user = new User();
        user.setId(1L);
        user.setUsername("admin");
        Order order = new Order();
        order.setId(1L);
        order.setName("order1");
        order.setUser(user);
        user.getOrders().add(order);
        ObjectMapper mapper = new ObjectMapper();
        mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
        String json = "";
        try {
            json = mapper.writeValueAsString(user);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }
        System.out.println(json);
    }
}

1.2 使用Hibernate的Hibernate.initialize()方法

可以使用Hibernate的Hibernate.initialize()方法预先初始化需要序列化的实体对象，从而避免延迟加载异常。此方法需要在事务环境下执行，否则会抛出以下异常：

org.hibernate.LazyInitializationException: could not initialize proxy - no Session

示例代码如下：

public class Main {
    public static void main(String[] args) {
        User user = new User();
        user.setId(1L);
        user.setUsername("admin");
        Order order = new Order();
        order.setId(1L);
        order.setName("order1");
        order.setUser(user);
        user.getOrders().add(order);
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("example");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        em.persist(user);
        em.persist(order);
        em.getTransaction().commit();
        em.close();
        em = emf.createEntityManager();
        em.getTransaction().begin();
        User user2 = em.find(User.class, 1L);
        Hibernate.initialize(user2.getOrders());
        ObjectMapper mapper = new ObjectMapper();
        mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
        String json = "";
        try {
            json = mapper.writeValueAsString(user2);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }
        System.out.println(json);
        em.getTransaction().commit();
        em.close();
        emf.close();
    }
}

2.示例

2.1 使用Jackson的ObjectMapper来序列化关联对象

假设我们有两个实体类Customer和Order，它们之间是一对多的关系：

@Entity
public class Customer {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @OneToMany(mappedBy = "customer")
    private List<Order> orders = new ArrayList<>();
    // getter and setter
}

@Entity
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "customer_id")
    private Customer customer;
    // getter and setter
}

如果我们暴力地将实体类转为JSON字符串，就会出现延迟加载异常：

ObjectMapper mapper = new ObjectMapper();
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
String json = "";
try {
    Customer customer = em.find(Customer.class, 1L);
    json = mapper.writeValueAsString(customer);
} catch (JsonProcessingException e) {
    e.printStackTrace();
}
System.out.println(json);

输出结果：

com.fasterxml.jackson.databind.JsonMappingException: could not initialize proxy - no Session (through reference chain: com.example.demo.entity.Customer["orders"]->java.util.ArrayList[0]->com.example.demo.entity.Order["customer"])

因此，我们需要在实体类上使用@JsonManagedReference对关系进行标注，然后在需要序列化关联对象的实体类上使用@JsonBackReference标注：

@Entity
public class Customer {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @OneToMany(mappedBy = "customer")
    @JsonManagedReference
    private List<Order> orders = new ArrayList<>();
    // getter and setter
}

@Entity
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "customer_id")
    @JsonBackReference
    private Customer customer;
    // getter and setter
}

这样就可以正常地将实体转为JSON字符串：

ObjectMapper mapper = new ObjectMapper();
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
String json = "";
try {
    Customer customer = em.find(Customer.class, 1L);
    json = mapper.writeValueAsString(customer);
} catch (JsonProcessingException e) {
    e.printStackTrace();
}
System.out.println(json);

输出结果：

{"id":1,"name":"customer1","orders":[{"id":1,"name":"order1"}]}

2.2 使用Hibernate的Hibernate.initialize()方法

假设我们有两个实体类Department和Employee，它们之间是多对一的关系：

@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @OneToMany(mappedBy = "department")
    private List<Employee> employees = new ArrayList<>();
    // getter and setter
}

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "department_id")
    private Department department;
    // getter and setter
}

如果我们暴力地将实体类转为JSON字符串，就会出现延迟加载异常：

ObjectMapper mapper = new ObjectMapper();
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
String json = "";
try {
    Department department = em.find(Department.class, 1L);
    json = mapper.writeValueAsString(department);
} catch (JsonProcessingException e) {
    e.printStackTrace();
}
System.out.println(json);

输出结果：

com.fasterxml.jackson.databind.JsonMappingException: could not initialize proxy - no Session (through reference chain: com.example.demo.entity.Department["employees"]->java.util.ArrayList[0]->com.example.demo.entity.Employee["department"])

因此，我们可以使用Hibernate的Hibernate.initialize()方法预先初始化需要序列化的实体对象，从而避免延迟加载异常：

Department department = em.find(Department.class, 1L);
Hibernate.initialize(department.getEmployees());
ObjectMapper mapper = new ObjectMapper();
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
String json = "";
try {
    json = mapper.writeValueAsString(department);
} catch (JsonProcessingException e) {
    e.printStackTrace();
}
System.out.println(json);

输出结果：

{"id":1,"name":"department1","employees":[{"id":1,"name":"employee1"},{"id":2,"name":"employee2"}]}

这样就可以正常地将实体转为JSON字符串了。