Summary of problems using Spring Boot

Question 1: When Spring Boot uses JPA, there is a problem when accessing the database table in the Controller and displaying it to the browser as (ResponseBody)

HTTP Status 500 - Could not write JSON: No serializer found for class org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationConfig.SerializationFeature.FAIL_ON_EMPTY_BEANS) )

----- Entity class User (corresponding to tb_user in the database)

//使用JPA
@Entity                    //是一个跟数据库对应的类
@Table(name="tb_user")    //指定生成的表名
public class User implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)     //主键id自增长
    private Integer id;

    @Column(name = "email")
    private String email;

    @Column(name = "last_name", length = 50)    //驼峰命名法, 并约定最长长度为50
    private String name;

    public Integer getId() {
        return id;
    }

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

    public String getEmail() {
        return email;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

//只需要继承JpaRepository类, 便可使用数据库的CRUD,分页以及排序操做
//<User,Integer> 表示的是操作的实体类, 以及主键类型
public interface UserRepository extends JpaRepository<User, Integer> {
}

application.yml configuration file

spring:
  datasource:
    username: root
    password: *****
    url: **************
    driver-class-name: com.mysql.jdbc.Driver
  # 更新或者创建表结构
  jpa:
    hibernate:
      ddl-auto: update
    show-sql: true

--UserController class, there is a method to find User by id, and return it to the browser for display in Json

@RestController
public class UserController {

    @Autowired
    UserRepository userRepository;

    @GetMapping("/user/{id}")
    public User getUser(@PathVariable("id") int id)
    {
        User user = userRepository.getOne(id);
        return user;
    }
}

Then... as soon as you visit, the browser appears

Intellij idea 中

大概就是: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: com.gdufe.entity.User_$$_jvst21_0["handler"])

Then Googled it a bit: I found that the problem seemed to be because the entity was loaded with laziness and serialization before it was fully loaded.

The solution is: add a comment on the User class

@JsonIgnoreProperties({
    
    "hibernateLazyInitializer", "handler"})
如下图

Delay loading by setting the hibernate proxy object. By annotating that the class has a private property of delayed loading to bypass it

problem solved

Summary of problems using Spring Boot

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