在使用jpa时，使用关联关系时，发现懒加载时会和json序列化冲突，导致懒加载失败。
创建三个表：
school 、teacher 、student
school和teacher是一对多的关系，teacher和student是多对一的关系。

sql

CREATE TABLE school(
    id INT AUTO_INCREMENT PRIMARY KEY,
    school_name VARCHAR(100) COMMENT '学校名称'
)
INSERT INTO school(school_name)VALUES("xx学校");

CREATE TABLE teacher(
    id INT AUTO_INCREMENT PRIMARY KEY,
    teacher_name VARCHAR(100) COMMENT '老师名称',
    school_id INT COMMENT '学校id'
)
INSERT INTO teacher(teacher_name,school_id)VALUES("老师1",1);
INSERT INTO teacher(teacher_name,school_id)VALUES("老师2",1);
INSERT INTO teacher(teacher_name,school_id)VALUES("老师3",1);

CREATE TABLE student(
    id INT AUTO_INCREMENT PRIMARY KEY,
    student_name VARCHAR(100) COMMENT '学生名称',
    teacher_id INT COMMENT '老师id'
)
INSERT INTO student(student_name,teacher_id)VALUES("学生1",1);
INSERT INTO student(student_name,teacher_id)VALUES("学生2",1);
INSERT INTO student(student_name,teacher_id)VALUES("学生3",1);

INSERT INTO student(student_name,teacher_id)VALUES("学生4",2);
INSERT INTO student(student_name,teacher_id)VALUES("学生5",2);
INSERT INTO student(student_name,teacher_id)VALUES("学生6",2);

INSERT INTO student(student_name,teacher_id)VALUES("学生7",3);
INSERT INTO student(student_name,teacher_id)VALUES("学生8",3);
INSERT INTO student(student_name,teacher_id)VALUES("学生9",3);

controller类

@RestController
public class SchoolController {
    @Autowired
    private SchoolRepo schoolRepo;
    @GetMapping("/list")
    public List<School> querySchool() {
        return schoolRepo.findAll();
    }
}

坑1：首先先遇到的是转json时互相引用，陷入死循环的问题！

解决方案如下：
使用@JsonIgnoreProperties，json序列化时忽略某些属性
修改代码:
school类

    @OneToMany(fetch=FetchType.LAZY,mappedBy="school")
    @JsonIgnoreProperties("school")
    private List<Teacher> teacherList;

teacher类

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="school_id",referencedColumnName="id")
    @JsonIgnoreProperties("teacherList")
    private School school;
    
    @OneToMany(fetch=FetchType.LAZY,mappedBy="teacher")
    @JsonIgnoreProperties("teacher")    
    private List<Student> studentList;

student类

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="teacher_id",referencedColumnName="id")
    @JsonIgnoreProperties("studentList")    
    @NotFound(action= NotFoundAction.IGNORE)
    private Teacher teacher;

坑2:No serializer found for class org.hibernate.proxy.pojo.bytebuddy.ByteBuddyInterceptor and no properties discovered to create BeanSerializer
在实体类中 发现有字段为null，在转化成json的时候，fasterxml.jackson将对象转换为json报错

解决方案如下：
使用@NotFound(action= NotFoundAction.IGNORE) 注解，找不到引用的外键数据时忽略，NotFound默认是exception
修改代码：
teacher类

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="school_id",referencedColumnName="id")
    @JsonIgnoreProperties("teacherList")
    @NotFound(action= NotFoundAction.IGNORE)
    private School school;

student类

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="teacher_id",referencedColumnName="id")
    @JsonIgnoreProperties("studentList")    
    @NotFound(action= NotFoundAction.IGNORE)
    private Teacher teacher;

坑3:运行发现懒加载并没有起作用！

解决方案如下：
映入pom文件

<dependency>
            <groupId>com.fasterxml.jackson.datatype</groupId>
            <artifactId>jackson-datatype-hibernate5</artifactId>
            <version>2.9.8</version>
</dependency>

yml文件开启懒加载

  jpa:
    open-in-view: true
    properties:
      hibernate:
        enable_lazy_load_no_trans: true

config文件

package com.ganlong.config;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.datatype.hibernate5.Hibernate5Module;

@Configuration
public class WebMvcConfiguration implements WebMvcConfigurer {
    /**
          * 解决jpa懒加载
     */
    @Bean
    public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter() {
        MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
        ObjectMapper mapper = converter.getObjectMapper();
        mapper.registerModule(hibernate5Module());
        return converter;
    }

    @Bean
    public Hibernate5Module hibernate5Module() {
        Hibernate5Module module = new Hibernate5Module();
        module.disable(Hibernate5Module.Feature.USE_TRANSIENT_ANNOTATION);
        module.enable(Hibernate5Module.Feature.SERIALIZE_IDENTIFIER_FOR_LAZY_NOT_LOADED_OBJECTS);
        return module;
    }
}

坑4：@ManyToOne(fetch=FetchType.LAZY)这样设置懒加载失效，要将optional=false。
@ManyToOne(fetch=FetchType.LAZY)和@NotFound(action= NotFoundAction.IGNORE)冲突，设置了@NotFound后@ManyToOne的懒加载就会失效！这个没找到解决方案。

坑5:hibernate会自动关闭session，这样，当要使用子数据的时候，系统会抛出lazyinit的错误。

解决方案如下：
yml文件：

open-in-view: true

config文件：

    @Bean
    public OpenEntityManagerInViewFilter openEntityManagerInViewFilter() {
        OpenEntityManagerInViewFilter openEntityManagerInViewFilter= new OpenEntityManagerInViewFilter();
        openEntityManagerInViewFilter.setEntityManagerFactoryBeanName("chexAdminJpaEntityManagerFactory");
        return  openEntityManagerInViewFilter;
    }

坑6：加载多条sql的问题！未解决