Why is the child collection is null in One-To-Many relationship of spring boot application? - spring

Question

I create a spring boot application with MySQL,JPA,Web dependencies,and manually config my database settings in .properties file of Spring boot. I passed compiling, and started application successfully, and adding one record is normal fine.
BUT, i use method 'findAll(Pageable pageable)' i got a problem, that was
Could not write JSON: failed to lazily initialize a collection of role，could not initialize proxy - no Session
I got confused, i started to debug my code, finally i found that the child collection of the result is null, and it contained an error, which is
"Exception occurred: com.sun.jdi.InvocationException occurred invoking method.."
I tried a lot to fix my code, but no use.
who can help me?
The entity relationship is a simple one to many:
TeacherInfo entity and ClassInfo entity, teacher manage multiple classes, just simple as this.
here is the enter point of my app:
#SpringBootApplication(exclude= {
DataSourceAutoConfiguration.class,
HibernateJpaAutoConfiguration.class,
DataSourceAutoConfiguration.class
})
#EnableTransactionManagement
public class OrmTestApplication {
public static void main(String[] args) {
SpringApplication.run(OrmTestApplication.class, args);
}
}
Database properties setting is here:
spring.datasource.primary.url=jdbc:mysql://localhost:3306/ormtest?useSSL=false
spring.datasource.primary.username=root
spring.datasource.primary.password=BlaNok2700
spring.datasource.primary.driver-class-name = com.mysql.jdbc.Driver
hibernate.hbm2ddl.auto = update
hibernate.show-sql = true
My Data base configure java code is here:
Configuration
#EnableJpaRepositories(basePackages = "com.lanjian.ormtest.repositories", entityManagerFactoryRef = "primaryEntityManagerFactory", transactionManagerRef = "primaryTransactionManager")
public class PrimaryDbConfig {
#Autowired
private Environment env;
#Bean
#ConfigurationProperties(prefix="spring.datasource.primary")
public DataSourceProperties primaryDataSourceProperties() {
return new DataSourceProperties();
}
#Bean
public DataSource primaryDataSource() {
DataSourceProperties dbProperty = primaryDataSourceProperties();
return DataSourceBuilder.create()
.driverClassName(dbProperty.getDriverClassName())
.url(dbProperty.getUrl())
.username(dbProperty.getUsername())
.password(dbProperty.getPassword())
.build();
}
#Bean
public LocalContainerEntityManagerFactoryBean primaryEntityManagerFactory() {
LocalContainerEntityManagerFactoryBean factory = new LocalContainerEntityManagerFactoryBean();
factory.setDataSource(primaryDataSource());
factory.setPackagesToScan("com.lanjian.ormtest.entities");
factory.setJpaVendorAdapter(new HibernateJpaVendorAdapter());
Properties jpaProperties = new Properties();
jpaProperties.put("hibernate.hbm2ddl.auto", env.getProperty("hibernate.hbm2ddl.auto"));
jpaProperties.put("hibernate.show-sql", env.getProperty("hibernate.show-sql"));
factory.setJpaProperties(jpaProperties);
return factory;
}
#Bean
public PlatformTransactionManager primaryTransactionManager() {
EntityManagerFactory factory = primaryEntityManagerFactory().getObject();
return new JpaTransactionManager(factory);
}
}
My REST controller method is here:
#Autowired
private TeacherRepository teacherRepository;
#GetMapping("/page")
public Page<TeacherInfo> page(Pageable pageable){
Page<TeacherInfo> list = teacherRepository.findAll(pageable);
return list;
}
What happened
After i started my application, and use postman send request, i got this:
got a 500 error
And i debugger my code, found this:
child collection is null
In the picture, 'classes' is a list collection, but it is null, i don't understand.
Here are the TeacherInfo entity I defined
#Entity
#Table(name = "teacher")
public class TeacherInfo {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
private byte age;
private boolean male;
#OneToMany(cascade = CascadeType.ALL,fetch = FetchType.LAZY, mappedBy="chargedTeacher")
private List<ClassInfo> classes = new ArrayList<>();
public void initialize() {
for (ClassInfo classInfo : classes) {
classInfo.setChargedTeacher(this);
for (StudentInfo studentInfo : classInfo.getStudents()) {
studentInfo.setClassInfo(classInfo);
}
}
}
//Setters and Getters}
Here is the ClassInfo Entity i defined
#Entity
#Table(name = "class_info")
public class ClassInfo {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
private int capacity;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "teacher_id",nullable=false)
#JsonIgnore
private TeacherInfo chargedTeacher;
#OneToMany(cascade = CascadeType.ALL,fetch=FetchType.LAZY,mappedBy="classInfo")
private List<StudentInfo> students = new ArrayList<>();
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getCapacity() {
return capacity;
}
public void setCapacity(int capacity) {
this.capacity = capacity;
}
public TeacherInfo getChargedTeacher() {
return chargedTeacher;
}
public void setChargedTeacher(TeacherInfo chargedTeacher) {
this.chargedTeacher = chargedTeacher;
}
public List<StudentInfo> getStudents() {
return students;
}
public void setStudents(List<StudentInfo> students) {
this.students = students;
}
}

Answer 1

I think that the problem may come from Transactionality and JPA Fetching types.
Your repository method is being invoked not using a transaction, which implies that the transaction is on the boundaries of the method invocation (which might not be wrong). Spring returns a Page with objects but when it tries to serialize them, transaction is gone so no way to access childs.
I would suggest to put the JPA relationship as EAGER fetching, allowing all the objects to be present on the repository result when the transaction ends.
EDIT:
Answer to comments
#Bean
public PlatformTransactionManager primaryTransactionManager(EntityManagerFactory factory) {
return new JpaTransactionManager(factory);
}