如何在hibernate中通过ArrayList保存多个对象？

Question

I have a Student entity. My idea is to collect multiple student objects in an ArrayList and save all objects from that list to the database. When do you use @ElementCollection annotation? Does it apply to situations like this?

Student :

package basic;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String name;

    public Student() {

    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + "]";
    }

    public Student(String name) {
        this.name = name;
    }

}

Runner :

package basic;

import java.util.ArrayList;
import java.util.List;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class Runner {

    public static void main(String[] args) {

        SessionFactory sessionFactory = new Configuration().configure("/basic/hibernate.cfg.xml").buildSessionFactory();
        Session session = sessionFactory.openSession();
        session.beginTransaction();

        List<Student> students = new ArrayList<>();

        students.add(new Student("Michael"));
        students.add(new Student("Dave"));
        students.add(new Student("Tom"));
        students.add(new Student("Dinesh"));
        students.add(new Student("Lakshman"));
        students.add(new Student("Cruise"));

        session.save(students);

        session.getTransaction().commit();
        session.close();
    }

}

Error

Exception in thread "main" org.hibernate.MappingException: Unknown entity: java.util.ArrayList
    at org.hibernate.metamodel.internal.MetamodelImpl.entityPersister(MetamodelImpl.java:620)
    at org.hibernate.internal.SessionImpl.getEntityPersister(SessionImpl.java:1596)
    at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:104)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:192)
    at org.hibernate.event.internal.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:38)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:177)
    at org.hibernate.event.internal.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:32)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:73)
    at org.hibernate.internal.SessionImpl.fireSave(SessionImpl.java:668)
    at org.hibernate.internal.SessionImpl.save(SessionImpl.java:660)
    at org.hibernate.internal.SessionImpl.save(SessionImpl.java:655)
    at basic.Runner.main(Runner.java:27)

Answer 1

You have to do something like this:

for(Student student : students) {
    session.save(student);
}

If you want to save entity you should map it. ArrayList<> is not mapped entity. Student has mapping so you should save it separately.

Answer 2

@ElementCollection you should use to define relation between object - here you have nice explenation https://en.wikibooks.org/wiki/Java_Persistence/ElementCollection

To save list of object, you need to iterate by objects, something like this -> How to insert multiple rows into database using hibernate?

Answer 3

Any Functions that we can store an array of objects at once rather than using loops. This is causing performance issues..

Answer 4

I would further recommend to use another Hibernate command to avoid an Out Of Memory error...

SessionFactory sessionFactory = config.buildSessionFactory();
    Session session = sessionFactory.openSession();
     
    Transaction transaction = session.beginTransaction();
     
    for (int i = 0 ; i < students.size(); i++) {
         session.save(students.get(i));
         
        if (i % 100 == 0) {//a batch size for safety
            session.flush();
            session.clear();
        }
    }
     
    transaction.commit();
    session.close();
    sessionFactory.close();