JavaPersistenceWithHibernate第二版笔记-第七章-001Mapping a set(@ElementCollection、@CollectionTable、@JoinColumn、)

一、结构

二、代码

1.

package org.jpwh.model.collections.setofstrings;

import org.jpwh.model.Constants;

import javax.persistence.CollectionTable;
import javax.persistence.Column;
import javax.persistence.ElementCollection;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import java.util.HashSet;
import java.util.Set;

@Entity
public class Item {

    @Id
    @GeneratedValue(generator = Constants.ID_GENERATOR)
    protected Long id;

    @ElementCollection
    @CollectionTable(
            name = "IMAGE", // Defaults to ITEM_IMAGES
            joinColumns = @JoinColumn(name = "ITEM_ID")) // Default, actually
    @Column(name = "FILENAME") // Defaults to IMAGES
    protected Set<String> images = new HashSet<String>(); // Initialize field here

    public Long getId() {
        return id;
    }

    public Set<String> getImages() {
        return images;
    }

    public void setImages(Set<String> images) {
        this.images = images;
    }

    // ...
}

2.

package org.jpwh.test.collections;

import org.jpwh.env.JPATest;
import org.jpwh.model.collections.setofstrings.Item;
import org.testng.annotations.BeforeClass;
import org.testng.annotations.Test;

import javax.persistence.EntityManager;
import javax.transaction.UserTransaction;

import static org.testng.Assert.assertEquals;

public class SetOfStrings extends JPATest {

    @Override
    public void configurePersistenceUnit() throws Exception {
        configurePersistenceUnit("SetOfStringsPU");
    }

    @Test
    public void storeLoadCollection() throws Exception {
        UserTransaction tx = TM.getUserTransaction();
        try {
            tx.begin();
            EntityManager em = JPA.createEntityManager();
            Item someItem = new Item();

            someItem.getImages().add("foo.jpg");
            someItem.getImages().add("bar.jpg");
            someItem.getImages().add("baz.jpg");
            someItem.getImages().add("baz.jpg"); // Duplicate, filtered at Java level by HashSet!

            em.persist(someItem);
            tx.commit();
            em.close();
            Long ITEM_ID = someItem.getId();

            tx.begin();
            em = JPA.createEntityManager();
            Item item = em.find(Item.class, ITEM_ID);
            assertEquals(item.getImages().size(), 3);
            tx.commit();
            em.close();
        } finally {
            TM.rollback();
        }
    }

}

It doesn’t seem likely that you’d allow the user to attach the same image more than once to the same item,