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,