主要实体:
@Entity public class KeyEntity
{
@Id @GeneratedValue(strategy = GenerationType.TABLE)
public Long id;
public String handle;
public boolean equals(Object o) {
KeyEntity oke = (KeyEntity) o;
return handle != null ? handle.equals(oke.handle) : oke.handle == null;
}
public int hashCode() {
return handle != null ? handle.hashCode() : 0;
}
}
价值实体:
@Entity public class ValueEntity
{
@Id @GeneratedValue(strategy = GenerationType.TABLE)
public Long id;
@ManyToOne
public KeyEntity key;
public String value;
public boolean equals(Object o) {
ValueEntity ove = (ValueEntity) o;
return key != null ? key.equals(ove.key) : ove.key == null;
}
public int hashCode() {
return key != null ? key.hashCode() : 0;
}
}
容器实体:
@Entity public class ContainerEntity
{
@Id @GeneratedValue(strategy = GenerationType.TABLE)
public Long id;
@OneToMany @MapKey(name = "key")
public Map<KeyEntity, ValueEntity> values = new HashMap<KeyEntity, ValueEntity>();
}
主:
KeyEntity k1 = new KeyEntity();
k1.handle = "k1";
em.persist(k1);
KeyEntity k2 = new KeyEntity();
k2.handle = "k2";
em.persist(k2);
ValueEntity v1 = new ValueEntity();
v1.key = k1;
v1.value = "v1";
em.persist(v1);
ValueEntity v2 = new ValueEntity();
v2.key = k2;
v2.value = "v2";
em.persist(v2);
ContainerEntity ce = new ContainerEntity();
ce.values.put(k1, v1);
ce.values.put(k2, v2);
em.persist(ce);
// display number of values
System.out.println(ce.values.size());
// create new transaction
em.getTransaction().commit();
em.close();
em = emf.createEntityManager();
em.getTransaction().begin();
// find our container and inspect the number of values
ce = em.find(ContainerEntity.class, ce.id);
System.out.println(ce.values.size());
如果我向ContainerEntity
添加几个键值对然后重新加载所述容器,则只会出现一个键值对。如果查看运行上述主函数的输出,首先打印'2',然后打印'1'。
我可以看到这是因为KeyEntity.hashCode
- 当插入HashMap
KeyEntity.handle
时null
,所以所有对都将具有相同的哈希码。此时填充了KeyEntity.id
- 如果我将哈希代码基于此字段,则一切正常。此外,如果我将密钥更改为String
,则会及时加载密钥,以便拨打hashCode
。
如何更改ContainerEntity
中的映射,以便在KeyEntity.handle
放入地图时加载hashCode
,以便{{1}}可以使用它?