Hibernate requires that persistent collection-valued fields be declared as an interface type. For example:
public class Product {
private String serialNumber;
private Set parts = new HashSet();
public Set getParts() { return parts; }
void setParts(Set parts) { this.parts = parts; }
public String getSerialNumber() { return serialNumber; }
void setSerialNumber(String sn) { serialNumber = sn; }
}The actual interface might be java.util.Set, java.util.Collection, java.util.List, java.util.Map, java.util.SortedSet, java.util.SortedMap or anything you like ("anything you like" means you will have to write an implementation of org.hibernate.usertype.UserCollectionType.)
Notice how the instance variable was initialized with an instance of HashSet. This is the best way to initialize collection valued properties of newly instantiated (non-persistent) instances. When you make the instance persistent, by calling persist() for example, Hibernate will actually replace the HashSet with an instance of Hibernate's own implementation of Set. Be aware of the following errors:
Cat cat = new DomesticCat(); Cat kitten = new DomesticCat(); .... Set kittens = new HashSet(); kittens.add(kitten); cat.setKittens(kittens); session.persist(cat); kittens = cat.getKittens(); // Okay, kittens collection is a Set (HashSet) cat.getKittens(); // Error!
The persistent collections injected by Hibernate behave like HashMap, HashSet, TreeMap, TreeSet or ArrayList, depending on the interface type.
Collections instances have the usual behavior of value types. They are automatically persisted when referenced by a persistent object and are automatically deleted when unreferenced. If a collection is passed from one persistent object to another, its elements might be moved from one table to another. Two entities cannot share a reference to the same collection instance. Due to the underlying relational model, collection-valued properties do not support null value semantics. Hibernate does not distinguish between a null collection reference and an empty collection.
Use persistent collections the same way you use ordinary Java collections. However, please ensure you understand the semantics of bidirectional associations (these are discussed later).
提示
There are quite a range of mappings that can be generated for collections that cover many common relational models. We suggest you experiment with the schema generation tool so that you understand how various mapping declarations translate to database tables.
The Hibernate mapping element used for mapping a collection depends upon the type of interface. For example, a <set> element is used for mapping properties of type Set.
<class name="Product">
<id name="serialNumber" column="productSerialNumber"/>
<set name="parts">
<key column="productSerialNumber" not-null="true"/>
<one-to-many class="Part"/>
</set>
</class>除了<set>,还有<list>, <map>, <bag>, <array> 和 <primitive-array> 映射元素。<map>具有代表性：
<map
name="propertyName" 
table="table_name" 
schema="schema_name" 
lazy="true|extra|false" 
inverse="true|false" 
cascade="all|none|save-update|delete|all-delete-orphan|delet
e-orphan"
sort="unsorted|natural|comparatorClass" 
order-by="column_name asc|desc" 
where="arbitrary sql where condition" 
fetch="join|select|subselect" 
batch-size="N" 
access="field|property|ClassName" 
optimistic-lock="true|false" 
mutable="true|false" 
node="element-name|."
embed-xml="true|false"
>
<key .... />
<map-key .... />
<element .... />
</map> |
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Collection instances are distinguished in the database by the foreign key of the entity that owns the collection. This foreign key is referred to as the collection key column, or columns, of the collection table. The collection key column is mapped by the <key> element.
There can be a nullability constraint on the foreign key column. For most collections, this is implied. For unidirectional one-to-many associations, the foreign key column is nullable by default, so you may need to specify not-null="true".
<key column="productSerialNumber" not-null="true"/>
The foreign key constraint can use ON DELETE CASCADE.
<key column="productSerialNumber" on-delete="cascade"/>
对<key> 元素的完整定义，请参阅前面的章节。
Collections can contain almost any other Hibernate type, including: basic types, custom types, components and references to other entities. This is an important distinction. An object in a collection might be handled with "value" semantics (its life cycle fully depends on the collection owner), or it might be a reference to another entity with its own life cycle. In the latter case, only the "link" between the two objects is considered to be a state held by the collection.
被包容的类型被称为集合元素类型（collection element type）。集合元素通过<element>或<composite-element>映射，或在其是实体引用的时候，通过<one-to-many> 或<many-to-many>映射。前两种用于使用值语义映射元素，后两种用于映射实体关联。
All collection mappings, except those with set and bag semantics, need an index column in the collection table. An index column is a column that maps to an array index, or List index, or Map key. The index of a Map may be of any basic type, mapped with <map-key>. It can be an entity reference mapped with <map-key-many-to-many>, or it can be a composite type mapped with <composite-map-key>. The index of an array or list is always of type integer and is mapped using the <list-index> element. The mapped column contains sequential integers that are numbered from zero by default.
<list-index
column="column_name"
base="0|1|..."/> | |
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<map-key
column="column_name"
formula="any SQL expression"
type="type_name"
node="@attribute-name"
length="N"/> | |
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<map-key-many-to-many
column="column_name"
formula="any SQL expression"
class="ClassName"
/> | |
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If your table does not have an index column, and you still wish to use List as the property type, you can map the property as a Hibernate <bag>. A bag does not retain its order when it is retrieved from the database, but it can be optionally sorted or ordered.
Any collection of values or many-to-many associations requires a dedicated collection table with a foreign key column or columns, collection element column or columns, and possibly an index column or columns.
For a collection of values use the <element> tag. For example:
<element
column="column_name"
formula="any SQL expression"
type="typename"
length="L"
precision="P"
scale="S"
not-null="true|false"
unique="true|false"
node="element-name"
/> | |
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A many-to-many association is specified using the <many-to-many> element.
<many-to-many
column="column_name"
formula="any SQL expression"
class="ClassName"
fetch="select|join"
unique="true|false"
not-found="ignore|exception"
entity-name="EntityName"
property-ref="propertyNameFromAssociatedClass"
node="element-name"
embed-xml="true|false"
/> | |
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Here are some examples.
A set of strings:
<set name="names" table="person_names">
<key column="person_id"/>
<element column="person_name" type="string"/>
</set>A bag containing integers with an iteration order determined by the order-by attribute:
<bag name="sizes"
table="item_sizes"
order-by="size asc">
<key column="item_id"/>
<element column="size" type="integer"/>
</bag>An array of entities, in this case, a many-to-many association:
<array name="addresses"
table="PersonAddress"
cascade="persist">
<key column="personId"/>
<list-index column="sortOrder"/>
<many-to-many column="addressId" class="Address"/>
</array>一个组件的列表：（下一章讨论）
<map name="holidays"
table="holidays"
schema="dbo"
order-by="hol_name asc">
<key column="id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date" type="date"/>
</map>A list of components (this is discussed in the next chapter):
<list name="carComponents"
table="CarComponents">
<key column="carId"/>
<list-index column="sortOrder"/>
<composite-element class="CarComponent">
<property name="price"/>
<property name="type"/>
<property name="serialNumber" column="serialNum"/>
</composite-element>
</list>A one-to-many association links the tables of two classes via a foreign key with no intervening collection table. This mapping loses certain semantics of normal Java collections:
An instance of the contained entity class cannot belong to more than one instance of the collection.
An instance of the contained entity class cannot appear at more than one value of the collection index.
An association from Product to Part requires the existence of a foreign key column and possibly an index column to the Part table. A <one-to-many> tag indicates that this is a one-to-many association.
<one-to-many
class="ClassName"
not-found="ignore|exception"
entity-name="EntityName"
node="element-name"
embed-xml="true|false"
/> |
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The <one-to-many> element does not need to declare any columns. Nor is it necessary to specify the table name anywhere.
警告
If the foreign key column of a <one-to-many> association is declared NOT NULL, you must declare the <key> mapping not-null="true" or use a bidirectional association with the collection mapping marked inverse="true". See the discussion of bidirectional associations later in this chapter for more information.
The following example shows a map of Part entities by name, where partName is a persistent property of Part. Notice the use of a formula-based index:
<map name="parts"
cascade="all">
<key column="productId" not-null="true"/>
<map-key formula="partName"/>
<one-to-many class="Part"/>
</map>Hibernate支持实现java.util.SortedMap和java.util.SortedSet的集合。 你必须在映射文件中指定一个比较器：
<set name="aliases"
table="person_aliases"
sort="natural">
<key column="person"/>
<element column="name" type="string"/>
</set>
<map name="holidays" sort="my.custom.HolidayComparator">
<key column="year_id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date" type="date"/>
</map>sort属性中允许的值包括unsorted,natural和某个实现了java.util.Comparator的类的名称。
分类集合的行为事实上象java.util.TreeSet或者java.util.TreeMap。
If you want the database itself to order the collection elements, use the order-by attribute of set, bag or map mappings. This solution is only available under JDK 1.4 or higher and is implemented using LinkedHashSet or LinkedHashMap. This performs the ordering in the SQL query and not in the memory.
<set name="aliases" table="person_aliases" order-by="lower(name) asc">
<key column="person"/>
<element column="name" type="string"/>
</set>
<map name="holidays" order-by="hol_date, hol_name">
<key column="year_id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date type="date"/>
</map>Note
The value of the order-by attribute is an SQL ordering, not an HQL ordering.
Associations can even be sorted by arbitrary criteria at runtime using a collection filter():
sortedUsers = s.createFilter( group.getUsers(), "order by this.name" ).list();
A bidirectional association allows navigation from both "ends" of the association. Two kinds of bidirectional association are supported:
- 一对多（one-to-many）
set or bag valued at one end and single-valued at the other
- 多对多（many-to-many）
两端都是set或bag值
You can specify a bidirectional many-to-many association by mapping two many-to-many associations to the same database table and declaring one end as inverse. You cannot select an indexed collection.
Here is an example of a bidirectional many-to-many association that illustrates how each category can have many items and each item can be in many categories:
<class name="Category">
<id name="id" column="CATEGORY_ID"/>
...
<bag name="items" table="CATEGORY_ITEM">
<key column="CATEGORY_ID"/>
<many-to-many class="Item" column="ITEM_ID"/>
</bag>
</class>
<class name="Item">
<id name="id" column="ITEM_ID"/>
...
<!-- inverse end -->
<bag name="categories" table="CATEGORY_ITEM" inverse="true">
<key column="ITEM_ID"/>
<many-to-many class="Category" column="CATEGORY_ID"/>
</bag>
</class>Changes made only to the inverse end of the association are not persisted. This means that Hibernate has two representations in memory for every bidirectional association: one link from A to B and another link from B to A. This is easier to understand if you think about the Java object model and how a many-to-many relationship in Javais created:
category.getItems().add(item); // The category now "knows" about the relationship item.getCategories().add(category); // The item now "knows" about the relationship session.persist(item); // The relationship won't be saved! session.persist(category); // The relationship will be saved
非反向端用于把内存中的表示保存到数据库中。
You can define a bidirectional one-to-many association by mapping a one-to-many association to the same table column(s) as a many-to-one association and declaring the many-valued end inverse="true".
<class name="Parent">
<id name="id" column="parent_id"/>
....
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<many-to-one name="parent"
class="Parent"
column="parent_id"
not-null="true"/>
</class>Mapping one end of an association with inverse="true" does not affect the operation of cascades as these are orthogonal concepts.
A bidirectional association where one end is represented as a <list> or <map>, requires special consideration. If there is a property of the child class that maps to the index column you can use inverse="true" on the collection mapping:
<class name="Parent">
<id name="id" column="parent_id"/>
....
<map name="children" inverse="true">
<key column="parent_id"/>
<map-key column="name"
type="string"/>
<one-to-many class="Child"/>
</map>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<property name="name"
not-null="true"/>
<many-to-one name="parent"
class="Parent"
column="parent_id"
not-null="true"/>
</class>If there is no such property on the child class, the association cannot be considered truly bidirectional. That is, there is information available at one end of the association that is not available at the other end. In this case, you cannot map the collection inverse="true". Instead, you could use the following mapping:
<class name="Parent">
<id name="id" column="parent_id"/>
....
<map name="children">
<key column="parent_id"
not-null="true"/>
<map-key column="name"
type="string"/>
<one-to-many class="Child"/>
</map>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<many-to-one name="parent"
class="Parent"
column="parent_id"
insert="false"
update="false"
not-null="true"/>
</class>Note that in this mapping, the collection-valued end of the association is responsible for updates to the foreign key.
There are three possible approaches to mapping a ternary association. One approach is to use a Map with an association as its index:
<map name="contracts">
<key column="employer_id" not-null="true"/>
<map-key-many-to-many column="employee_id" class="Employee"/>
<one-to-many class="Contract"/>
</map><map name="connections">
<key column="incoming_node_id"/>
<map-key-many-to-many column="outgoing_node_id" class="Node"/>
<many-to-many column="connection_id" class="Connection"/>
</map>A second approach is to remodel the association as an entity class. This is the most common approach.
A final alternative is to use composite elements, which will be discussed later.
The majority of the many-to-many associations and collections of values shown previously all map to tables with composite keys, even though it has been have suggested that entities should have synthetic identifiers (surrogate keys). A pure association table does not seem to benefit much from a surrogate key, although a collection of composite values might. It is for this reason that Hibernate provides a feature that allows you to map many-to-many associations and collections of values to a table with a surrogate key.
The <idbag> element lets you map a List (or Collection) with bag semantics. For example:
<idbag name="lovers" table="LOVERS">
<collection-id column="ID" type="long">
<generator class="sequence"/>
</collection-id>
<key column="PERSON1"/>
<many-to-many column="PERSON2" class="Person" fetch="join"/>
</idbag>An <idbag> has a synthetic id generator, just like an entity class. A different surrogate key is assigned to each collection row. Hibernate does not, however, provide any mechanism for discovering the surrogate key value of a particular row.
The update performance of an <idbag> supersedes a regular <bag>. Hibernate can locate individual rows efficiently and update or delete them individually, similar to a list, map or set.
在目前的实现中，还不支持使用identity标识符生成器策略来生成<idbag>集合的标识符。
This section covers collection examples.
The following class has a collection of Child instances:
package eg;
import java.util.Set;
public class Parent {
private long id;
private Set children;
public long getId() { return id; }
private void setId(long id) { this.id=id; }
private Set getChildren() { return children; }
private void setChildren(Set children) { this.children=children; }
....
....
}If each child has, at most, one parent, the most natural mapping is a one-to-many association:
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>在以下的表定义中反应了这个映射关系：
create table parent ( id bigint not null primary key ) create table child ( id bigint not null primary key, name varchar(255), parent_id bigint ) alter table child add constraint childfk0 (parent_id) references parent
如果父亲是必须的, 那么就可以使用双向one-to-many的关联了：
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
<many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/>
</class>
</hibernate-mapping>请注意NOT NULL的约束:
create table parent ( id bigint not null primary key )
create table child ( id bigint not null
primary key,
name varchar(255),
parent_id bigint not null )
alter table child add constraint childfk0 (parent_id) references parentAlternatively, if this association must be unidirectional you can declare the NOT NULL constraint on the <key> mapping:
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children">
<key column="parent_id" not-null="true"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>On the other hand, if a child has multiple parents, a many-to-many association is appropriate:
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children" table="childset">
<key column="parent_id"/>
<many-to-many class="Child" column="child_id"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>表定义：
create table parent ( id bigint not null primary key )
create table child ( id bigint not null primary key, name varchar(255) )
create table childset ( parent_id bigint not null,
child_id bigint not null,
primary key ( parent_id, child_id ) )
alter table childset add constraint childsetfk0 (parent_id) references parent
alter table childset add constraint childsetfk1 (child_id) references childFor more examples and a complete explanation of a parent/child relationship mapping, see 第 21 章 示例：父子关系(Parent Child Relationships) for more information.
Even more complex association mappings are covered in the next chapter.