类图和ORM关系(三)

技术2022-05-11 77

3 关联映射 3.1 单向ManyToOne关联这是实体间最基本、最重要的关联，在数据库表间建立外键关系。理论上可以只使用ManyToOne关联而不用其他关联来实现整个应用；另一方向的关联可以通过查询(query)实现。 Employee类和Company类是ManyToOne的关系，例子如下： @Entity public class Company1 { @Id private int id; private String companyName;} @Entity public class Employee1 { @Id private int id; private String name; @ManyToOne(optional = false) private Company1 company;} 数据库表如下：create table " COMPANY1 " ( " ID " INTEGER not null , " COMPANYNAME " VARCHAR( 255 ), constraint " SYS_PK_1992 " primary key ( " ID " ));create unique index " SYS_PK_1992 " on " COMPANY1 " ( " ID " );create table " EMPLOYEE1 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), " COMPANY_ID " INTEGER not null , constraint " SYS_PK_2006 " primary key ( " ID " ));alter table " EMPLOYEE1 " add constraint " FK14AC0F233D3817C " foreign key ( " COMPANY_ID " ) references " COMPANY1 " ( " ID " );create unique index " SYS_PK_2006 " on " EMPLOYEE1 " ( " ID " );create index " SYS_IDX_2075 " on " EMPLOYEE1 " ( " COMPANY_ID " ); 3.2 单向OneToMany关联 OneToMany是涉及到集合的最重要的关联，可以采用Set、Collection和List来表示集合。 Set实现集合的例子如下： @Entity public class Employee2 { @Id private int id; private String name;} @Entity public class Company2 { @Id private int id; private String companyName; @OneToMany private Set<Employee2> employees = new HashSet(); } 数据库表如下：create table " EMPLOYEE2 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), constraint " SYS_PK_2007 " primary key ( " ID " ));create unique index " SYS_PK_2007 " on " EMPLOYEE2 " ( " ID " );create table " COMPANY2 " ( " ID " INTEGER not null , " COMPANYNAME " VARCHAR( 255 ), constraint " SYS_PK_1993 " primary key ( " ID " ));create unique index " SYS_PK_1993 " on " COMPANY2 " ( " ID " );create table " COMPANY2_EMPLOYEE2 " ( " COMPANY2_ID " INTEGER not null , " EMPLOYEES_ID " INTEGER not null , constraint " SYS_PK_1995 " primary key ( " COMPANY2_ID " , " EMPLOYEES_ID " ));alter table " COMPANY2_EMPLOYEE2 " add constraint " FK4BD4B2DA17FCDBA5 " foreign key ( " COMPANY2_ID " ) references " COMPANY2 " ( " ID " );alter table " COMPANY2_EMPLOYEE2 " add constraint " FK4BD4B2DA1F5EA2AE " foreign key ( " EMPLOYEES_ID " ) references " EMPLOYEE2 " ( " ID " );create unique index " SYS_IDX_SYS_CT_1994_1996 " on " COMPANY2_EMPLOYEE2 " ( " EMPLOYEES_ID " );create unique index " SYS_PK_1995 " on " COMPANY2_EMPLOYEE2 " ( " COMPANY2_ID " , " EMPLOYEES_ID " );create index " SYS_IDX_2065 " on " COMPANY2_EMPLOYEE2 " ( " COMPANY2_ID " );create index " SYS_IDX_2067 " on " COMPANY2_EMPLOYEE2 " ( " EMPLOYEES_ID " ); 采用Collection实现集合只是用Collection代替Set。如果采用List实现集合，集合中的元素包括位置信息，但是需要Hibernate特有的Annotation支持： @Entity public class Employee3 { @Id private int id; private String name;} @Entity public class Company3 { @Id private int id; private String companyName; @OneToMany @org.hibernate.annotations.IndexColumn(name = "EMPLOYEE_POSITION") private List<Employee3> employees = new ArrayList<Employee3>(); } 数据库表与上面的例子比，COMPANY3_EMPLOYEE3表中多了EMPLOYEE_POSITION字段。 3.3 双向ManyToOne关联如果需要实现双向关联，可以增加另一个方向的关联。 @Entity public class Employee4 { @Id private int id; private String name; @ManyToOne Company4 company;} @Entity public class Company4 { @Id private int id; private String name; @OneToMany(mappedBy="company") private Set<Employee4> employees = new HashSet(); } 数据库表如下：create table " COMPANY4 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), constraint " SYS_PK_2001 " primary key ( " ID " ));create unique index " SYS_PK_2001 " on " COMPANY4 " ( " ID " );create table " EMPLOYEE4 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), " COMPANY_ID " INTEGER, constraint " SYS_PK_2009 " primary key ( " ID " ));alter table " EMPLOYEE4 " add constraint " FK14AC0F263D3817F " foreign key ( " COMPANY_ID " ) references " COMPANY4 " ( " ID " );create unique index " SYS_PK_2009 " on " EMPLOYEE4 " ( " ID " );create index " SYS_IDX_2077 " on " EMPLOYEE4 " ( " COMPANY_ID " ); 需要注意的是，为了正确持久化，双方对象需要相互引用；mappedBy属性说明ManyToOne和OneToMany是镜像关系，如果没有该属性Hibernate可能会有不必要的更新。还有正确cascade属性，保证关联对象自动更新。对于可选的Many关系(Zero或Many关系)，如果不希望Employee类中的company字段可以是null，则可以通过@JoinTable采用中间表实现： @Entity public class Company5 { @Id private int id; private String companyName; @OneToMany(mappedBy = "company") private Set<Employee5> employees = new HashSet<Employee5>(); } @Entity public class Employee5 { @Id private int id; private String name; @ManyToOne @JoinTable(name = "COMPANY5_EMPLOYEE5", joinColumns = {@JoinColumn(name = "EMPLOYEE_ID")}, inverseJoinColumns = {@JoinColumn(name = "COMPANY_ID")}) private Company5 company; } 数据库表如下：create table " EMPLOYEE5 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), constraint " SYS_PK_2010 " primary key ( " ID " ));create unique index " SYS_PK_2010 " on " EMPLOYEE5 " ( " ID " );create table " COMPANY5 " ( " ID " INTEGER not null , " COMPANYNAME " VARCHAR( 255 ), constraint " SYS_PK_2002 " primary key ( " ID " ));create unique index " SYS_PK_2002 " on " COMPANY5 " ( " ID " );create table " COMPANY5_EMPLOYEE5 " ( " EMPLOYEE_ID " INTEGER not null , " COMPANY_ID " INTEGER, constraint " SYS_PK_1982 " primary key ( " EMPLOYEE_ID " ));alter table " COMPANY5_EMPLOYEE5 " add constraint " FKB3F547403D38180 " foreign key ( " COMPANY_ID " ) references " COMPANY5 " ( " ID " );alter table " COMPANY5_EMPLOYEE5 " add constraint " FKB3F54740BCD0EEE8 " foreign key ( " EMPLOYEE_ID " ) references " EMPLOYEE5 " ( " ID " );create unique index " SYS_PK_1982 " on " COMPANY5_EMPLOYEE5 " ( " EMPLOYEE_ID " );create index " SYS_IDX_2057 " on " COMPANY5_EMPLOYEE5 " ( " EMPLOYEE_ID " );create index " SYS_IDX_2059 " on " COMPANY5_EMPLOYEE5 " ( " COMPANY_ID " ); 3.4 单向OneToOne关联 OneToOne关联不是常用的关联。一般地如User类和Address类采用值类型映射；如果Address类还被其他类关联，则Address类需要实现为实体。这时候User类和Address类就是OneToOne关联。 @Entity public class Address6 { @Id private int id; private String street;} @Entity public class User6 { @Id private int id; private String name; @OneToOne private Address6 homeAddress; } 数据库表如下：create table " ADDRESS6 " ( " ID " INTEGER not null , " STREET " VARCHAR( 255 ), constraint " SYS_PK_1967 " primary key ( " ID " ));create unique index " SYS_PK_1967 " on " ADDRESS6 " ( " ID " );create table " USER6 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), " HOMEADDRESS_ID " INTEGER, constraint " SYS_PK_2026 " primary key ( " ID " ));alter table " USER6 " add constraint " FK4E39DAB932C1432 " foreign key ( " HOMEADDRESS_ID " ) references " ADDRESS6 " ( " ID " );create unique index " SYS_PK_2026 " on " USER6 " ( " ID " );create index " SYS_IDX_2085 " on " USER6 " ( " HOMEADDRESS_ID " ); OneToOne的一方如果是可选的，也就是说One to Zero或One。这种关联可以采用中间表方式实现，又可以分为JointTable和SecondaryTable方式。JoinTable方式例子如下： @Entity public class Address7 { @Id private int id; private String street;} @Entity public class User7 { @Id private int id; private String name; @OneToOne @JoinTable( name="User7_Address7", joinColumns = @JoinColumn(name = "User_ID"), inverseJoinColumns = @JoinColumn(name = "Address_ID") ) private Address7 homeAddress;} 数据库表如下：create table " ADDRESS7 " ( " ID " INTEGER not null , " STREET " VARCHAR( 255 ), constraint " SYS_PK_1968 " primary key ( " ID " ));create unique index " SYS_PK_1968 " on " ADDRESS7 " ( " ID " );create table " USER7 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), constraint " SYS_PK_2027 " primary key ( " ID " ));create unique index " SYS_PK_2027 " on " USER7 " ( " ID " );create table " USER7_ADDRESS7 " ( " USER_ID " INTEGER not null , " ADDRESS_ID " INTEGER, constraint " SYS_PK_2029 " primary key ( " USER_ID " ));alter table " USER7_ADDRESS7 " add constraint " FKB6D07C1661646A14 " foreign key ( " ADDRESS_ID " ) references " ADDRESS7 " ( " ID " );alter table " USER7_ADDRESS7 " add constraint " FKB6D07C16BC1189B0 " foreign key ( " USER_ID " ) references " USER7 " ( " ID " );create unique index " SYS_PK_2029 " on " USER7_ADDRESS7 " ( " USER_ID " );create index " SYS_IDX_2087 " on " USER7_ADDRESS7 " ( " USER_ID " );create index " SYS_IDX_2089 " on " USER7_ADDRESS7 " ( " ADDRESS_ID " ); SecondaryTable方式的例子如下： @Entity public class Address8 { @Id private int id; private String street;} @Entity@SecondaryTable(name = " User8_Address8 " ) public class User8 { @Id private int id; private String name; @OneToOne @JoinColumn(table = "User8_Address8", name = "Address_ID") private Address8 homeAddress; } 数据库表结构和上面的例子类似，不再重复。 3.5 双向OneToOne关联双向OneToOne关联采用外键关联，例子如下： public class Address9 { @Id private int id; private String street; @OneToOne(mappedBy="homeAddress") private User9 user;} @Entity public class User9 { @Id private int id; private String name; @OneToOne private Address9 homeAddress; } 数据库表如下：create table " ADDRESS9 " ( " ID " INTEGER not null , " STREET " VARCHAR( 255 ), constraint " SYS_PK_1970 " primary key ( " ID " ));create unique index " SYS_PK_1970 " on " ADDRESS9 " ( " ID " );create table " USER9 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), " HOMEADDRESS_ID " INTEGER, constraint " SYS_PK_2032 " primary key ( " ID " ));alter table " USER9 " add constraint " FK4E39DAE932C1435 " foreign key ( " HOMEADDRESS_ID " ) references " ADDRESS9 " ( " ID " );create unique index " SYS_PK_2032 " on " USER9 " ( " ID " );create index " SYS_IDX_2095 " on " USER9 " ( " HOMEADDRESS_ID " ); 3.6 单向ManyToMany关联单向ManyToMany采用中间表实现，包括Set、Collection和List三种方式，其中Set中没有重复元素，Collection可以包括重复元素，List可以记录元素的位置。Collection和List方式都要用到Hibernate特有的Annotation。Set的例子如下： @Entity public class Item1 { @Id private int id; private String description;} @Entity public class Category1 { @Id private int id; private String name; @ManyToMany @JoinTable( name = "CATEGORY1_ITEM1", joinColumns = {@JoinColumn(name = "CATEGORY_ID")}, inverseJoinColumns = {@JoinColumn(name = "ITEM_ID")} ) private Set<Item1> items = new HashSet<Item1>(); } 数据库表如下：create table " ITEM1 " ( " ID " INTEGER not null , " DESCRIPTION " VARCHAR( 255 ), constraint " SYS_PK_2011 " primary key ( " ID " ));create unique index " SYS_PK_2011 " on " ITEM1 " ( " ID " );create table " CATEGORY1 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), constraint " SYS_PK_1985 " primary key ( " ID " ));create unique index " SYS_PK_1985 " on " CATEGORY1 " ( " ID " );create table " CATEGORY1_ITEM1 " ( " CATEGORY_ID " INTEGER not null , " ITEM_ID " INTEGER not null , constraint " SYS_PK_1977 " primary key ( " CATEGORY_ID " , " ITEM_ID " ));alter table " CATEGORY1_ITEM1 " add constraint " FK43420332A218AA44 " foreign key ( " CATEGORY_ID " ) references " CATEGORY1 " ( " ID " );alter table " CATEGORY1_ITEM1 " add constraint " FK434203324250AF1A " foreign key ( " ITEM_ID " ) references " ITEM1 " ( " ID " );create unique index " SYS_PK_1977 " on " CATEGORY1_ITEM1 " ( " CATEGORY_ID " , " ITEM_ID " );create index " SYS_IDX_2035 " on " CATEGORY1_ITEM1 " ( " ITEM_ID " );create index " SYS_IDX_2037 " on " CATEGORY1_ITEM1 " ( " CATEGORY_ID " ); Collection的例子如下： @Entity public class Item2 { @Id private int id; private String description;} @Entity public class Category2 { @Id private int id; private String name; @ManyToMany @org.hibernate.annotations.CollectionId( columns = @Column(name = "CATEGORY_ITEM_ID"), type = @org.hibernate.annotations.Type(type = "long"), generator = "native") @JoinTable( name = "CATEGORY2_ITEM2", joinColumns = {@JoinColumn(name = "CATEGORY_ID")}, inverseJoinColumns = {@JoinColumn(name = "ITEM_ID")}) private Collection<Item2> items = new ArrayList<Item2>(); } 数据库与前面的例子类似，不再重复。 List的例子如下： @Entity public class Item3 { @Id private int id; private String description;} @Entity public class Category3 { @Id private int id; private String name; @ManyToMany @JoinTable( name = "CATEGORY3_ITEM3", joinColumns = {@JoinColumn(name = "CATEGORY_ID")}, inverseJoinColumns = {@JoinColumn(name = "ITEM_ID")}) @org.hibernate.annotations.IndexColumn(name = "ITEM_POSITION") private List<Item3> items = new ArrayList<Item3>(); } 数据库和上面的例子类似，不再给出。 3.7 双向ManyToMany关联双向ManyToMany关联最好采用两个OneToMany来实现。当然采用ManyToMany关联也是可以的。例子如下： @Entity public class Item4 { @Id private int id; private String description; @ManyToMany(mappedBy = "items") private Set<Category4> categories = new HashSet<Category4>(); } @Entity public class Category4 { @Id private int id; private String name; @ManyToMany @JoinTable( name = "CATEGORY4_ITEM4", joinColumns = {@JoinColumn(name = "CATEGORY_ID")}, inverseJoinColumns = {@JoinColumn(name = "ITEM_ID")} ) private Set<Item4> items = new HashSet<Item4>(); } 数据库表和前面的例子类似，不再给出。如果在双向关联中，还有其他字段信息，则可以采用值对象JoinTable或中间实体JoinTable实现。下面的例子假定在Category类和Item类关联时还需要增加日期信息。值对象JoinTable方式相对简单，但是需要用到Hibernate特有的Annotation，例子如下： @Entity public class Item5 { @Id private int id; private String description;} @Embeddable public class CategorizedItem5 { @org.hibernate.annotations.Parent // Optional back-pointer private Category5 category; @ManyToOne @JoinColumn(name = "ITEM_ID", nullable = false, updatable = false) private Item5 item; private Date dateAdded; public Category5 getCategory() { return category;} public void setCategory(Category5 category) { this.category = category; }} @Entity public class Category5 { @Id private int id; private String name; @org.hibernate.annotations.CollectionOfElements @JoinTable( name = "CategorizedItem5", joinColumns = @JoinColumn(name = "CATEGORY_ID")) private Set<CategorizedItem5> categorizedItems = new HashSet<CategorizedItem5>(); } 数据库表如下：create table " ITEM5 " ( " ID " INTEGER not null , " DESCRIPTION " VARCHAR( 255 ), constraint " SYS_PK_2015 " primary key ( " ID " ));create unique index " SYS_PK_2015 " on " ITEM5 " ( " ID " ); create table " CATEGORY5 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), constraint " SYS_PK_1989 " primary key ( " ID " ) );create unique index " SYS_PK_1989 " on " CATEGORY5 " ( " ID " );create table " CATEGORIZEDITEM5 " ( " CATEGORY_ID " INTEGER not null , " ITEM_ID " INTEGER not null , " DATEADDED " TIMESTAMP, constraint " SYS_PK_1983 " primary key ( " CATEGORY_ID " , " ITEM_ID " ));alter table " CATEGORIZEDITEM5 " add constraint " FKD6EE8B77A218AA48 " foreign key ( " CATEGORY_ID " ) references " CATEGORY5 " ( " ID " );alter table " CATEGORIZEDITEM5 " add constraint " FKD6EE8B774250AF1E " foreign key ( " ITEM_ID " ) references " ITEM5 " ( " ID " );create unique index " SYS_PK_1983 " on " CATEGORIZEDITEM5 " ( " CATEGORY_ID " , " ITEM_ID " );create index " SYS_IDX_2061 " on " CATEGORIZEDITEM5 " ( " ITEM_ID " );create index " SYS_IDX_2063 " on " CATEGORIZEDITEM5 " ( " CATEGORY_ID " ); 中间实体JoinTable方式的例子如下： @Entity public class Item6 { @Id private int id; private String description; @OneToMany(mappedBy = "item") private Set<CategorizedItem6> categorizedItems = new HashSet<CategorizedItem6>(); } @Entity public class Category6 { @Id private int id; private String name; @OneToMany(mappedBy = "category") private Set<CategorizedItem6> categorizedItems = new HashSet<CategorizedItem6>(); } @Entity public class CategorizedItem6 { @Embeddable public static class Id implements Serializable { @Column(name = "CATEGORY_ID") private Long categoryId; @Column(name = "ITEM_ID") private Long itemId; public Id() {} public Id(Long categoryId, Long itemId) { this.categoryId = categoryId; this.itemId = itemId; } public boolean equals(Object o) { if (o != null && o instanceof Id) { Id that = (Id)o; return this.categoryId.equals(that.categoryId) && this.itemId.equals(that.itemId); } else { return false; } } public int hashCode() { return categoryId.hashCode() + itemId.hashCode(); } } @EmbeddedId private Id id = new Id(); private Date dateAdded = new Date(); @ManyToOne @JoinColumn(name="ITEM_ID", insertable = false,updatable = false) private Item6 item; @ManyToOne @JoinColumn(name="CATEGORY_ID", insertable = false, updatable = false) private Category6 category; } 数据库表和上面的例子类似，不再给出。 3.8 三元关联三元关联（三个表的关联）可以采用Map实现。下面的例子建立Category类、Item类和User类的三元关联： @Entity public class Item7 { @Id private int id; private String description;} @Entity public class User7_3 { @Id private int id; private String name;} @Entity public class Category7 { @Id private int id; private String name; @ManyToMany @org.hibernate.annotations.MapKeyManyToMany( joinColumns = @JoinColumn(name = "ITEM_ID")) @JoinTable( name = "CATEGORY7_ITEM7", joinColumns = @JoinColumn(name = "CATEGORY_ID"), inverseJoinColumns = @JoinColumn(name = "USER_ID") ) private Map<Item7,User7_3> itemsAndUser = new HashMap<Item7,User7_3>(); } 数据库表如下：create table " ITEM7 " ( " ID " INTEGER not null , " DESCRIPTION " VARCHAR( 255 ), constraint " SYS_PK_2017 " primary key ( " ID " ));create unique index " SYS_PK_2017 " on " ITEM7 " ( " ID " );create table " USER7_3 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), constraint " SYS_PK_2028 " primary key ( " ID " ));create unique index " SYS_PK_2028 " on " USER7_3 " ( " ID " );create table " CATEGORY7 " ( " ID " INTEGER not null , " NAME " VARCHAR( 255 ), constraint " SYS_PK_1991 " primary key ( " ID " ));create unique index " SYS_PK_1991 " on " CATEGORY7 " ( " ID " );create table " CATEGORY7_ITEM7 " ( " CATEGORY_ID " INTEGER not null , " USER_ID " INTEGER not null , " ITEM_ID " INTEGER not null , constraint " SYS_PK_1981 " primary key ( " CATEGORY_ID " , " ITEM_ID " ));alter table " CATEGORY7_ITEM7 " add constraint " FK80A766BEA218AA4A " foreign key ( " CATEGORY_ID " ) references " CATEGORY7 " ( " ID " );alter table " CATEGORY7_ITEM7 " add constraint " FK80A766BE4250AF20 " foreign key ( " ITEM_ID " ) references " ITEM7 " ( " ID " );alter table " CATEGORY7_ITEM7 " add constraint " FK80A766BEE0D93124 " foreign key ( " USER_ID " ) references " USER7_3 " ( " ID " );create unique index " SYS_PK_1981 " on " CATEGORY7_ITEM7 " ( " CATEGORY_ID " , " ITEM_ID " );create index " SYS_IDX_2051 " on " CATEGORY7_ITEM7 " ( " USER_ID " );create index " SYS_IDX_2053 " on " CATEGORY7_ITEM7 " ( " ITEM_ID " );create index " SYS_IDX_2055 " on " CATEGORY7_ITEM7 " ( " CATEGORY_ID " ); 最后，正如UML类图只是描述系统的静态结构，这里小鸡射手仅描述了ORM映射部分。如果要编写高质量的ORM程序，还需要了解实体的生命期、事务和并发、Query、Fetch机制和缓存等动态特性。全文完。

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