one-to-one雙向關聯之載入

關於hibernate的載入方式有兩種，一種是get載入，一種是load載入，load屬於消極式載入，使用了動態代理。這不是我們所關心的，我們看一看一對一雙向關聯在載入時有什麼特點，想要觀察特點，不得不看hibernate為我們產生的sql語句。

husband類與上一篇沒有變化如下：

/** *  */package com.maybe.test_1;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.OneToOne;import javax.persistence.Table;/** * @author MayBe * * function: */@Entity@Table(name="t_husband")public class Husband {private Integer id;private String name;private Wife wife;@Id@GeneratedValuepublic Integer getId() {return id;}public String getName() {return name;}public void setId(Integer id) {this.id = id;}public void setName(String name) {this.name = name;}@OneToOne(mappedBy="husband")public Wife getWife() {return wife;}public void setWife(Wife wife) {this.wife = wife;}}

wife類也沒有變化：

/** *  */package com.maybe.test_1;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.OneToOne;import javax.persistence.Table;/** * @author MayBe * * function: */@Entity@Table(name="t_wife")public class Wife {private Integer id;private String name;private Husband husband;@Id@GeneratedValuepublic Integer getId() {return id;}public String getName() {return name;}public void setId(Integer id) {this.id = id;}public void setName(String name) {this.name = name;}@OneToOnepublic Husband getHusband() {return husband;}public void setHusband(Husband husband) {this.husband = husband;}}

junit測試方法如下：

@Testpublic void One_to_oneLoadTest1(){//一對一雙向關聯Session s = sessionFacotry.getCurrentSession();s.beginTransaction();Husband hus = (Husband)s.get(Husband.class, 2);                System.out.println("*****************");Wife wif = (Wife)s.get(Wife.class, 3);s.getTransaction().commit();}

假設我們在資料中存在一個id為2的husband，他與id為2的wife是關聯的。在資料庫有一個id為3的wife，她與id為3的husband是關聯的，我們運行一下這個測試，輸出sql語句如下：

2014-02-04 16:52:20 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update   2014-02-04 16:52:20 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:193 - HHH000102: Fetching database metadata   2014-02-04 16:52:20 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:205 - HHH000396: Updating schema   2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_husband   2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, name]   2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: []   2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_husban__3213e83f0f624af8]   2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_wife   2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, husband_id, name]   2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: [fk_fi3kodkmubgryyblf4935y4dk]   2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_wife__3213e83f1332dbdc]   2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:242 - HHH000232: Schema update complete   Hibernate:     select        husband0_.id as id1_0_0_,        husband0_.name as name2_0_0_,        wife1_.id as id1_1_1_,        wife1_.husband_id as husband_3_1_1_,        wife1_.name as name2_1_1_     from        t_husband husband0_     left outer join        t_wife wife1_             on husband0_.id=wife1_.husband_id     where        husband0_.id=?*****************Hibernate:     select        wife0_.id as id1_1_0_,        wife0_.husband_id as husband_3_1_0_,        wife0_.name as name2_1_0_,        husband1_.id as id1_0_1_,        husband1_.name as name2_0_1_     from        t_wife wife0_     left outer join        t_husband husband1_             on wife0_.husband_id=husband1_.id     where        wife0_.id=?Hibernate:     select        wife0_.id as id1_1_1_,        wife0_.husband_id as husband_3_1_1_,        wife0_.name as name2_1_1_,        husband1_.id as id1_0_0_,        husband1_.name as name2_0_0_     from        t_wife wife0_     left outer join        t_husband husband1_             on wife0_.husband_id=husband1_.id     where        wife0_.husband_id=?

首先載入husband對象時會把相關聯的欄位全部載入出來。

但是可以看出來載入wife表的時候會執行兩次查詢，雖然這兩次查詢的內容是一樣的，但是第一次是查詢出的是主表的資訊，第二次是為了查出從表資訊，這是hibernate的預設載入策略，因為t_wife表有外鍵，如果載入的表有從表，他會預設載入同時夾在兩個表的資訊。是否載入從表資訊是由fetch進行設定的。

我們把wife類one-to-one新添加一個屬性

/** *  */package com.maybe.test_1;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.OneToOne;import javax.persistence.Table;/** * @author MayBe * * function: */@Entity@Table(name="t_wife")public class Wife {private Integer id;private String name;private Husband husband;@Id@GeneratedValuepublic Integer getId() {return id;}public String getName() {return name;}public void setId(Integer id) {this.id = id;}public void setName(String name) {this.name = name;}@OneToOne(fetch=FetchType.LAZY)public Husband getHusband() {return husband;}public void setHusband(Husband husband) {this.husband = husband;}}

我們把抓取方式改成懶抓取，載入輸出如下所示：

2014-02-04 17:07:38 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
   2014-02-04 17:07:38 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:193 - HHH000102: Fetching database metadata
   2014-02-04 17:07:38 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:205 - HHH000396: Updating schema
   2014-02-04 17:07:39 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_husband
   2014-02-04 17:07:39 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, name]
   2014-02-04 17:07:39 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: []
   2014-02-04 17:07:39 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_husban__3213e83f0f624af8]
   2014-02-04 17:07:40 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_wife
   2014-02-04 17:07:40 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, husband_id, name]
   2014-02-04 17:07:40 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: [fk_fi3kodkmubgryyblf4935y4dk]
   2014-02-04 17:07:40 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_wife__3213e83f1332dbdc]
   2014-02-04 17:07:40 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:242 - HHH000232: Schema update complete
   Hibernate:
    select
        husband0_.id as id1_0_0_,
        husband0_.name as name2_0_0_,
        wife1_.id as id1_1_1_,
        wife1_.husband_id as husband_3_1_1_,
        wife1_.name as name2_1_1_
    from
        t_husband husband0_
    left outer join
        t_wife wife1_
            on husband0_.id=wife1_.husband_id
    where
        husband0_.id=?
*****************
Hibernate:
    select
        wife0_.id as id1_1_0_,
        wife0_.husband_id as husband_3_1_0_,
        wife0_.name as name2_1_0_
    from
        t_wife wife0_
    where
        wife0_.id=?

這樣wife類只載入自己主表的資訊，不考慮從表的資訊了( 當然把husband也是如此，可以自己去試試)。

hibernate預設的載入策略為eager載入，也就是急抓取，會把所關聯的資訊一次性全部得到，而lazy則都是在你用到從表才會去載入。

我們改變一下測試類別內容：

public void One_to_oneLoadTest1() {// 一對一雙向關聯Session s = sessionFacotry.getCurrentSession();s.beginTransaction();Husband hus = (Husband) s.get(Husband.class, 2);    System.out.println("*****************");Wife wif = (Wife) s.get(Wife.class, 3);System.out.println(wif.getHusband().getName());s.getTransaction().commit();}

輸出如下：

2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:193 - HHH000102: Fetching database metadata
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:205 - HHH000396: Updating schema
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_husband
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, name]
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: []
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_husban__3213e83f0f624af8]
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_wife
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, husband_id, name]
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: [fk_fi3kodkmubgryyblf4935y4dk]
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_wife__3213e83f1332dbdc]
   2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:242 - HHH000232: Schema update complete
   Hibernate:
    select
        husband0_.id as id1_0_0_,
        husband0_.name as name2_0_0_,
        wife1_.id as id1_1_1_,
        wife1_.husband_id as husband_3_1_1_,
        wife1_.name as name2_1_1_
    from
        t_husband husband0_
    left outer join
        t_wife wife1_
            on husband0_.id=wife1_.husband_id
    where
        husband0_.id=?
*****************
Hibernate:
    select
        wife0_.id as id1_1_0_,
        wife0_.husband_id as husband_3_1_0_,
        wife0_.name as name2_1_0_
    from
        t_wife wife0_
    where
        wife0_.id=?
Hibernate:
    select
        husband0_.id as id1_0_0_,
        husband0_.name as name2_0_0_,
        wife1_.id as id1_1_1_,
        wife1_.husband_id as husband_3_1_1_,
        wife1_.name as name2_1_1_
    from
        t_husband husband0_
    left outer join
        t_wife wife1_
            on husband0_.id=wife1_.husband_id
    where
        husband0_.id=?
Hibernate:
    select
        wife0_.id as id1_1_0_,
        wife0_.husband_id as husband_3_1_0_,
        wife0_.name as name2_1_0_
    from
        t_wife wife0_
    where
        wife0_.husband_id=?
GossipMan

這樣才會載入從表的資訊，對於一些其他的語句，可能是hibernate自動產生的其他語句，但只要理解其中的思想就可以了，不要在意這些細節。