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                現在大家都不推薦使用聯合主鍵,關鍵是因為其需要自己手工維護,比較麻煩。但是一個項目可能因為歷史遺留原因,你不得不面對聯合主鍵。? Hibernate聯合主鍵問題解決如下: 可以使用一個組件作為一個實體類的標識符。你的組件類必須滿足以下要求:? (1)它必須實現 java.io.Serializable 接口? (2)它必須重新實現 equals() 和 hashCode() 方法,始終和組合關鍵字在數據庫中的概念保持一致? 注意:在 Hibernate3 中,第二個要求并非是 Hibernate 強制必須的。但最好這樣做。? 不能使用一個 IdentifierGenerator 產生組合關鍵字。一個應用程序必須分配它自己的標識符。 將主鍵所對應屬性提取出一個類(稱之為主鍵類),并且主鍵類需要實現Serializable接口,重寫equals方法與hashCode方法,原因: 在于Hibernate要根據數據庫的聯合主鍵來判斷某兩行記錄是否是一樣,如果一樣那么就認為是同一個對象,如果不一樣,那么就認為是不同的對象。 ### 一 .XML配置方式 具體做法如下步驟: (1)將主鍵所對應屬性提取出一個類(稱之為主鍵類),并且主鍵類需要實現Serializable接口,重寫equals方法與hashCode方法? 實例如下: ~~~ package com.model; import java.io.Serializable; //ScoreInfo主鍵類 //composite-id class must implement Serializable: com.model.ScorePK public class ScorePK implements Serializable{ private static final long serialVersionUID = 1L; private Integer studentID; private Integer courseID; public int getStudentID() { return studentID; } public void setStudentID(int studentID) { this.studentID = studentID; } public int getCourseID() { return courseID; } public void setCourseID(int courseID) { this.courseID = courseID; } @Override public boolean equals(Object o){ if(o instanceof ScorePK){ ScorePK scorePK = (ScorePK)o; if(this.studentID == scorePK.studentID && this.courseID == scorePK.courseID){ return true; } } return false; } @Override public int hashCode(){ int result = 1; result = result + ((this.studentID == null) ? 0 : this.studentID.hashCode()); result = result + ((this.courseID == null) ? 0 : this.courseID.hashCode()); return result; } } ~~~ 這里要特別注意,如hibernate API文檔所述,主鍵類必須實現java.io.Serializable接口,而且重寫equals和hashCode方法,保證主鍵的唯一性. (2)實體類(這里不用包含主鍵所對應的屬性) ~~~ package com.model; public class ScoreInfo { //聯合主鍵 private ScorePK scorePK; private int score; public ScorePK getScorePK() { return scorePK; } public void setScorePK(ScorePK scorePK) { this.scorePK = scorePK; } public int getScore() { return score; } public void setScore(int score) { this.score = score; } } ~~~ (3)配置Hibernate配置文件ScoreInfo.hbm.xml,實例如下: ~~~ <?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name="com.model.ScoreInfo" table="ScoreInfo"> <composite-id name = "ScorePK" class = "com.model.ScorePK"> <key-property name="StudentID"></key-property> <key-property name="CourseID"></key-property> </composite-id> <property name="score" column = "Score"/> </class> </hibernate-mapping> ~~~ 如果表使用聯合主鍵,你可以映射類的多個屬性為標識符屬性。 <composite-id>元素接受<key-property> 屬性映射和<key-many-to-one>屬性映射作為子元素。 ~~~ <composite-id name = "ScorePK" class = "com.model.ScorePK"> <key-property name="StudentID"></key-property> <key-property name="CourseID"></key-property> </composite-id> ~~~ 這個聯合主鍵就是由StudentID和CourseID構成。 (4)不要忘記在hibernate配置文件中引用映射文件。 ~~~ <!-- XML配置方式 --> <mapping resource="com/model/ScoreInfo.hbm.xml"/> ~~~ (5)測試 ~~~ package com.test; import org.hibernate.Session; import com.model.ScoreInfo; import com.model.ScorePK; import com.util.HibernateUtil; public class Test { public static void main(String[] args) { Test mgr = new Test(); mgr.createAndStoreEvent(); HibernateUtil.getSessionFactory().close(); } private void createAndStoreEvent() { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction(); ScorePK scorePK = new ScorePK(); scorePK.setStudentID(2); scorePK.setCourseID(1); ScoreInfo score = new ScoreInfo(); score.setScorePK(scorePK); score.setScore(98); session.save(score); session.getTransaction().commit(); } } ~~~ 輔助類:HibernateUtil ~~~ package com.util; import org.hibernate.SessionFactory; import org.hibernate.cfg.Configuration; public class HibernateUtil { private static final SessionFactory sessionFactory; static { try { // Create the SessionFactory from hibernate.cfg.xml sessionFactory = new Configuration().configure().buildSessionFactory(); } catch (Throwable ex) { // Make sure you log the exception, as it might be swallowed System.err.println("Initial SessionFactory creation failed." + ex); throw new ExceptionInInitializerError(ex); } } public static SessionFactory getSessionFactory() { return sessionFactory; } } ~~~ ### 二 .Annotation注解方式 下面是定義聯合主鍵的幾種方式: (1)將組件類(主鍵類)注解為@Embeddable,并在實體類中將組件的屬性注解為@Id (2)在實體類中將組件的屬性注解為@EmbeddedId (3)將實體類注解為@IdClass(value = ScorePk.class),并將該實體中所有屬于主鍵的屬性都注解為@Id 對于EJB2的開發人員來說@IdClass是很常見的,但是對于Hibernate用戶來說就是一個嶄新的用法。 組合主鍵類對應一個實體類的多個字段或屬性,而且主鍵類中定義主鍵的字段或屬性和實體類中對應的字段或屬性早類型上必須一致。 實例如下: (1)將組件類(主鍵類)注解為@Embeddable ~~~ package com.model; import java.io.Serializable; import javax.persistence.Embeddable; //ScoreInfo主鍵類 //composite-id class must implement Serializable: com.model.ScorePK @Embeddable public class ScorePK implements Serializable{ private static final long serialVersionUID = 1L; private Integer studentID; private Integer courseID; public int getStudentID() { return studentID; } public void setStudentID(int studentID) { this.studentID = studentID; } public int getCourseID() { return courseID; } public void setCourseID(int courseID) { this.courseID = courseID; } @Override public boolean equals(Object o){ if(o instanceof ScorePK){ ScorePK scorePK = (ScorePK)o; if(this.studentID == scorePK.studentID && this.courseID == scorePK.courseID){ return true; } } return false; } @Override public int hashCode(){ int result = 1; result = result + ((this.studentID == null) ? 0 : this.studentID.hashCode()); result = result + ((this.courseID == null) ? 0 : this.courseID.hashCode()); return result; } } ~~~ 將在實體類中組件的屬性注解為@Id ~~~ public class ScoreInfo { private int studentID; private ScorePK scorePK; @Id public ScorePK getScorePK() { return scorePK; } public void setScorePK(ScorePK scorePK) { this.scorePK = scorePK; } public int getScore() { return score; } public void setScore(int score) { this.score = score; } } ~~~ (2)在實體類中將組件的屬性注解為@EmbeddedId ~~~ public class ScoreInfo { private int studentID; private ScorePK scorePK; @EmbeddedId public ScorePK getScorePK() { return scorePK; } public void setScorePK(ScorePK scorePK) { this.scorePK = scorePK; } public int getScore() { return score; } public void setScore(int score) { this.score = score; } } ~~~ (3)將實體類注解為@IdClass(value = ScorePk.class),并將該實體中所有屬于主鍵的屬性都注解為@Id ~~~ @Entity @IdClass(value = ScorePk.class) public class ScoreInfo { private int studentID; private int courseID; private int score; @Id public int getStudentID() { return studentID; } public void setStudentID(int studentID) { this.studentID = studentID; } @Id public int getCourseID() { return courseID; } public void setCourseID(int courseID) { this.courseID = courseID; } public int getScore() { return score; } public void setScore(int score) { this.score = score; } } ~~~
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