Hibernate中用到联合主键的使用方法，为何要序列化，为何要重写hashcode 和 equals 方法...

联合主键用Hibernate注解映射方式主要有三种：

第一、将联合主键的字段单独放在一个类中，该类需要实现java.io.Serializable接口并重写equals和hascode，再将该类注解为@Embeddable,最后在主类中(该类不包含联合主键类中的字段)保存该联合主键类的一个引用，并生成set和get方法，并将该引用注解为@Id

package com.test.entity;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Embeddable;

@Embeddable
public class TestPk implements Serializable{
   private static final long serialVersionUID = 1L;
   @Column(name="id")
   private int id;
   @Column(name="sid")
   private String sid;

   public int getId() {
       return id;
   }
   public void setId(int id) {
       this.id = id;
   }
   public String getSid() {
       return sid;
   }
   public void setSid(String sid) {
       this.sid = sid;
   }
}

package com.test.entity;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;
@Table(name="test")
@Entity
public class Test1 {
   @Column(name="date")
   private String date;
   @Id
   private TestPk testPk;
   public String getDate() {
       return date;
   }
   public void setDate(String date) {
       this.date = date;
   }
   public TestPk getTestPk() {
       return testPk;
   }
   public void setTestPk(TestPk testPk) {
       this.testPk = testPk;
   }


}

第二、将联合主键的字段单独放在一个类中，该类需要实现java.io.Serializable接口并重写equals和hascode，最后在主类中(该类不包含联合主键类中的字段)保存该联合主键类的一个引用，并生成set和get方法，并将该引用注解为@EmbeddedId

package com.test.entity;

import java.io.Serializable;

import javax.persistence.Column;

public class TestPk implements Serializable{
   private static final long serialVersionUID = 1L;
   @Column(name="id")
   private int id;
   @Column(name="sid")
   private String sid;

   public int getId() {
       return id;
   }
   public void setId(int id) {
       this.id = id;
   }
   public String getSid() {
       return sid;
   }
   public void setSid(String sid) {
       this.sid = sid;
   }
}
package com.test.entity;
import java.util.Date;

import javax.persistence.Column;
import javax.persistence.EmbeddedId;
import javax.persistence.Entity;
import javax.persistence.Table;
@Table(name="test")
@Entity
public class Test1 {
   @Column(name="date")
   private Date date;
   @EmbeddedId
   private TestPk testPk;
   public Date getDate() {
       return date;
   }
   public void setDate(Date date) {
       this.date = date;
   }
   public TestPk getTestPk() {
       return testPk;
   }
   public void setTestPk(TestPk testPk) {
       this.testPk = testPk;
   }


}

第三、将联合主键的字段单独放在一个类中，该类需要实现java.io.Serializable接口并要重写equals和hashcode.最后在主类中(该类包含联合主键类中的字段)将联合主键字段都注解为@Id,并在该类上方将上这样的注解：@IdClass(联合主键类.class)

package com.test.entity;

import java.io.Serializable;

import javax.persistence.Column;

public class TestPk implements Serializable{
   private static final long serialVersionUID = 1L;
   @Column(name="id")
   private int id;
   @Column(name="sid")
   private String sid;

   public int getId() {
       return id;
   }
   public void setId(int id) {
       this.id = id;
   }
   public String getSid() {
       return sid;
   }
   public void setSid(String sid) {
       this.sid = sid;
   }
}
package com.test.entity;
import java.util.Date;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.IdClass;
import javax.persistence.Table;
@Table(name="test")
@IdClass(TestPk.class)
@Entity
public class Test1 {
   public int getId() {
       return id;
   }
   public void setId(int id) {
       this.id = id;
   }
   public String getSid() {
       return sid;
   }
   public void setSid(String sid) {
       this.sid = sid;
   }
   @Column(name="date")
   private Date date;
   @Id
   private int id;
   @Id
   private String sid;
   public Date getDate() {
       return date;
   }
   public void setDate(Date date) {
       this.date = date;
   }



}