1>.学过hibernate的都知道用<id>来配置主键生成策略，显然，它只能配置主键是单列的表，对于联合主键的情况<id>就无能为力了。今天翻到个复合主键映射的资料，觉得还是有一定价值的，就写了下来，以备后用。
    2>.hibernate处理复合主键映射是基于这样一种做法：把目标类中所有主键关联属性抽出来，单独写成一个类(我暂且叫它主键类)，目标类就只需持有主键类对象，而不必再包含各个主键属性；在映射文件中使用<composite-id>标签来配置主键对象并指出关联属性，普通属性照常配置；hibernate只会创建一张表在，并且把主键设置为主键类各属性的联合主键，存储和加载数据时，会自然的把关联对象各属性跟表中的主键字段对应起来。在这个操作过程中，它要求主键类必须是序列化的，并且要覆盖equals方法，最好覆盖hashCode方法。(我试验过了，不覆盖equals和hashCode，结果没有出现任何问题，而且从输出的sql语句看，也没有影响数据库操作，不知道它是否还有更深的含义？还希望高手给指点下)
    3>.例子。一个规模较大公司的部门表(hibernate_dept_compositePK)，由所在区域(area)，部门名(name)，本部门人数(empCount),组建时间(birthday)等字段组成，我们使用所在区域和部门名做联合主键：

1.目标类：Department.java

public class Department { 
  /** 把主键关联属性抽象出来单独写成一个类 */ 
  //private String area; 
  //private String name; 
  /**把主键类对象作为成员变量*/ 
  private DepartmentPK departmentPK; 
  private int empCount; 
  private Date birthday;

//  public String getArea() { 
//    return area; 
//  } 
// 
//  public void setArea(String area) { 
//    this.area = area; 
//  } 
// 
//  public String getName() { 
//    return name; 
//  } 
// 
//  public void setName(String name) { 
//    this.name = name; 
//  } 
    
  public int getEmpCount() { 
    return empCount; 
  }

public void setEmpCount(int empCount) { 
    this.empCount = empCount; 
  }

public Date getBirthday() { 
    return birthday; 
  }

public void setBirthday(Date birthday) { 
    this.birthday = birthday; 
  }

public DepartmentPK getDepartmentPK() { 
    return departmentPK; 
  }

public void setDepartmentPK(DepartmentPK departmentPK) { 
    this.departmentPK = departmentPK; 
  }

}

2.主键类：DepartmentPK.java

public class DepartmentPK implements Serializable {

private static final long serialVersionUID = -288002855915204255L; 
  private String area; 
  private String name; 
  /** 
    * 覆盖hashCode方法(根据area和name判断) 
    */ 
  //@Override 
  public int hashCode() { 
    final int prime = 31; 
    int result = 1; 
    result = prime * result + ((area == null) ? 0 : area.hashCode()); 
    result = prime * result + ((name == null) ? 0 : name.hashCode()); 
    return result; 
  } 
  /** 
    * 覆盖equals(根据area和name判断) 
    */ 
  @Override 
  public boolean equals(Object obj) { 
    if (this == obj) 
      return true; 
    if (obj == null) 
      return false; 
    if (getClass() != obj.getClass()) 
      return false; 
    final DepartmentPK other = (DepartmentPK) obj; 
    if (area == null) { 
      if (other.area != null) 
        return false; 
    } else if (!area.equals(other.area)) 
      return false; 
    if (name == null) { 
      if (other.name != null) 
        return false; 
    } else if (!name.equals(other.name)) 
      return false; 
    return true; 
  }

public String getArea() { 
    return area; 
  }

public void setArea(String area) { 
    this.area = area; 
  }

public String getName() { 
    return name; 
  }

public void setName(String name) { 
    this.name = name; 
  } 
}

3.映射文件Department.hbm.xml

<?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.yangfei.hibernate.compositePk.entity.Department" table="hibernate_dept_compositePK"> 
    <!-- 联合主键 --> 
    <!-- name指的是主键对象属性 --> 
    <composite-id name="departmentPK"> 
      <!-- 这里是主键关联属性 --> 
      <key-property name="area" /> 
      <key-property name="name" /> 
    </composite-id> 
    <!-- 其它属性 --> 
    <property name="empCount" length="4" /> 
    <property name="birthday" type="date" /> 
  </class> 
</hibernate-mapping>

4.hibernate配置文件hibernate.cfg.xml

<?xml version='1.0' encoding='UTF-8'?> 
<!DOCTYPE hibernate-configuration PUBLIC 
                    "-//Hibernate/Hibernate Configuration DTD 3.0//EN" 
                    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<!-- Generated by MyEclipse Hibernate Tools.                                     --> 
<hibernate-configuration> 
        <session-factory> 
                <property name="dialect">org.hibernate.dialect.Oracle9Dialect</property> 
                <property name="connection.url">jdbc:oracle:thin:@127.0.0.1:1521:orcl10</property> 
                <property name="connection.username">scott</property> 
                <property name="connection.password">yf123</property> 
                <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property> 
                <property name="hibernate.show_sql">true</property> 
            
          <mapping resource="com/yangfei/hibernate/compositePk/entity/Department.hbm.xml"/> 
        </session-factory> 
</hibernate-configuration>

5.测试类：DepartmentTest.java

public class DepartmentTest extends TestCase { 
  /** 
    * 测试插入数据 
    */ 
  public void save() { 
    Session session = HibernateUtils.getSession(); 
    Transaction t = session.beginTransaction(); 
    try { 
      Department dept = new Department(); 
      /** 生成主键对象 */ 
      DepartmentPK deptPK = new DepartmentPK(); 
      deptPK.setArea("北京"); 
      deptPK.setName("研发部"); 
      dept.setDepartmentPK(deptPK);

dept.setEmpCount(100); 
      dept.setBirthday(new Date());

session.save(dept); 
      t.commit(); 
    } catch (HibernateException e) { 
      e.printStackTrace(); 
      t.rollback(); 
    } finally { 
      HibernateUtils.closeSession(session); 
    } 
  } 
  /** 
    * 测试加载数据 
    */ 
  public void load() { 
    Session session = HibernateUtils.getSession(); 
    Transaction t = session.beginTransaction(); 
    try { 
      /** 生成主键对象 */ 
      DepartmentPK deptPK = new DepartmentPK(); 
      deptPK.setArea("北京"); 
      deptPK.setName("研发部");

Department dept=(Department)session.load(Department.class, deptPK); 
      System.out.println(dept.getDepartmentPK().getArea()+","+dept.getDepartmentPK().getName()+","+dept.getEmpCount()+","+dept.getBirthday()); 
    } catch (HibernateException e) { 
      e.printStackTrace(); 
      t.rollback(); 
    } finally { 
      HibernateUtils.closeSession(session); 
    } 
  } 
    
  /** 
    * 测试修改数据 
    */ 
  public void update() { 
    Session session = HibernateUtils.getSession(); 
    Transaction t = session.beginTransaction(); 
    try { 
      /** 生成主键对象 */ 
      DepartmentPK deptPK = new DepartmentPK(); 
      deptPK.setArea("北京"); 
      deptPK.setName("研发部");

Department emp=(Department)session.load(Department.class, deptPK); 
      System.out.println(emp.getDepartmentPK().getArea()+","+emp.getDepartmentPK().getName()+","+emp.getEmpCount()+","+emp.getBirthday()); 
      emp.setEmpCount(100); 
      session.saveOrUpdate(emp); 
        
      /** 生成主键对象 */ 
      DepartmentPK deptPK2 = new DepartmentPK(); 
      deptPK2.setArea("北京"); 
      deptPK2.setName("研发部"); 
      Department dept=(Department)session.load(Department.class, deptPK2); 
      System.out.println(dept.getDepartmentPK().getArea()+","+dept.getDepartmentPK().getName()+","+dept.getEmpCount()+","+dept.getBirthday()); 
      t.commit(); 
    } catch (HibernateException e) { 
      e.printStackTrace(); 
      t.rollback(); 
    } finally { 
      HibernateUtils.closeSession(session); 
    } 
  } 
    
  /** 
    * 测试删除数据 
    */ 
  public void delete() { 
    Session session = HibernateUtils.getSession(); 
    Transaction t = session.beginTransaction(); 
    try { 
      /** 生成主键对象 */ 
      DepartmentPK deptPK = new DepartmentPK(); 
      deptPK.setArea("北京"); 
      deptPK.setName("研发部"); 
        
      Department dept=(Department)session.load(Department.class, deptPK); 
      session.delete(dept); 
      t.commit(); 
    } catch (HibernateException e) { 
      e.printStackTrace(); 
      t.rollback(); 
    } finally { 
      HibernateUtils.closeSession(session); 
    } 
  } 
}

4.一般来说，复合主键映射用起来是很复杂的，无论是开发时还是升级时。所以，人们往往宁愿选择增加个主键字段，也不尽量不采用它。

     本文转自NightWolves 51CTO博客，原文链接：http://blog.51cto.com/yangfei520/310555，如需转载请自行联系原作者