hibernate注解方式实现复合主键

有时一个实体的主键可能同时为多个，例如同样是之前使用的“CustomerEO”实体，需要通过name和email来查找指定实体，当且仅当name和email的值完全相同时，才认为是相同的实体对象。要配置这样的复合主键，步骤如以下所示。（1）编写一个复合主键的类CustomerPK，代码如下。

package com.petrochina.hibernate.compose;

import java.io.Serializable;

public class CustomerPK implements Serializable {
/**
*
*/
private static final long serialVersionUID = 1L;
private String email;
private String name;

public CustomerPK() {
super();
// TODO Auto-generated constructor stub
}

public CustomerPK(String email, String name) {
super();
this.email = email;
this.name = name;
}

public String getId() {
return email;
}

public void setId(String email) {
this.email = email;
}

public String getName() {
return name;
}

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

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((email == null) ? 0 : email.hashCode());
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
CustomerPK other = (CustomerPK) obj;
if (email == null) {
if (other.email != null)
return false;
} else if (!email.equals(other.email))
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}

<span style="font-family: Verdana, Geneva, Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">作为符合主键类，要满足以下几点要求。</span>

zl 必须实现Serializable接口。l 必须有默认的public无参数的构造方法。l 必须覆盖equals和hashCode方法。equals方法用于判断两个对象是否相同，EntityManger通过find方法来查找Entity时，是根据equals的返回值来判断的。本例中，只有对象的name和email值完全相同时或同一个对象时则返回true，否则返回false。hashCode方法返回当前对象的哈希码，生成的hashCode相同的概率越小越好，算法可以进行优化。（2）通过@IdClass注释在实体中标注复合主键，实体代码如下。

package com.petrochina.hibernate.compose;import javax.persistence.Entity;import javax.persistence.Id;import javax.persistence.IdClass;import javax.persistence.Table;@Entity@Table(name = "customer")@IdClass(CustomerPK.class)public class Customer {private String id;@Idprivate String name;@Idprivate String email;public Customer() {super();// TODO Auto-generated constructor stub}public Customer(String id, String name, String email) {super();this.id = id;this.name = name;this.email = email;}public String getId() {return id;}public void setId(String id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getEmail() {return email;}public void setEmail(String email) {this.email = email;}}

标注复合主键时需要注意以下几个问题。l @IdClass标注用于标注实体所使用主键规则的类。它的定义如下所示。@Target({TYPE}) @Retention(RUNTIME)public @interface IdClass {Class value();}属性Class表示符合主键所使用的类，本例中使用CustomerPK这个复合主键类。l 在实体中同时标注主键的属性。本例中在email和name的getter方法前标注@Id，表示符合主键使用这两个属性。（3）这样定义实体的复合主键后，通过以下代码便可以获得指定的实体对象：CustomerPK cpk = new CustomerPK("Janet","janetvsfei@yahoo.com.cn");CustomerEO instance = entityManager.find(CustomerEO.class, cpk);本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/EJB_JPA/archive/2008/05/09/2422540.aspx