复合主键@IdClass
有時一個實體的主鍵可能同時為多個，例如同樣是之前使用的“CustomerEO”實體，需要通過name和email來查找指定實體，當且僅當name和email的值完全相同時，才認為是相同的實體對象。要配置這樣的復合主鍵，步驟如以下所示。
（1）編寫一個復合主鍵的類CustomerPK，代碼如下。
CustomerPK.java
import java.io.Serializable;
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public class CustomerPK?implements Serializable?{
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?????????public CustomerPK() {
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?????????}
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?????????public CustomerPK(String name, String email) {
???????????????????this.name = name;
???????????????????this.email = email;
?????????}
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?????????private String email;
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?????????public String getEmail() {
???????????????????return email;
?????????}
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?????????public void setEmail(String email) {
???????????????????this.email = email;
?????????}
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?????????private String name;
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?????????public String getName() {
???????????????????return name;
?????????}
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?????????public void setName(String name) {
???????????????????this.name = name;
?????????}
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?????????@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;
?????????}
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?????????@Override
?????????public boolean equals(Object obj) {
???????????????????if (this == obj)
????????????????????????????return true;
???????????????????if (obj == null)
????????????????????????????return false;
???????????????????if (getClass() != obj.getClass())
????????????????????????????return false;
???????????????????final 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;
?????????}
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}
作為符合主鍵類，要滿足以下幾點要求。
l?????????必須實現Serializable接口。
l?????????必須有默認的public無參數的構造方法。
l?????????必須覆蓋equals和hashCode方法。equals方法用于判斷兩個對象是否相同，EntityManger通過find方法來查找Entity時，是根據equals的返回值來判斷的。本例中，只有對象的name和email值完全相同時或同一個對象時則返回true，否則返回false。hashCode方法返回當前對象的哈希碼，生成的hashCode相同的概率越小越好，算法可以進行優化。
（2）通過@IdClass注釋在實體中標注復合主鍵，實體代碼如下。
@Entity
@Table(name = "customer")
@IdClass(CustomerPK.class)
public class CustomerEO implements java.io.Serializable {
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?????????private Integer id;
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?????????public Integer getId() {
???????????????????return this.id;
?????????}
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?????????public void setId(Integer id) {
???????????????????this.id = id;
?????????}
????????
?????????private String name;
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@Id
?????????public String getName() {
???????????????????return this.name;
?????????}
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?????????public void setName(String name) {
???????????????????this.name = name;
?????????}
????????
?????????private String email;
@Id
?????????public String getEmail() {
???????????????????return email;
?????????}
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?????????public void setEmail(String email) {
???????????????????this.email = email;
?????????}
}
標注復合主鍵時需要注意以下幾個問題。
l?????????@IdClass標注用于標注實體所使用主鍵規則的類。它的定義如下所示。
@Target({TYPE}) @Retention(RUNTIME)
public @interface IdClass {
Class value();
}
屬性Class表示符合主鍵所使用的類，本例中使用CustomerPK這個復合主鍵類。
l?????????在實體中同時標注主鍵的屬性。本例中在email和name的getter方法前標注@Id，表示符合主鍵使用這兩個屬性。
（3）這樣定義實體的復合主鍵后，通過以下代碼便可以獲得指定的實體對象：
????CustomerPK cpk = new CustomerPK("Janet","janetvsfei@yahoo.com.cn");
????CustomerEO instance = entityManager.find(CustomerEO.class, cpk);
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