Hibernate註解聯合主鍵 如何使用

解答：Hibernate的註解方式可以用來定義聯合主鍵，使用@Embeddable和@EmbeddedId註解。

一、@Embeddable和@EmbeddedId註解

在Hibernate註解中，使用@Embeddable註解的類中可以嵌入其他屬性，而這些屬性可以被@EmbeddedId註解的類所使用。@Embeddable註解的類中的getter和setter方法可以直接被使用@EmbeddedId註解的類所訪問和修改。

示例代碼：

@Embeddable
public class UserId implements Serializable {

    private String firstName;

    private String lastName;

    public UserId() {}

    public String getFirstName() { return firstName; }
    public void setFirstName(String firstName) { this.firstName = firstName; }

    public String getLastName() { return lastName; }
    public void setLastName(String lastName) { this.lastName = lastName; }
}

@Entity
public class User {

    @EmbeddedId
    private UserId id;

    private String email;
    private String password;

    // getters and setters ...

}

二、自定義主鍵生成策略

在@EmbeddedId註解的類中，可以自定義主鍵生成策略，支持的策略包括IDENTITY、SEQUENCE、TABLE、AUTO、NONE等。在@Id註解中也可以定義主鍵生成策略。

示例代碼：

@Entity
@IdClass(CompositePrimaryKey.class)
public class User {

    @Id
    private String firstName;

    @Id
    private String lastName;

    private String email;
    private String password;

    // getters and setters ...
}

public class CompositePrimaryKey implements Serializable {

    private String firstName;
    private String lastName;

    public CompositePrimaryKey() {}

    public CompositePrimaryKey(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    // getters and setters ...
}

@Entity
@TableGenerator(
    name = "user_gen",
    table = "id_gen",
    pkColumnName = "gen_name",
    valueColumnName = "gen_val",
    allocationSize = 100
)
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "user_gen")
    private long id;

    @EmbeddedId
    private UserIdentity userIdentity;

    // other fields
}

三、複合主鍵的繼承關係

在複合主鍵的繼承關係中，子類需要重新定義父類的主鍵屬性，並在@EmbeddedId註解中定義子類的主鍵屬性。

示例代碼：

@Entity
public class Vehicle {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    @Embedded
    private VehicleRegistration registration;

    // getters and setters ...
}

@Embeddable
public class VehicleRegistration {

    private String registrationNumber;
    private String registrationCountry;

    // getters and setters ...
}

@Entity
@AttributeOverrides({
    @AttributeOverride(name = "registrationNumber", column = @Column(name = "bike_reg_no")),
    @AttributeOverride(name = "registrationCountry", column = @Column(name = "bike_reg_country"))
})
public class Bike extends Vehicle {

    @EmbeddedId
    private VehicleRegistration registration;

    // getters and setters ...
}

@Entity
@AttributeOverrides({
    @AttributeOverride(name = "registrationNumber", column = @Column(name = "car_reg_no")),
    @AttributeOverride(name = "registrationCountry", column = @Column(name = "car_reg_country"))
})
public class Car extends Vehicle {

    @EmbeddedId
    private VehicleRegistration registration;

    // getters and setters ...
}

以上就是關於Hibernate註解聯合主鍵的詳細闡述，其中包括了@Embeddable和@EmbeddedId註解、自定義主鍵生成策略以及複合主鍵的繼承關係。掌握了這些知識，您將能夠更好地使用Hibernate註解方式定義聯合主鍵。