Hibernate笔记(映射可嵌入组件)

组件

他们没有自己的标识符,其主键是它所属实体拥有映射的数据库标识符,内嵌的组件有独立的生命周期:在保存所拥有的实体实例时.也会保存该组件实例,当其所拥有实体实例被删除时,也会删除该组件实例,Hibernate甚至不必为此执行任何特殊的SQL,所有数据都位于一行中.
　Java没有组合的概念,类或属性不能被标记为组件或组合生命周期,与实体的唯一区别是数据库标识符:组件类没有独立的标识;因此,组件类不需要标识符属性或标识符映射,他是一个简单的POJO.代码:

@Embeddable
public class Address {
    @NotNull
    @Column(nullable = false)
    private String street;

    @NotNull
    @Column(nullable = false, length = 5)
    private String zipcode;

    @NotNull
    @Column(nullable = false)
    private String city;

    public Address() {
    }

    public Address(String street, String zipcode, String city) {
        this.street = street;
        this.zipcode = zipcode;
        this.city = city;
    }

    public String getStreet() {
        return street;
    }

    public void setStreet(String street) {
        this.street = street;
    }

    public String getZipcode() {
        return zipcode;
    }

    public void setZipcode(String zipcode) {
        this.zipcode = zipcode;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }
}

使用方法:

@Entity
@Table(name = "t_Users")
public class User implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    //Address是@Embeddable  这里不需要注解
    private Address homeAddress;

    public Long getId() {
        return id;
    }


    public Address getHomeAddress() {
        return homeAddress;
    }

    public void setHomeAddress(Address homeAddress) {
        this.homeAddress = homeAddress;
    }
}

Hibernate会检测Address类是否使用@Embeddable进行了注解, street, zipcode,city列是在t_Users表上映射的,即所属实体的表,
　在更为现实的场景中,一个用户可能会将不同的地址用于不同的目的,例如User有家庭地址,还有账单地址:

重写嵌入属性

billingAddress是User类的另一个嵌入组件属性,所以必须在t_Users表中保存另一个address,这就造成了映射冲突,到目前为止,架构中只有在street,zipcode,city.中存储一个address列,需要额外的列来存储用于每个User行的另一个Address.映射billingAddress时,要重写列的名称

@Entity
@Table(name = "t_Users")
public class User implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;


    private Address homeAddress;
    @Embedded
    @AttributeOverrides({
            @AttributeOverride(name = "street",
                    column = @Column(name = "billing_street")),
            @AttributeOverride(name = "zipcode",
                    column = @Column(name = "billing_zipcode", length = 5)),
            @AttributeOverride(name = "city",
                    column = @Column(name = "billing_city"))
    })
    private Address billingAddress;

    public Address getBillingAddress() {
        return billingAddress;
    }

    public void setBillingAddress(Address billingAddress) {
        this.billingAddress = billingAddress;
    }

    public Long getId() {
        return id;
    }


    public Address getHomeAddress() {
        return homeAddress;
    }

    public void setHomeAddress(Address homeAddress) {
        this.homeAddress = homeAddress;
    }
}

数据库实体:t_Users

image.png

　@AttributeOverrides会选择地重写嵌入类的属性映射;在这个示例中,重写了全部3个属性并且提供了不同的列名,现在可以在t_Users表中存储两个Address实例了,每个实例位于不同的列中.
用于组件
　用于组件属性的每个@AttributeOverride都完成了:重写的属性上的任何JPA或Hibernate都会忽略,这意味着Address类上的@Column注解会被忽略-----所有的billing_*列都可为null,(不过 Bean验证依旧会识别组件属性上的@NotNull注解;Hibernate只会重写持久化注解).

映射嵌套的可嵌入组件

考虑一下Address类以及它如何封装地址详情,不使用简单的city字符串.而是将这一个详情移动到一个新的City可嵌入类中,代码如下:

City类

@Embeddable
public class City {
    @NotNull
    @Column(nullable = false, length = 5)
    private String zipcode;
    @NotNull
    @Column(nullable = false)
    private String name;

    @NotNull
    @Column(nullable = false)
    private String country;

    public City() {
    }

    public City(String zipcode, String name, String country) {
        this.zipcode = zipcode;
        this.name = name;
        this.country = country;
    }

    public String getZipcode() {
        return zipcode;
    }

    public void setZipcode(String zipcode) {
        this.zipcode = zipcode;
    }

    public String getName() {
        return name;
    }

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

    public String getCountry() {
        return country;
    }

    public void setCountry(String country) {
        this.country = country;
    }
}

Address类

@Embeddable
public class Address {
    @NotNull
    @Column(nullable = false)
    private String street;


    @NotNull
    @AttributeOverrides(
            @AttributeOverride(name = "name",
                    column = @Column(name = "city", nullable = false))
    )
    private City city;

    public Address() {
    }

    public Address(String street, City city) {
        this.street = street;
        this.city = city;
    }

    public String getStreet() {
        return street;
    }

    public void setStreet(String street) {
        this.street = street;
    }


    public City getCity() {
        return city;
    }

    public void setCity(City city) {
        this.city = city;
    }


}

User类

@Entity
@Table(name = "t_Users")
public class User implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;


    private Address homeAddress;
    @Embedded
    @AttributeOverrides({
            @AttributeOverride(name = "street",
                    column = @Column(name = "billing_street")),
            @AttributeOverride(name = "city.zipcode",
                    column = @Column(name = "billing_zipcode", length = 5)),
            @AttributeOverride(name = "city.name",
                    column = @Column(name = "billing_city")),
            @AttributeOverride(name = "city.country",
                    column = @Column(name = "billing_country"))
    })
    private Address billingAddress;

    public Address getBillingAddress() {
        return billingAddress;
    }

    public void setBillingAddress(Address billingAddress) {
        this.billingAddress = billingAddress;
    }

    public Long getId() {
        return id;
    }


    public Address getHomeAddress() {
        return homeAddress;
    }

    public void setHomeAddress(Address homeAddress) {
        this.homeAddress = homeAddress;
    }
}

数据库实体:t_Users

image.png

　可以在任何级别声明＠AttributeOverrides，正如City类的name属性所做的那样,将它映射到City列,这可以使用(所示的)Address中的@AttributeOverride或者根实体类User中重写来实现,嵌套属性可以使用.符号来引用:例如,@AttributeOverride(name = "city.country",column = @Column(name = "billing_country"))引用了Address的#City#name属性.

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