Hibernate筆記(映射可嵌入組件)

組件

他們沒有自己的標(biāo)識符,其主鍵是它所屬實體擁有映射的數(shù)據(jù)庫標(biāo)識符,內(nèi)嵌的組件有獨立的生命周期:在保存所擁有的實體實例時.也會保存該組件實例,當(dāng)其所擁有實體實例被刪除時,也會刪除該組件實例,Hibernate甚至不必為此執(zhí)行任何特殊的SQL,所有數(shù)據(jù)都位于一行中.
　Java沒有組合的概念,類或?qū)傩圆荒鼙粯?biāo)記為組件或組合生命周期,與實體的唯一區(qū)別是數(shù)據(jù)庫標(biāo)識符:組件類沒有獨立的標(biāo)識;因此,組件類不需要標(biāo)識符屬性或標(biāo)識符映射,他是一個簡單的POJO.代碼:

@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  這里不需要注解
    private Address homeAddress;

    public Long getId() {
        return id;
    }


    public Address getHomeAddress() {
        return homeAddress;
    }

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

Hibernate會檢測Address類是否使用@Embeddable進(jìn)行了注解, street, zipcode,city列是在t_Users表上映射的,即所屬實體的表,
　在更為現(xiàn)實的場景中,一個用戶可能會將不同的地址用于不同的目的,例如User有家庭地址,還有賬單地址:

重寫嵌入屬性

billingAddress是User類的另一個嵌入組件屬性,所以必須在t_Users表中保存另一個address,這就造成了映射沖突,到目前為止,架構(gòu)中只有在street,zipcode,city.中存儲一個address列,需要額外的列來存儲用于每個User行的另一個Address.映射billingAddress時,要重寫列的名稱

@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;
    }
}

數(shù)據(jù)庫實體:t_Users

image.png

　@AttributeOverrides會選擇地重寫嵌入類的屬性映射;在這個示例中,重寫了全部3個屬性并且提供了不同的列名,現(xiàn)在可以在t_Users表中存儲兩個Address實例了,每個實例位于不同的列中.
用于組件
　用于組件屬性的每個@AttributeOverride都完成了:重寫的屬性上的任何JPA或Hibernate都會忽略,這意味著Address類上的@Column注解會被忽略-----所有的billing_*列都可為null,(不過 Bean驗證依舊會識別組件屬性上的@NotNull注解;Hibernate只會重寫持久化注解).

映射嵌套的可嵌入組件

考慮一下Address類以及它如何封裝地址詳情,不使用簡單的city字符串.而是將這一個詳情移動到一個新的City可嵌入類中,代碼如下:

City類

@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類

@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類

@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;
    }
}

數(shù)據(jù)庫實體:t_Users

image.png

　可以在任何級別聲明＠AttributeOverrides，正如City類的name屬性所做的那樣,將它映射到City列,這可以使用(所示的)Address中的@AttributeOverride或者根實體類User中重寫來實現(xiàn),嵌套屬性可以使用.符號來引用:例如,@AttributeOverride(name = "city.country",column = @Column(name = "billing_country"))引用了Address的#City#name屬性.

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