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EJB3.0之映射（轉(zhuǎn)）

一對(duì)一映射

雙向一對(duì)一關(guān)系需要在關(guān)系維護(hù)端（owner side）的one2one Annotition定義mappedBy屬性。建表時(shí)在關(guān)系被維護(hù)端（inverse side）建立外鍵列指向關(guān)系維護(hù)端的主鍵列。

假設(shè)Country 和 Capital 是雙向一對(duì)一的關(guān)系，具體元數(shù)據(jù)聲明如下:

				
public class Country {
	@OneToOne(optional = true,cascade = CascadeType.ALL, mappedBy = "country")
	private Capital capital;
}

public class Capital {
	@OneToOne(optional = false, cascade = CascadeType.ALL)
	@JoinColumn(name = "COUNTRY_ID", referencedColumnName = "id")
	private Country country;
                
			

代碼中元數(shù)據(jù)的說(shuō)明：

元數(shù)據(jù)描述:

@OneToOne(optional = true,cascade = CascadeType.ALL, mappedBy = "country")

optional聲明關(guān)系是否是必須存在的，即是否允許其中一端為null。

cascade聲明級(jí)聯(lián)操作。

@JoinColumn(name = "COUNTRY_ID", referencedColumnName = "id")

name聲明外鍵列的名字，referencedColumnName聲明外鍵指向列的列名。

一對(duì)多映射

雙向一對(duì)多關(guān)系，一是關(guān)系維護(hù)端（owner side），多是關(guān)系被維護(hù)端（inverse side）。 建表時(shí)在關(guān)系被維護(hù)端建立外鍵列指向關(guān)系維護(hù)端的主鍵列。

假設(shè)Father 和 Child 是雙向一對(duì)多的關(guān)系，具體元數(shù)據(jù)聲明如下:

				
public class Father {
	@OneToMany(targetEntity = Child.class, cascade = CascadeType.ALL, mappedBy = "father")
	public List<Child> getChildren() {
		return children;
	}
}

public class Child {
	@ManyToOne
	@JoinColumn(name = "FATHER_ID", referencedColumnName = "id")
	public Father getFather() {
		return father;
	}
}

                
			

代碼中元數(shù)據(jù)的說(shuō)明：

元數(shù)據(jù)描述:

@OneToMany(targetEntity = Child.class, cascade = CascadeType.ALL, mappedBy = "father")

targetEntity = Child.class表明關(guān)系另一端的實(shí)體類(lèi)型

cascade聲明級(jí)聯(lián)操作。

mappedBy聲明關(guān)系維護(hù)端的字段（field）名。

@ManyToOne

@JoinColumn(name = "FATHER_ID", referencedColumnName = "id")

name聲明外鍵列的名字，referencedColumnName聲明外鍵指向列的列名。

多對(duì)多映射

多對(duì)多映射采取中間表連接的映射策略，建立的中間表將分別引入兩邊的主鍵作為外鍵。

EJB3對(duì)于中間表的元數(shù)據(jù)提供了可配置的方式，用戶可以自定義中間表的表名，列名。

假設(shè)Teacher 和 Student是多對(duì)多的關(guān)系，具體元數(shù)據(jù)聲明如下:

				
pubic class Teacher{
	@ManyToMany(targetEntity = Student.class, cascade = CascadeType.PERSIST)
	@JoinTable(table = @Table(name = "M2M_TEACHER_STUDENT"),
        joinColumns = @JoinColumn(name = "TEACHER_ID", referencedColumnName = "ID"),
        inverseJoinColumns = @JoinColumn(name = "STUDENT_ID", referencedColumnName = "ID"))
	public List<Student> getStudents() {return students;}
					
}
public class Student{
	@ManyToMany(targetEntity = Teacher.class, mappedBy = "students")
	public List<Teacher> getTeachers() {
		return teachers;
	}
}
			
			

代碼中元數(shù)據(jù)的說(shuō)明：

元數(shù)據(jù)描述:

@ManyToMany(targetEntity = Student.class, cascade = CascadeType.PERSIST)

targetEntity = Student.class表明關(guān)系另一端的實(shí)體類(lèi)型。cascade聲明級(jí)聯(lián)操作。

@JoinTable(table = @Table(name = "M2M_TEACHER_STUDENT"),

joinColumns = @JoinColumn(name = "TEACHER_ID", referencedColumnName = "ID"),

inverseJoinColumns = @JoinColumn(name = "STUDENT_ID", referencedColumnName = "ID"))

JoinTable配置中間表信息，它由3個(gè)部分組成:

1) table = @Table(name = "M2M_TEACHER_STUDENT") ,聲明中間表的名字

2） joinColumns ，定義中間表與關(guān)系維護(hù)端的外鍵關(guān)系。

3） inverseJoinColumns，定義中間表與inverse端的外鍵關(guān)系.

				
@Entity
@Table( name="inheritance_Employee" )
@Inheritance(strategy=InheritanceType.SINGLE_TABLE,
		discriminatorType=DiscriminatorType.STRING,
		discriminatorValue="employee")
public class Employee {...}
					
@Entity
@Inheritance(discriminatorValue="fullTimeEmp")
public class FullTimeEmployee extends Employee {...}

@Entity
@Inheritance(discriminatorValue="partTimeEmp")
public class PartTimeEmployee extends Employee {...}
                
			

posted on 2006-07-03 00:52 liaojiyong 閱讀(734) 評(píng)論(0)  編輯  收藏 所屬分類(lèi): EJB