EJB3.0开发之多对多和一对一

EJB3.0开发之多对多和一对一在前面的例子中，我们演示了一对多和多对一的例子，在本章将演示多对多和一对一的关系。

学生和老师就是多对多的关系。

一个学生有多个老师，一个老师教多个学生。

学生和档案就是一对一的关系（不知道国外的学生有没有档案？）。

为了实现多对多的关系，数据库中需要关联表，用以在两个实体间建立关联。

JBoss可以自动生成关联表，你也可以@AssociationTable来指定关联表的信息。

如:@ManyToMany(cascade = {CascadeType.CREATE, CascadeType.MERGE}, fetch = FetchType.EAGER, isInverse = true)@AssociationTable(table = @Table(name = "STUDENT_TEACHER"),joinColumns = {@JoinColumn(name = "TEACHER_ID")},inverseJoinColumns = {@JoinColumn(name = "STUDENT_ID")})@ AssociationTable的注释声明如下：@Target({METHOD, FIELD})public @interface AssociationTable {Table table() default @Table(specified=false);JoinColumn[] joinColumns() default {};JoinColumn[] inverseJoinColumns() default {};}关联表注释指定了关联表的名称、主表的列和从表的列。

为了实现一对一的关系，需要用@OneToOne来注释。

如:@OneToOne(cascade = {CascadeType.ALL})@JoinColumn(name = "DOS SIER_ID")public Dossier getDossier(){return dossier;}这定义了一个单向的一对一的关系。

如果在Dossier也定义了相关的关联，那么它就是双向的。

双向的意思就是通过一个Student实体就可以查找到一个Dossier，通过一个Dossier 就可以查找到一个Student。

@ OneToOne的注释声明如下：@Target({METHOD, FIELD}) @Retention(RUNTIME)public @interface OneToOne {String targetEntity() default "";CascadeType[] cascade() default {};FetchType fetch() default EAGER;boolean optional() default true;}这个例子主要有以下几个文件，这个例子主要实现了学生和老师、学生和档案之间的关系。

Student、Teacher、Dossier都是实体Bean。

Student和Dossier是一个双向的OneToOne 之间的关系，Student和Teacher是ManyToMany的关系，也是双向的。

和前面的例子一样，我们还是使用Client测试。

Student.java：实体Bean。

Dossier.java：实体Bean所依赖的类。

Teacher.java：实体Bean所依赖的类。

EntityTest.java：会话Bean的业务接口EntityTest Bean.java：会话Bean的实现类Client.java：测试EJB的客户端类。

jndi.properties：jndi属性文件，提供访问jdni的基本配置属性。

Build.xml：ant 配置文件，用以编译、发布、测试、清除EJB。

下面针对每个文件的内容做一个介绍。

Student.javapackage com.kuaff.ejb3.relationships;import javax.ejb.CascadeType;import javax.ejb.Entity;import javax.ejb.FetchType;import javax.ejb.GeneratorType;import javax.ejb.Id;import javax.ejb.JoinColumn;import javax.ejb.OneToOne;import javax.ejb.ManyToMany;import javax.ejb.Table;import javax.ejb.AssociationTable;import java.util.ArrayList;import java.util.Set;import java.util.Collection;import java.io.Serializable;@Entity@Table(name = "STUDENT")public class Student implements Serializable{private int id;private String first;private String last;private Dossier dossier;private Setteachers;@Id(generate = GeneratorType.AUTO)public int getId(){return id;}public void setId(int id){this.id = id;}public void setFirst(String first){this.first = first;}public String getFirst(){return first;}public void setLast(String last){st = last;}public String getLast(){return last;}public void setDossier(Dossier dossier){this.dossier = dossier;}@OneToOne(cascade = {CascadeType.ALL}) @JoinColumn(name = "DOS SIER_ID")public Dossier getDossier(){return dossier;}public void setTeacher(Set teachers)this.teachers = teachers;}@ManyToMany(cascade = {CascadeType.CREATE, CascadeType.MERGE}, fetch = FetchType.EAGER, isInverse = true)@AssociationTable(table = @Table(name = "STUDENT_TEACHER"),joinColumns = {@JoinColumn(name = "TEACHER_ID")},inverseJoinColumns = {@JoinColumn(name = "STUDENT_ID")})public Set getTeacher(){return teachers;}}Dossier.javapackage com.kuaff.ejb3.relationships;import javax.ejb.Entity;import javax.ejb.GeneratorType;import javax.ejb.Id;@Entitypublic class Dossier implements java.io.Serializable{private Long id;private String resume;@Id(generate = GeneratorType.AUTO)public Long getId(){return id;}public void setId(Long id){this.id = id;}public void setResume(String resume)this.resume = resume;}public String getResume(){return resume;}}Teacher.javapackage com.kuaff.ejb3.relationships;import javax.ejb.AssociationTable;import javax.ejb.Basic;import javax.ejb.CascadeType;import javax.ejb.Column;import javax.ejb.Entity;import javax.ejb.FetchType;import javax.ejb.Id;import javax.ejb.JoinColumn;import javax.ejb.ManyToMany;import javax.ejb.Table;import javax.ejb.Transient;import javax.ejb.Version;import java.util.Set;import javax.ejb.GeneratorType;@Entitypublic class Teacher implements java.io.Serializable {private Long id;private String resume;private String name;private String info;private Set students;@Id(generate = GeneratorType.IDENTITY)public Long getId(){return id;public void setId(Long id){this.id = id;}public void setName(String name){ = name;}public String getName(){return name;}public void setInfo(String info){ = info;}public String getInfo(){return info;}public void setStudents(Set students){this.students = students;}@ManyToMany(cascade = {CascadeType.CREATE, CascadeType.MERGE}, fetch = FetchType.EAGER)@AssociationTable(table = @Table(name = "STUDENT_TEACHER"),joinColumns = {@JoinColumn(name = "TEACHER_ID",referencedColumnName="ID")},inverseJoinColumns = {@JoinColumn(name = "STUDENT_ID",referencedColumnName="ID")})public Set getStudents(){return students;}EntityTest.javapackage com.kuaff.ejb3.relationships;import javax.ejb.Remote;import java.util.List;@Remotepublic interface EntityTest{public void createData();public List findByName(String name);}EntityTestBean.javapackage com.kuaff.ejb3.relationships;import javax.ejb.EntityManager;import javax.ejb.Inject;import javax.ejb.Stateless;import java.util.HashSet;import java.util.Set;import java.util.List;@Statelesspublic class EntityTestBean implements EntityTest {private @Inject EntityManager manager;public void createData(){Teacher teacher1 = new Teacher();Teacher teacher2 = new Teacher();Set students1 = new HashSet();Set students2 = new HashSet();Student student1 = new Student();Student student2 = new Student();Student student3 = new Student();Dossier dossier1 = new Dossier();Dossier dossier2 = new Dossier();Dossier dossier3 = new Dossier();teacher1.setId(new Long(1));teacher1.setName("hushisheng");teacher1.setInfo("胡时胜教授，博士生导师");manager.create(teacher1);teacher2.setId(new Long(2));teacher2.setName("liyongchi");teacher2.setInfo("李永池教授，博士生导师");manager.create(teacher2);student1.setFirst("晁");student1.setLast("岳攀");dossier1.setResume("这是晁岳攀的档案");student1.setDossier(dossier1);students1.add(student1);student2.setFirst("赵");student2.setLast("志伟");dossier2.setResume("这是赵志伟的档案");student2.setDossier(dossier2);students1.add(student2);student3.setFirst("田");student3.setLast("明");dossier3.setResume("这是田明的档案");student3.setDossier(dossier3);students2.add(student3);teacher1.setStudents(students1);teacher2.setStudents(students2);}public List findByName(String name){return manager.createQuery("from Teacher t where = :name").setParameter("name", name).listResults();}}在这个会话Bean中提供了创建各个实体Bean的方法，并提供了查找老师的方法。