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EJB3.0开发指南之多对多和一对一


  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 = "DOSSIER_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.java
  
  package 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 Set<Teacher> teachers;
  
  @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)
  {
  this.last = last;
  }
  
  public String getLast()
  {
  return last;
  }
  
  public void setDossier(Dossier dossier)
  {
  this.dossier = dossier;
  }
  
  @OneToOne(cascade = {CascadeType.ALL})
  @JoinColumn(name = "DOSSIER_ID")
  
  public Dossier getDossier()
  {
  return dossier;
  }
  
  public void setTeacher(Set<Teacher> 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<Teacher> getTeacher()
  {
  return teachers;
  }
  }
  
  
  Dossier.java
  
  package com.kuaff.ejb3.relationships;
  
  import javax.ejb.Entity;
  import javax.ejb.GeneratorType;
  import javax.ejb.Id;
  
  @Entity
  
  public 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.java
  
  package 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;
  
  @Entity
  
  public class Teacher implements java.io.Serializable
  {
  private Long id;
  private String resume;
  private String name;
  private String info;
  private Set<Student> students;
  
  @Id(generate = GeneratorType.IDENTITY)
  
  public Long getId()
  {
  return id;
  }
  
  public void setId(Long id)
  {
  this.id = id;
  }
  
  public void setName(String name)
  {
  this.name = name;
  }
  
  public String getName()
  {
  return name;
  }
  
  public void setInfo(String info)
  {
  this.info = info;
  }
  
  public String getInfo()
  {
  return info;
  }
  
  public void setStudents(Set<Student> 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<Student> getStudents()
  {
  return students;
  }
  }
  
  EntityTest.java
  
  package com.kuaff.ejb3.relationships;
  
  import javax.ejb.Remote;
  import java.util.List;
  
  @Remote
  
  public interface EntityTest
  {
  public void createData();
  public List findByName(String name);
  }
  
  EntityTestBean.java
  
  package 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.
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