实体关系之@ManyToMany

来源：互联网发布：开淘宝需要营业执照吗编辑：程序博客网时间：2024/05/01 01:57

转自http://lym6520.iteye.com/blog/314065

Student.java

package com.entity;    import java.io.Serializable;  import java.util.HashSet;  import java.util.Set;    import javax.persistence.Column;  import javax.persistence.Entity;  import javax.persistence.GeneratedValue;  import javax.persistence.Id;  import javax.persistence.ManyToMany;  import javax.persistence.Table;    @Entity  @Table(name = "Student")  public class Student implements Serializable {            private Integer studentid;//学生ID      private String studentName;//学生姓名      private Set<Teacher> teachers = new HashSet<Teacher>();//对应的教师集合        public Student() {      }        public Student(String studentName) {          this.studentName = studentName;      }        @Id      @GeneratedValue      public Integer getStudentid() {          return studentid;      }        public void setStudentid(Integer studentid) {          this.studentid = studentid;      }        @Column(nullable = false, length = 32)      public String getStudentName() {          return studentName;      }        public void setStudentName(String studentName) {          this.studentName = studentName;      }            /*      * @ManyToMany 注释表示Student是多对多关系的一边，mappedBy 属性定义了Student 为双向关系的维护端      */      @ManyToMany(mappedBy = "students")      public Set<Teacher> getTeachers() {          return teachers;      }        public void setTeachers(Set<Teacher> teachers) {          this.teachers = teachers;      }  }

Teacher.java

package com.entity;    import java.io.Serializable;  import java.util.HashSet;  import java.util.Set;    import javax.persistence.CascadeType;  import javax.persistence.Column;  import javax.persistence.Entity;  import javax.persistence.FetchType;  import javax.persistence.GeneratedValue;  import javax.persistence.Id;  import javax.persistence.JoinTable;  import javax.persistence.ManyToMany;  import javax.persistence.Table;  import javax.persistence.JoinColumn;    @Entity  @Table  public class Teacher implements Serializable {        private Integer teacherid;// 教师ID      private String teacherName;// 教师姓名      private Set<Student> students = new HashSet<Student>();// 对应的学生集合        public Teacher() {        }        public Teacher(String teacherName) {          this.teacherName = teacherName;      }        @Id      @GeneratedValue      public Integer getTeacherid() {          return teacherid;      }        public void setTeacherid(Integer teacherid) {          this.teacherid = teacherid;      }        @Column(nullable = false, length = 32)      public String getTeacherName() {          return teacherName;      }        public void setTeacherName(String teacherName) {          this.teacherName = teacherName;      }      /*      * @ManyToMany 注释表示Teacher 是多对多关系的一端。      * @JoinTable 描述了多对多关系的数据表关系，name属性指定中间表名称。      * joinColumns 定义中间表与Teacher 表的外键关系，中间表Teacher_Student的Teacher_ID 列是Teacher 表的主键列对应的外键列。      * inverseJoinColumns 属性定义了中间表与另外一端(Student)的外键关系。      */      @ManyToMany(cascade = CascadeType.PERSIST, fetch = FetchType.LAZY)      @JoinTable(name = "Teacher_Student",               joinColumns ={@JoinColumn(name = "teacher_ID", referencedColumnName = "teacherid") },               inverseJoinColumns = { @JoinColumn(name = "student_ID", referencedColumnName = "studentid")       })      public Set<Student> getStudents() {          return students;      }        public void setStudents(Set<Student> students) {          this.students = students;      }            public void addStudent(Student student) {          if (!this.students.contains(student)) {//检测在该散列表中某些键是否映射到指定值,value 查找的值。如果某些键映射到该散列表中的值为true，否则false              this.students.add(student);          }      }        public void removeStudent(Student student) {          this.students.remove(student);      }    }

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