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Hibernate多对多(many-to-many):在操作和性能方面都不太理想,所以多对多的关系使用的比较少,实际使用最好转换成一对多的对象模型,实际上Hibernate可以帮我们创建中间关联表,转换成两个一对多的关系.

比如老师和学生是比较常见的多对多关系,我们可以创建一个中间表,用中间表去记录老师和学生之间多对多的关系.

Teacher.java

@Entity
@Table(name = "TB_TEACHER")
public class Teacher {private int teacherId;private String teacherName;private Set<Student> students = new HashSet<Student>();public Teacher(){}public Teacher(String teacherName){this.teacherName = teacherName;}@Id@GeneratedValue(strategy = GenerationType.AUTO)@Column(name = "TEACHER_ID", unique = true, nullable = false)public int getTeacherId() {return teacherId;}public void setTeacherId(int teacherId) {this.teacherId = teacherId;}@Column(name = "TEACHER_NAME")public String getTeacherName() {return teacherName;}public void setTeacherName(String teacherName) {this.teacherName = teacherName;}@ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)@JoinTable(name = "TB_TEACHER_STUDENT", joinColumns = {@JoinColumn(name = "TEACHER_ID", nullable = false, updatable = false) }, inverseJoinColumns = {@JoinColumn(name = "STUDENT_ID", nullable = false, updatable = false) })public Set<Student> getStudents() {return students;}public void setStudents(Set<Student> students) {this.students = students;}}

Student.java

@Entity
@Table(name = "TB_STUDENT")
public class Student {private int studentId;private String studentName;private Set<Teacher> teachers = new HashSet<Teacher>();public Student(){}public Student(String studentName){this.studentName = studentName;}@Id@GeneratedValue(strategy = GenerationType.AUTO)@Column(name = "STUDENT_ID", unique = true, nullable = false)public int getStudentId() {return studentId;}public void setStudentId(int studentId) {this.studentId = studentId;}@Column(name = "STUDENT_NAME")public String getStudentName() {return studentName;}public void setStudentName(String studentName) {this.studentName = studentName;}@ManyToMany(fetch = FetchType.LAZY, mappedBy = "students")public Set<Teacher> getTeachers() {return teachers;}public void setTeachers(Set<Teacher> teachers) {this.teachers = teachers;}}

通过设置Hibernate自动生成代码

hibernate.show_sql = true
hibernate.format_sql = true
hibernate.hbm2ddl.auto = create

自动生成的DDL SQL语句

CREATE TABLE `tb_teacher` (`TEACHER_ID` INT(11) NOT NULL AUTO_INCREMENT,`TEACHER_NAME` VARCHAR(255) NULL DEFAULT NULL,PRIMARY KEY (`TEACHER_ID`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB;CREATE TABLE `tb_student` (`STUDENT_ID` INT(11) NOT NULL AUTO_INCREMENT,`STUDENT_NAME` VARCHAR(255) NULL DEFAULT NULL,PRIMARY KEY (`STUDENT_ID`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB;CREATE TABLE `tb_teacher_student` (`TEACHER_ID` INT(11) NOT NULL,`STUDENT_ID` INT(11) NOT NULL,PRIMARY KEY (`TEACHER_ID`, `STUDENT_ID`),INDEX `FK_9484qoyn1n7mq08lbv4q2wni0` (`STUDENT_ID`),CONSTRAINT `FK_9484qoyn1n7mq08lbv4q2wni0` FOREIGN KEY (`STUDENT_ID`) REFERENCES `tb_student` (`STUDENT_ID`),CONSTRAINT `FK_l1jwjq45c5vucjgivby1e41a5` FOREIGN KEY (`TEACHER_ID`) REFERENCES `tb_teacher` (`TEACHER_ID`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB;

A. 测试插入操作:

Teacher teacher1 = new Teacher("teacher1");
Teacher teacher2 = new Teacher("teacher2");
Student student1 = new Student("student1");
Student student2 = new Student("student2");student1.getTeachers().add(teacher1);
student1.getTeachers().add(teacher2);
student2.getTeachers().add(teacher1);session.save(teacher1);
session.save(teacher2);
session.save(student1);
session.save(student2);

用上面的语句执行插入操作,后台console打印出来的SQL:

Hibernate: insert intoTB_TEACHER(TEACHER_NAME) values(?)
Hibernate: insert intoTB_TEACHER(TEACHER_NAME) values(?)
Hibernate: insert intoTB_STUDENT(STUDENT_NAME) values(?)
Hibernate: insert intoTB_STUDENT(STUDENT_NAME) values(?)

数据库中数据

TB_STUDENT

TB_TEACHER

TB_TEACHER_STUDENT (NULL)

B.测试插入操作:

Teacher teacher1 = new Teacher("teacher1");
Teacher teacher2 = new Teacher("teacher2");
Student student1 = new Student("student1");
Student student2 = new Student("student2");teacher1.getStudents().add(student1);
teacher1.getStudents().add(student2);
teacher2.getStudents().add(student1);session.save(teacher1);
session.save(teacher2);
session.save(student1);
session.save(student2);

用上面的语句执行插入操作,后台console打印出来的SQL:

Hibernate: insert intoTB_TEACHER(TEACHER_NAME) values(?)
Hibernate: insert intoTB_STUDENT(STUDENT_NAME) values(?)
Hibernate: insert intoTB_STUDENT(STUDENT_NAME) values(?)
Hibernate: insert intoTB_TEACHER(TEACHER_NAME) values(?)
Hibernate: insert intoTB_TEACHER_STUDENT(TEACHER_ID, STUDENT_ID) values(?, ?)
Hibernate: insert intoTB_TEACHER_STUDENT(TEACHER_ID, STUDENT_ID) values(?, ?)
Hibernate: insert intoTB_TEACHER_STUDENT(TEACHER_ID, STUDENT_ID) values(?, ?)

数据库中数据

TB_STUDENT

TB_TEACHER

TB_TEACHER_STUDENT

A 和 B操作的唯一区别是

//A
student1.getTeachers().add(teacher1);
student1.getTeachers().add(teacher2);
student2.getTeachers().add(teacher1);//B
teacher1.getStudents().add(student1);
teacher1.getStudents().add(student2);
teacher2.getStudents().add(student1);

需要注意的是Student.java, @ManyToMany的属性mappedBy = "students"

@Entity
@Table(name = "TB_STUDENT")
public class Student {...@ManyToMany(fetch = FetchType.LAZY, mappedBy = "students")public Set<Teacher> getTeachers() {return teachers;}...
}

当配置了这句话的时候就不可以再配置@JoinTable和@JoinColumn等,同时Teacher就是owning side, Student就是non-owning side,就是Teacher和Student之间的关系是通过Teacher端来维护的.