@ManyToMany Mapping – Java Tutorials for dummies

February 27th, 2023

@ManyToMany Mapping

We use ManyToMany when Multiple relations are between tables.

				
					<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>3.0.2</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>
	<groupId>com.brains.jpa.hibernate</groupId>
	<artifactId>jpa-hibernate-demo</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>jpa-hibernate-demo</name>
	<description>Demo project for Spring Boot</description>
	<properties>
		<java.version>17</java.version>
	</properties>
	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-data-jpa</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>

		<dependency>
			<groupId>com.h2database</groupId>
			<artifactId>h2</artifactId>
			<scope>runtime</scope>
		</dependency>
		<dependency>
			<groupId>org.projectlombok</groupId>
			<artifactId>lombok</artifactId>
			<optional>true</optional>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
				<configuration>
					<excludes>
						<exclude>
							<groupId>org.projectlombok</groupId>
							<artifactId>lombok</artifactId>
						</exclude>
					</excludes>
				</configuration>
			</plugin>
		</plugins>
	</build>

</project>

				
					spring.datasource.url=jdbc:h2:mem:testdb;NON_KEYWORDS=USER;DB_CLOSE_ON_EXIT=FALSE
spring.h2.console.enabled=true
spring.jpa.defer-datasource-initialization=true

# Turn Statistics ON
#spring.jpa.properties.hibernate.generate_statistics=true
logging.level.org.hibernate.stat=debug
#logging.level.root=trace
# Show all queries
spring.jpa.show-sql=true

# Format the queries
spring.jpa.properties.hibernate.format_sql=true

				
					SELECT * FROM STUDENT_COURSE, STUDENT, COURSE
where STUDENT_COURSE.student_id = STUDENT.id
and STUDENT_COURSE.course_id = COURSE.id

One Course has many students and One Student can enroll many courses.

@Setter(value = AccessLevel.NONE) – This removes the setter method
@JoinTable(name = "STUDENT_COURSE)" – Give a custom name to the join table
joinColumns = @JoinColumn(name = "STUDENT_ID") – Give a custom name to the owning column
inverseJoinColumns = @JoinColumn(name = "COURSE_ID") – Give a custom name to the referenced column
mappedBy is always added to the non-owning side of the relationship.

Here between Student and Course, Student is the owner, that’s the reason mappedBy is added in the Course.

				
					package com.brains.jpa.hibernate.jpahibernatedemo.entity;

import java.util.ArrayList;
import java.util.List;

import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.FetchType;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.Id;
import jakarta.persistence.JoinColumn;
import jakarta.persistence.JoinTable;
import jakarta.persistence.ManyToMany;
import jakarta.persistence.OneToOne;
import lombok.AccessLevel;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.NonNull;
import lombok.RequiredArgsConstructor;
import lombok.Setter;
import lombok.ToString;

@Entity
@Data @NoArgsConstructor(access = AccessLevel.PROTECTED)
@AllArgsConstructor
@RequiredArgsConstructor
public class Student {

	@Id
	@GeneratedValue
	private Long id;
	
	@NonNull
	@Column(nullable = false)
	private String name;
	
	@OneToOne(fetch = FetchType.LAZY)
	private Passport passport;
	
	@ToString.Exclude
	@ManyToMany
	@Setter(value = AccessLevel.NONE)
	@JoinTable(name = "STUDENT_COURSE", 
			joinColumns = @JoinColumn(name = "STUDENT_ID"),
			inverseJoinColumns = @JoinColumn(name = "COURSE_ID")
			)
	private List<Course> courses = new ArrayList<>();
	
	public void addCourse(Course course) {
		this.courses.add(course);
	}
}

				
					package com.brains.jpa.hibernate.jpahibernatedemo.entity;

import java.time.LocalDateTime;
import java.util.ArrayList;
import java.util.List;

import org.hibernate.annotations.CreationTimestamp;
import org.hibernate.annotations.UpdateTimestamp;

import jakarta.persistence.Entity;
import jakarta.persistence.FetchType;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.Id;
import jakarta.persistence.ManyToMany;
import jakarta.persistence.OneToMany;
import lombok.AccessLevel;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.Setter;

@Entity
@Data
@NoArgsConstructor(access = AccessLevel.PROTECTED)
public class Course {
	
	@Id
	@GeneratedValue
	private Long id;
	
	private String name;
	
	@Setter(AccessLevel.NONE)
	@OneToMany(mappedBy = "course", fetch = FetchType.LAZY)
	private List<Review> reviews = new ArrayList<>();
	
	@ManyToMany(mappedBy = "courses")
	@Setter(value = AccessLevel.NONE)
	private List<Student> students = new ArrayList<>();
	
	public void addStudent(Student student) {
		this.students.add(student);
	}
	
	public void addReview(Review review) {
		this.reviews.add(review);
	}
	
	public void removeReview(Review review) {
		this.reviews.remove(review);
	}
	
	@UpdateTimestamp
	private LocalDateTime lastUpdatedDate;

	@CreationTimestamp
	private LocalDateTime createdDate;
	
	public Course(String name) {
		this.name = name;
	}
}

				
					package com.brains.jpa.hibernate.jpahibernatedemo.entity;

import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.FetchType;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.Id;
import jakarta.persistence.OneToOne;
import lombok.AccessLevel;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.NonNull;
import lombok.RequiredArgsConstructor;
import lombok.ToString;

@Entity
@Data @NoArgsConstructor(access = AccessLevel.PROTECTED)
@AllArgsConstructor
@RequiredArgsConstructor
public class Passport {

	@Id
	@GeneratedValue
	private Long id;
	
	@NonNull
	@Column(nullable = false)
	private String number;
	
	@ToString.Exclude
	@OneToOne(fetch = FetchType.LAZY, mappedBy = "passport")
	private Student student;
}

				
					package com.brains.jpa.hibernate.jpahibernatedemo.entity;

import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.Id;
import jakarta.persistence.ManyToOne;
import lombok.AccessLevel;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.NonNull;
import lombok.RequiredArgsConstructor;
import lombok.ToString;

@Entity
@Data @NoArgsConstructor(access = AccessLevel.PROTECTED)
@AllArgsConstructor
@RequiredArgsConstructor
public class Review {

	@Id
	@GeneratedValue
	private Long id;
	
	@NonNull
	private String rating;
	
	@NonNull
	private String description;
	
	@ToString.Exclude
	@ManyToOne
	private Course course;
}

				
					insert into course(id, name, created_date, last_updated_date) 
values(10001, 'JPA in 5 steps', LOCALTIMESTAMP, LOCALTIMESTAMP);
insert into course(id, name, created_date, last_updated_date) 
values(10002, 'JDBC in 10 steps', LOCALTIMESTAMP, LOCALTIMESTAMP);
insert into course(id, name, created_date, last_updated_date) 
values(10003, 'JPQL in 50 steps', LOCALTIMESTAMP, LOCALTIMESTAMP);

insert into passport(id, number)
values(40001,'E12345');
insert into passport(id, number)
values(40002,'F212345');
insert into passport(id, number)
values(40003,'G12345');

insert into student(id, name, passport_id)
values(20001,'Ranga',40001);
insert into student(id, name, passport_id)
values(20002,'Adam',40002);
insert into student(id, name, passport_id)
values(20003,'Jane',40003);

insert into review(id, rating, description, course_id)
values(50001,'1', 'Great course', 10001);
insert into review(id, rating, description, course_id)
values(50002,'3', 'Nice course', 10001);
insert into review(id, rating, description, course_id)
values(50004,'5', 'Good you are writing', 10003);

insert into student_course(student_id, course_id)
values(20001, 10001);
insert into student_course(student_id, course_id)
values(20002, 10001);
insert into student_course(student_id, course_id)
values(20003, 10001);
insert into student_course(student_id, course_id)
values(20001, 10003);

				
					package com.brains.jpa.hibernate.jpahibernatedemo.repository;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Repository;

import com.brains.jpa.hibernate.jpahibernatedemo.entity.Course;
import com.brains.jpa.hibernate.jpahibernatedemo.entity.Passport;
import com.brains.jpa.hibernate.jpahibernatedemo.entity.Student;

import jakarta.persistence.EntityManager;
import jakarta.transaction.Transactional;

@Repository
@Transactional
public class StudentRepository {

	private Logger logger = LoggerFactory.getLogger(getClass());
	
	@Autowired
	EntityManager em;
	
	public Student findById(Long id) {
		return em.find(Student.class, id);
	}
	
	public void saveStudentWithPassport() {
		Passport passport = new Passport("Z12345");
		em.persist(passport);
		Student student1 = new Student("Mike");
		student1.setPassport(passport);
		passport.setStudent(student1);
		em.persist(student1);
	}
	
	public void someOperationToUnderstandPersistanceContext() {
		Student student = em.find(Student.class, 20001L);
		logger.info("Student -> {}", student);
		
		Passport passport = student.getPassport();
		logger.info("Passport -> {}", passport);
		
		passport.setNumber("E12345-0");
		student.setName("Ranga - updated");
		logger.info("Student -> {}", student);
	}
	
	public void insertHardcodedStudentCourse() {
		Student student = new Student("Jack");
		Course course = new Course("Microservices in 100 Steps");
		em.persist(student);
		em.persist(course);
		
		student.addCourse(course);
		course.addStudent(student);
	}
	
	public void insertStudentCourse(Student student, Course course) {
		em.persist(student);
		em.persist(course);
		
		student.addCourse(course);
		course.addStudent(student);
	}
}

				
					package com.brains.jpa.hibernate.jpahibernatedemo.repository;

import org.junit.jupiter.api.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import com.brains.jpa.hibernate.jpahibernatedemo.JpaHibernateDemoApplication;
import com.brains.jpa.hibernate.jpahibernatedemo.entity.Course;
import com.brains.jpa.hibernate.jpahibernatedemo.entity.Student;

import jakarta.persistence.EntityManager;
import jakarta.transaction.Transactional;

@SpringBootTest(classes = JpaHibernateDemoApplication.class)
class StudentCourseRepositoryTest {

	private Logger logger = LoggerFactory.getLogger(getClass());
	
	@Autowired
	EntityManager em;
	
	@Test
	@Transactional
	void retrieveStudentAndCourse() {
		Student student = em.find(Student.class, 20001L);
		logger.info("Student: -> {}", student);
		logger.info("Courses: -> {}", student.getCourses());
	}
	
	@Test
	@Transactional
	void retrieveCourseAndStudent() {
		Course course = em.find(Course.class, 10001L);
		logger.info("Courses: -> {}", course);
		logger.info("Student: -> {}", course.getStudents());
	}

}

				
					package com.brains.jpa.hibernate.jpahibernatedemo;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import com.brains.jpa.hibernate.jpahibernatedemo.entity.Course;
import com.brains.jpa.hibernate.jpahibernatedemo.entity.Student;
import com.brains.jpa.hibernate.jpahibernatedemo.repository.StudentRepository;

@SpringBootApplication
public class JpaHibernateDemoApplication implements CommandLineRunner {

	public static void main(String[] args) {
		SpringApplication.run(JpaHibernateDemoApplication.class, args);
	}

	private Logger logger = LoggerFactory.getLogger(this.getClass());
	
	@Autowired
	private StudentRepository studentRepository;
	
	@Override
	public void run(String... args) throws Exception {
		studentRepository.insertHardcodedStudentCourse();
		Student student = new Student("Jack1");
		Course course = new Course("Microservices in 10 Steps");
		studentRepository.insertStudentCourse(student, course);
	}
}

Related Tutorials

NamedQuery Annotations

Entities Relationships

@OneToOne Mapping

@ManyToOne Mapping

JPA Inheritance Hierarchies and Mapping

JPQL Queries

JOIN Query

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