Many to Many mapping in Spring boot

In this blog, I will explain how to use many-to-many mapping in Spring boot Application

What you need?

• JAVA
• MySql
• Eclipse IDE ( whatever you like IDE, I'm using Eclipse for this example)
• Maven ( you can use the Gradle as well)

Initial Plan

spring boot application will be created using spring initializer web tool and imported as a maven project to the eclipse ide. Scenarios will be post and tags. one post have more than one tags and one tags have more than one posts. Two model classes will be created for Post and Tags. Finally, the command line runner interface will be used to run the application.

Below diagram is the database model diagram which we going to install using the spring boot application.

Let's Start to Code.

You need to configure the application.properties file for database connections. add the following content to the src/main/resources/application.properties

spring.datasource.url=jdbc:mysql://localhost:3306/learning
spring.datasource.username=root
spring.datasource.password=root

spring.jpa.properties.hibernate.dialect = org.hibernate.dialect.MySQL5InnoDBDialect
spring.jpa.hibernate.ddl-auto = update

Change the database username and password according to your MySql configurations.


1. AuditModel Class

This class will automatically create create_at and update_at fields while persisting the entities.

package lk.hjsoft.springbootmanytomany.model;

import java.io.Serializable;
import java.util.Date;

import javax.persistence.Column;
import javax.persistence.EntityListeners;
import javax.persistence.MappedSuperclass;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

import org.springframework.data.annotation.CreatedDate;
import org.springframework.data.annotation.LastModifiedDate;
import org.springframework.data.jpa.domain.support.AuditingEntityListener;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@MappedSuperclass
@EntityListeners(AuditingEntityListener.class)
@JsonIgnoreProperties(value = { "createdAt", "updatedAt" }, allowGetters = true)
public abstract class AuditModel implements Serializable {

 @Temporal(TemporalType.TIMESTAMP)
 @Column(name = "createAt", nullable = false)
 @CreatedDate
 public Date createAt;

 @Temporal(TemporalType.TIMESTAMP)
 @Column(name = "updateAt", nullable = false)
 @LastModifiedDate
 public Date updateAt;

 public Date getCreateAt() {
  return createAt;
 }

 public void setCreateAt(Date createAt) {
  this.createAt = createAt;
 }

 public Date getUpdateAt() {
  return updateAt;
 }

 public void setUpdateAt(Date updateAt) {
  this.updateAt = updateAt;
 }
}

Auditing, JsonIgonreProperties. JsonIgnore annotations are discussed in this blog post

2. Post class

package lk.hjsoft.springbootmanytomany.model;

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.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.Lob;
import javax.persistence.ManyToMany;
import javax.persistence.Table;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;

@Entity
@Table(name = "POSTS")
public class Post extends AuditModel {

 @Id
 @GeneratedValue(strategy = GenerationType.IDENTITY)
 private Long id;

 @NotNull
 @Size(max = 200)
 @Column(name = "title")
 private String title;

 @NotNull
 @Size(max = 200)
 @Column(name = "description")
 private String description;

 @NotNull
 @Lob
 private String content;
 
 @ManyToMany(fetch = FetchType.LAZY, cascade = {
   CascadeType.PERSIST,
   CascadeType.MERGE
 })
 @JoinTable(name="POSTS_TAGS", joinColumns = {
   @JoinColumn(name="post_id")
 }, inverseJoinColumns = {
   @JoinColumn(name="tag_id")
 })
 private Set tags = new HashSet<>();

 public Set getTags() {
  return tags;
 }

 public void setTags(Set tags) {
  this.tags = tags;
 }

 public Long getId() {
  return id;
 }

 public void setId(Long id) {
  this.id = id;
 }

 public String getTitle() {
  return title;
 }

 public void setTitle(String title) {
  this.title = title;
 }

 public String getDescription() {
  return description;
 }

 public void setDescription(String description) {
  this.description = description;
 }

 public String getContent() {
  return content;
 }

 public void setContent(String content) {
  this.content = content;
 }

}

3. Tag Class

package lk.hjsoft.springbootmanytomany.model;

import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;

@Entity
@Table(name = "TAGS")
public class Tag implements Serializable {

 @Id
 @GeneratedValue(strategy = GenerationType.IDENTITY)
 private Long id;

 @NotNull
 @Size(max = 100)
 private String name;
 
 @ManyToMany( fetch = FetchType.LAZY, cascade = {
  CascadeType.PERSIST,
  CascadeType.MERGE
 }, mappedBy = "tags")
 private Set posts = new HashSet<>();

 public Set getPosts() {
  return posts;
 }

 public void setPosts(Set posts) {
  this.posts = posts;
 }

 public Long getId() {
  return id;
 }

 public void setId(Long id) {
  this.id = id;
 }

 public String getName() {
  return name;
 }

 public void setName(String name) {
  this.name = name;
 }

}

Now Repository interfaces will be created for database operations.

1. PostRepository interface

package lk.hjsoft.springbootmanytomany.repository;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

import lk.hjsoft.springbootmanytomany.model.Post;

@Repository
public interface PostRepository extends JpaRepository<Post, Long>{

}

2. TagRepository interface

package lk.hjsoft.springbootmanytomany.repository;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

import lk.hjsoft.springbootmanytomany.model.Tag;

@Repository
public interface TagRepository extends JpaRepository<Tag, Long>{

}

Codes to test the application

Main-Class will be replaced with the following codes.

package lk.hjsoft.springbootmanytomany;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.data.jpa.repository.config.EnableJpaAuditing;

import lk.hjsoft.springbootmanytomany.model.Post;
import lk.hjsoft.springbootmanytomany.model.Tag;
import lk.hjsoft.springbootmanytomany.repository.PostRepository;
import lk.hjsoft.springbootmanytomany.repository.TagRepository;

@SpringBootApplication
@EnableJpaAuditing
public class Application implements CommandLineRunner {
 
 @Autowired
 private PostRepository postRepo;
 
 @Autowired
 private TagRepository tagRepo;

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

 @Override
 public void run(String... args) throws Exception {
  
  Post p1 = new Post();
  p1.setTitle("first title");
  p1.setDescription("first description");
  p1.setContent("this is first content");
  
  Tag t1 = new Tag();
  t1.setName("spring");
  
  Tag t2 = new Tag();
  t2.setName("java");
  
  p1.getTags().add(t1);
  p1.getTags().add(t2);
  
  t1.getPosts().add(p1);
  t2.getPosts().add(p1);
  
  postRepo.save(p1);
 }

}

Running the application

mvn spring-boot:run

Check the database and data will be in the relevant tables.

You can download the source code from Github