Spring jpa: Many To Many mapping example

In Many to Many relationship, records in the first table are mapped to multiple rows in the second table and similarly rows in the second table are mapped to multiple rows in the first table.

For example, Take two tables, employees and projects. An employee can work in one or more projects and similarly a project can contain one or more employees.

We can model this many to many relationship using a mapping table.

employees Table

emp_id	first_name	last_name

projects Table

pjt_id	project_name	Team_size

employee_project_mapping table

emp_id	pjt_id

We can model above relationships using ManyToMany annotation like below.

@Entity
@Table(name = "employees")
public class Employee {
 @Id
 @GeneratedValue
 @Column(name = "emp_id")
 private int id;
 private String firstName;
 private String lastName;

 @JoinTable(name = "employee_project_mapping", joinColumns = @JoinColumn(name = "employee_id", referencedColumnName = "emp_id"), inverseJoinColumns = @JoinColumn(name = "project_id", referencedColumnName = "pjt_id"))
 @JsonManagedReference
 @ManyToMany(cascade = CascadeType.ALL)
 Set<Project> projects = new HashSet<Project>();

 ......
 ......

}

@Entity
@Table(name = "projects")
public class Project {
 @Id
 @GeneratedValue
 @Column(name = "pjt_id")
 private int projectId;

 private String projectName;

 private int teamSize;

 @ManyToMany(mappedBy = "projects")
 @JsonBackReference
 private Set<Employee> emp = new HashSet<Employee>();

 ......
 ......
}

Find the below working application.

Employee.java

package com.sample.app.entity;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

import com.fasterxml.jackson.annotation.JsonManagedReference;

@Entity
@Table(name = "employees")
public class Employee {
 @Id
 @GeneratedValue
 @Column(name = "emp_id")
 private int id;
 private String firstName;
 private String lastName;

 @JoinTable(name = "employee_project_mapping", joinColumns = @JoinColumn(name = "employee_id", referencedColumnName = "emp_id"), inverseJoinColumns = @JoinColumn(name = "project_id", referencedColumnName = "pjt_id"))
 @JsonManagedReference
 @ManyToMany(cascade = CascadeType.ALL)
 Set<Project> projects = new HashSet<Project>();

 public int getId() {
  return id;
 }

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

 public String getFirstName() {
  return firstName;
 }

 public void setFirstName(String firstName) {
  this.firstName = firstName;
 }

 public String getLastName() {
  return lastName;
 }

 public void setLastName(String lastName) {
  this.lastName = lastName;
 }

 public Set<Project> getProjects() {
  return projects;
 }

 public void setProjects(Set<Project> projects) {
  this.projects = projects;
 }

}

Project.java

package com.sample.app.entity;

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;

import com.fasterxml.jackson.annotation.JsonBackReference;

@Entity
@Table(name = "projects")
public class Project {
 @Id
 @GeneratedValue
 @Column(name = "pjt_id")
 private int projectId;

 private String projectName;

 private int teamSize;

 @ManyToMany(mappedBy = "projects")
 @JsonBackReference
 private Set<Employee> emp = new HashSet<Employee>();

 public int getProjectId() {
  return projectId;
 }

 public void setProjectId(int projectId) {
  this.projectId = projectId;
 }

 public String getProjectName() {
  return projectName;
 }

 public void setProjectName(String projectName) {
  this.projectName = projectName;
 }

 public Set<Employee> getEmp() {
  return emp;
 }

 public void setEmp(Set<Employee> emp) {
  this.emp = emp;
 }

 public int getTeamSize() {
  return teamSize;
 }

 public void setTeamSize(int teamSize) {
  this.teamSize = teamSize;
 }

}

EmployeeDto.java

package com.sample.app.dto;

import java.util.List;

public class EmployeeDto {
 private String firstName;
 private String lastName;
 private List<ProjectDto> projects;

 public String getFirstName() {
  return firstName;
 }

 public void setFirstName(String firstName) {
  this.firstName = firstName;
 }

 public String getLastName() {
  return lastName;
 }

 public void setLastName(String lastName) {
  this.lastName = lastName;
 }

 public List<ProjectDto> getProjects() {
  return projects;
 }

 public void setProjects(List<ProjectDto> projects) {
  this.projects = projects;
 }

}

ProjectDto.java

package com.sample.app.dto;

public class ProjectDto {

 private String projectName;

 private int teamSize;

 public String getProjectName() {
  return projectName;
 }

 public void setProjectName(String projectName) {
  this.projectName = projectName;
 }

 public int getTeamSize() {
  return teamSize;
 }

 public void setTeamSize(int teamSize) {
  this.teamSize = teamSize;
 }

}

EmployeeRepository.java

package com.sample.app.repository;

import org.springframework.data.repository.CrudRepository;

import com.sample.app.entity.Employee;

public interface EmployeeRepository extends CrudRepository<Employee, Integer> {

}

EmployeeService.java

package com.sample.app.service;

import com.sample.app.dto.EmployeeDto;
import com.sample.app.entity.Employee;

public interface EmployeeService {

 Employee save(EmployeeDto emp);

 Employee getById(int id);

}

EmployeeServiceImpl.java

package com.sample.app.service.impl;

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

import javax.transaction.Transactional;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import com.sample.app.dto.EmployeeDto;
import com.sample.app.dto.ProjectDto;
import com.sample.app.entity.Employee;
import com.sample.app.entity.Project;
import com.sample.app.repository.EmployeeRepository;
import com.sample.app.service.EmployeeService;

@Service
public class EmployeeServiceImpl implements EmployeeService {

 @Autowired
 private EmployeeRepository empRepo;

 @Override
 @Transactional
 public Employee save(EmployeeDto empDto) {

  String firstName = empDto.getFirstName();

  String lastName = empDto.getLastName();

  Employee emp = new Employee();
  emp.setFirstName(firstName);
  emp.setLastName(lastName);

  List<Project> projectToBeSaved = new ArrayList<>();

  for (ProjectDto dto : empDto.getProjects()) {
   Project tempPjt = new Project();

   tempPjt.setProjectName(dto.getProjectName());
   tempPjt.setTeamSize(dto.getTeamSize());

   projectToBeSaved.add(tempPjt);

  }

  emp.setProjects(new HashSet<>(projectToBeSaved));

  return empRepo.save(emp);
 }

 @Override
 public Employee getById(int id) {
  return empRepo.findById(id).get();
 }

}

EmployeeController.java

package com.sample.app.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

import com.sample.app.dto.EmployeeDto;
import com.sample.app.entity.Employee;
import com.sample.app.service.EmployeeService;

import io.swagger.annotations.Api;
import io.swagger.annotations.ApiOperation;

@RestController
@Api(tags = "Employee Controller", value = "This section contains all Employee Speicifc Operations")
public class EmployeeController {

 @Autowired
 private EmployeeService empService;

 @ApiOperation(value = "Create new Employee", notes = "Create new Employee")
 @PostMapping(value = "/employees", produces = MediaType.APPLICATION_JSON_VALUE)
 public ResponseEntity<Employee> saveEmployee(@RequestBody EmployeeDto empDto) {

  Employee persistedEmp = empService.save(empDto);

  return new ResponseEntity<>(persistedEmp, HttpStatus.CREATED);
 }

 @ApiOperation(value = "Get Employee", notes = "Get employee by id")
 @GetMapping(value = "/employees/{id}", produces = MediaType.APPLICATION_JSON_VALUE)
 public ResponseEntity<Employee> getEmployees(@PathVariable final Integer id) {

  Employee persistedEmp = empService.getById(id);

  return new ResponseEntity<>(persistedEmp, HttpStatus.CREATED);
 }

}

SwaggerConfig.java

package com.sample.app.config;

import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.context.annotation.Bean;
import org.springframework.stereotype.Component;

import springfox.documentation.builders.ApiInfoBuilder;
import springfox.documentation.builders.PathSelectors;
import springfox.documentation.builders.RequestHandlerSelectors;
import springfox.documentation.service.ApiInfo;
import springfox.documentation.spi.DocumentationType;
import springfox.documentation.spring.web.plugins.Docket;
import springfox.documentation.swagger2.annotations.EnableSwagger2;

@Component
@EnableAutoConfiguration
@EnableSwagger2
public class SwaggerConfig {

 @Bean
 public Docket userApi() {

  return new Docket(DocumentationType.SWAGGER_2).apiInfo(apiInfo()).select().paths(PathSelectors.any())
    .apis(RequestHandlerSelectors.basePackage("com.sample.app.controller")).build();
 }

 private ApiInfo apiInfo() {

  return new ApiInfoBuilder().title("Query builder").description("Query builder using spring specification")
    .version("2.0").build();
 }
}

App.java

package com.sample.app;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class App {

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

}

pom.xml

<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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>
 <groupId>com.sample.app</groupId>
 <artifactId>manyToMany</artifactId>
 <version>1</version>

 <parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-parent</artifactId>
  <version>2.1.6.RELEASE</version>
 </parent>

 <properties>
  <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 </properties>


 <dependencies>
  <!-- https://mvnrepository.com/artifact/org.springframework.boot/spring-boot-starter-data-jpa -->
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-data-jpa</artifactId>
  </dependency>

  <!-- https://mvnrepository.com/artifact/com.h2database/h2 -->
  <dependency>
   <groupId>com.h2database</groupId>
   <artifactId>h2</artifactId>
  </dependency>

  <!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-envers -->
  <dependency>
   <groupId>org.hibernate</groupId>
   <artifactId>hibernate-envers</artifactId>
  </dependency>

  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-web</artifactId>
  </dependency>

  <dependency>
   <groupId>io.springfox</groupId>
   <artifactId>springfox-swagger2</artifactId>
   <version>2.9.2</version>
  </dependency>


  <dependency>
   <groupId>io.springfox</groupId>
   <artifactId>springfox-swagger-ui</artifactId>
   <version>2.9.2</version>
  </dependency>

 </dependencies>
</project>

Create application.properties file under src/main/resources folder.

application.properties

logging.level.root=WARN
logging.level.org.hibernate=ERROR

## H2 specific properties
spring.h2.console.enabled=true
spring.h2.console.path=/h2

spring.datasource.url=jdbc:h2:file:~/db/myOrg.db;DB_CLOSE_ON_EXIT=FALSE;DB_CLOSE_DELAY=-1;

spring.datasource.username=krishna
spring.datasource.password=password123

spring.datasource.driverClassName=org.h2.Driver

## JPA specific properties
# Creates the schema, destroying previous data.
spring.jpa.hibernate.ddl-auto=create

spring.jpa.database-platform=org.hibernate.dialect.H2Dialect

spring.jpa.show-sql=true
#spring.jpa.properties.hibernate.format_sql=true

## Database connection pooling properties
# Number of ms to wait before throwing an exception if no connection is available.
spring.datasource.max-wait=10000

# Maximum number of active connections that can be allocated from this pool at the same time.
spring.datasource.tomcat.max-active=10
spring.datasource.tomcat.max-idle=5
spring.datasource.tomcat.min-idle=3

spring.jackson.serialization.FAIL_ON_EMPTY_BEANS=false

Total Project structure looks like below.

Run App.java.

Open the url ‘http://localhost:8080/swagger-ui.html’ to see the swagger User Interface.

Open the url ‘http://localhost:8080/h2/login.do’, to see H2 database console.

You can download complete working application from this link.

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