last modified July 27, 2023
In this article we show how to create one-to-many relationship in a Spring Boot application.
Spring is a popular Java application framework and Spring Boot is an evolution of Spring which helps create stand-alone, production-grade Spring based applications easily.
Entity is a Java object that is going to be persisted. Entity classes
are decorated with annotations such as @Id, @Table, or
@Column. There are relationships between entities (also called
associations.) The basic associations are one-to-one, one-to-many, and
many-to-many.
In a one-to-many relationship, one record in a table can be associated with one or more records in another table. For example, one customer can have many orders.
Relationships may be unidirectional or bidirectional. In a unidirectional relationship we have a navigational access in one direction, in a bidirectional relationship, we have access in both directions. Suppose we have two entities: User and Post. There is a one-to-many relationship between the entities; one user may have multiple posts. In a unidirectional relationship, we can get posts from a user. In a bidirectional, we can get the user from the posts as well.
In the following example we create a simple Spring Boot application with a unidirectional one-to-many relationship between two entities.
In the example, we have User and Post entities.
A user can have multiple posts; so there is a one-to-many relationship
between the two entities.
build.gradle ... src ├───main │ ├───java │ │ └───com │ │ └───zetcode │ │ │ Application.java │ │ │ MyRunner.java │ │ ├───model │ │ │ Post.java │ │ │ User.java │ │ └───repository │ │ PostRepository.java │ │ UserRepository.java │ └───resources │ application.properties └───test └───java
This is the project structure.
plugins {
id 'org.springframework.boot' version '3.1.1'
id 'io.spring.dependency-management' version '1.1.0'
id 'java'
}
group = 'com.zetcode'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '17'
repositories {
mavenCentral()
}
dependencies {
implementation 'org.springframework.boot:spring-boot-starter-data-jpa'
implementation 'com.h2database:h2'
}
This is the Gradle build.gradle file. We add the
spring-boot-starter-data-jpa starter and the in-memory H2 database.
spring.main.banner-mode=off
spring.jpa.show-sql=true
logging.pattern.console=%clr(%d{yy-MM-dd E HH:mm:ss.SSS}){blue} %clr(%-5p) %clr(%logger{0}){blue} %clr(%m){faint}%n
In the application.properties, we turn off the Spring banner,
enable SQL logging, and customize overall logging patterns.
package com.zetcode.model;
import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.JoinColumn;
import jakarta.persistence.OneToMany;
import jakarta.persistence.Table;
import java.util.Objects;
import java.util.Set;
@Entity
@Table(name="users")
public class User {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String firstName;
private String lastName;
@OneToMany
@JoinColumn
private Set<Post> posts;
public User() {}
public User(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
User user = (User) o;
return Objects.equals(id, user.id) &&
Objects.equals(firstName, user.firstName) &&
Objects.equals(lastName, user.lastName) &&
Objects.equals(posts, user.posts);
}
@Override
public int hashCode() {
return Objects.hash(id, firstName, lastName, posts);
}
public Long getId() {
return 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<Post> getPosts() {
return posts;
}
public void setPosts(Set<Post> posts) {
this.posts = posts;
}
@Override
public String toString() {
final StringBuilder sb = new StringBuilder("User{");
sb.append("id=").append(id);
sb.append(", firstName='").append(firstName).append('\'');
sb.append(", lastName='").append(lastName).append('\'');
sb.append(", posts=").append(posts);
sb.append('}');
return sb.toString();
}
}
This is the User model class. It has the following
fields: id, firstName, and lastName.
@OneToMany @JoinColumn private Set<Post> posts;
With the @OneToMany annotation and the Set collection,
we create a one-to-many relationship. The @JoinColumn creates a foreign
key in the corresponding users table. Without the @JoinColumn
annotation, Hibernate would create a users_posts join table.
package com.zetcode.model;
import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.Table;
import java.util.Objects;
@Entity
@Table(name="tasks")
public class Post {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
public Post() {}
public Post(String name) {
this.name = name;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Post post = (Post) o;
return Objects.equals(id, post.id) &&
Objects.equals(name, post.name);
}
@Override
public int hashCode() {
return Objects.hash(id, name);
}
public Long getId() {
return id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
final StringBuilder sb = new StringBuilder("Post{");
sb.append("id=").append(id);
sb.append(", name='").append(name).append('\'');
sb.append('}');
return sb.toString();
}
}
This is the Post model class. It has the following attributes:
id and name. It has no extra annotations for the
relationship.
package com.zetcode.repository;
import com.zetcode.model.Post;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;
@Repository
public interface PostRepository extends JpaRepository<Post, Long> {
}
The PostRepository defines a storage place for post objects.
package com.zetcode.repository;
import com.zetcode.model.User;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;
@Repository
public interface UserRepository extends JpaRepository<User, Long> {
}
The UserRepository defines a storage place for user objects.
package com.zetcode;
import com.zetcode.model.Post;
import com.zetcode.model.User;
import com.zetcode.repository.PostRepository;
import com.zetcode.repository.UserRepository;
import jakarta.transaction.Transactional;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;
import java.util.List;
import java.util.Set;
@Component
public class MyRunner implements CommandLineRunner {
private static final Logger logger = LoggerFactory.getLogger(MyRunner.class);
private final UserRepository userRepository;
private final PostRepository postRepository;
@Autowired
public MyRunner(UserRepository userRepository, PostRepository postRepository) {
this.userRepository = userRepository;
this.postRepository = postRepository;
}
@Override
@Transactional
public void run(String... args) throws Exception {
logger.info("Saving data");
var p1 = new Post("Post 1");
var p2 = new Post("Post 2");
var p3 = new Post("Post 3");
var p4 = new Post("Post 4");
var p5 = new Post("Post 5");
var u1 = new User("John", "Doe");
var u2 = new User("Bobby", "Brown");
var u3 = new User("Lucy", "Smith");
u1.setPosts(Set.of(p1, p2, p3));
u2.setPosts(Set.of(p4));
u3.setPosts(Set.of(p5));
postRepository.saveAll(List.of(p1, p2, p3, p4, p5));
userRepository.saveAll(List.of(u1, u2, u3));
userRepository.findById(1L).ifPresent(user ->
logger.info("{} {} has Posts: {}", user.getFirstName(),
user.getLastName(), user.getPosts()));
userRepository.findById(2L).ifPresent(user ->
logger.info("{} {} has Posts: {}", user.getFirstName(),
user.getLastName(), user.getPosts()));
}
}
In the MyRunner, we create users and posts. We define relationships
between users and posts.
private final UserRepository userRepository;
private final PostRepository postRepository;
@Autowired
public MyRunner(UserRepository userRepository, PostRepository postRepository) {
this.userRepository = userRepository;
this.postRepository = postRepository;
}
We inject the two repositories.
@Override
@Transactional
public void run(String... args) throws Exception {
The database operations must be decorated with the @Transactional
annotation.
var p1 = new Post("Post 1");
var p2 = new Post("Post 2");
var p3 = new Post("Post 3");
var p4 = new Post("Post 4");
var p5 = new Post("Post 5");
var u1 = new User("John", "Doe");
var u2 = new User("Bobby", "Brown");
var u3 = new User("Lucy", "Smith");
We create five posts and three users.
u1.setPosts(Set.of(p1, p2, p3)); u2.setPosts(Set.of(p4)); u3.setPosts(Set.of(p5));
We associate posts with users.
postRepository.saveAll(List.of(p1, p2, p3, p4, p5)); userRepository.saveAll(List.of(u1, u2, u3));
The user and post objects are saved.
userRepository.findById(1L).ifPresent(User ->
logger.info("{} {} has Posts: {}", User.getFirstName(),
User.getLastName(), User.getPosts()));
We find all posts of user with Id 1.
package com.zetcode;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}
This code sets up the Spring Boot application.
In this article we have worked with one-to-many relationship.
My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.