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JoinColumn annotation in JPA

JoinColumn annotation in JPA

The @JoinColumn annotation in JPA is used to specify the column that is used for joining an entity association. It is often used in the context of defining relationships between entities, such as @OneToOne, @OneToMany, @ManyToOne, and @ManyToMany.

JoinColumn annotation in JPA

Key Purposes of @JoinColumn:

  • 1.  Column Specification :
    • Specifies the column in the owning entity’s table that joins the related entity’s table.
  • 2.  Foreign Key :
    • The column specified is often a foreign key referencing the primary key of the associated entity.
  • 3.  Customization :
    • Allows customization of the column name, uniqueness, nullability, and other constraints.

Example

Let’s illustrate the use of @JoinColumn with a simple example involving two entities: Author and Book. In this example, each book is written by one author, so the Book entity will have a foreign key column referencing the Author entity.

Step 1: Define the Author Entity
java
package com.example.demo.model;

import javax.persistence.*;
import java.util.List;

@Entity
public class Author {

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

    @OneToMany(mappedBy = "author")
    private List<Book> books;

    // Constructors, getters, and setters

    public Author() {}

    public Author(String name) {
        this.name = name;
    }

    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;
    }

    public List<Book> getBooks() {
        return books;
    }

    public void setBooks(List<Book> books) {
        this.books = books;
    }
}

Step 2: Define the Book Entity
java
package com.example.demo.model;

import javax.persistence.*;

@Entity
public class Book {

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

    @ManyToOne
    @JoinColumn(name = "author_id", nullable = false)
    private Author author;

    // Constructors, getters, and setters

    public Book() {}

    public Book(String title, Author author) {
        this.title = title;
        this.author = author;
    }

    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 Author getAuthor() {
        return author;
    }

    public void setAuthor(Author author) {
        this.author = author;
    }
}

Step 3: Define the Repository
java
package com.example.demo.repository;

import com.example.demo.model.Author;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface AuthorRepository extends JpaRepository<Author, Long> {
}

package com.example.demo.repository;

import com.example.demo.model.Book;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface BookRepository extends JpaRepository<Book, Long> {
}

Step 4: Define the Service
java
package com.example.demo.service;

import com.example.demo.model.Author;
import com.example.demo.repository.AuthorRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class AuthorService {

    @Autowired
    private AuthorRepository authorRepository;

    public List<Author> getAllAuthors() {
        return authorRepository.findAll();
    }

    public Author getAuthorById(Long id) {
        return authorRepository.findById(id).orElse(null);
    }

    public void saveAuthor(Author author) {
        authorRepository.save(author);
    }

    public void deleteAuthor(Long id) {
        authorRepository.deleteById(id);
    }
}

package com.example.demo.service;

import com.example.demo.model.Book;
import com.example.demo.repository.BookRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class BookService {

    @Autowired
    private BookRepository bookRepository;

    public List<Book> getAllBooks() {
        return bookRepository.findAll();
    }

    public Book getBookById(Long id) {
        return bookRepository.findById(id).orElse(null);
    }

    public void saveBook(Book book) {
        bookRepository.save(book);
    }

    public void deleteBook(Long id) {
        bookRepository.deleteById(id);
    }
}

Step 5: Define the Controller
java
package com.example.demo.controller;

import com.example.demo.model.Author;
import com.example.demo.service.AuthorService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/api/authors")
public class AuthorController {

    @Autowired
    private AuthorService authorService;

    @GetMapping
    public List<Author> getAllAuthors() {
        return authorService.getAllAuthors();
    }

    @GetMapping("/{id}")
    public Author getAuthorById(@PathVariable Long id) {
        return authorService.getAuthorById(id);
    }

    @PostMapping
    public void saveAuthor(@RequestBody Author author) {
        authorService.saveAuthor(author);
    }

    @DeleteMapping("/{id}")
    public void deleteAuthor(@PathVariable Long id) {
        authorService.deleteAuthor(id);
    }
}

package com.example.demo.controller;

import com.example.demo.model.Book;
import com.example.demo.service.BookService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/api/books")
public class BookController {

    @Autowired
    private BookService bookService;

    @GetMapping
    public List<Book> getAllBooks() {
        return bookService.getAllBooks();
    }

    @GetMapping("/{id}")
    public Book getBookById(@PathVariable Long id) {
        return bookService.getBookById(id);
    }

    @PostMapping
    public void saveBook(@RequestBody Book book) {
        bookService.saveBook(book);
    }

    @DeleteMapping("/{id}")
    public void deleteBook(@PathVariable Long id) {
        bookService.deleteBook(id);
    }
}

Explanation of Example

  • Book Entity (Book.java) :
    • Annotated with @Entity to mark it as a JPA entity.
    • Uses @ManyToOne to define the relationship with the Author entity.
    • The @JoinColumn(name = “author_id”, nullable = false) annotation specifies that the author_id column in the Book table is used for the join and it cannot be null.
  • Author Entity (Author.java) :
    • Annotated with @Entity to mark it as a JPA entity.
    • Uses @OneToMany(mappedBy = “author”) to define the one-to-many relationship with the Book entity.

Summary

The @JoinColumn annotation is crucial in defining the column used for joining related entities in JPA. It specifies the column name, constraints, and other properties for the join column, ensuring proper mapping and relationship management between entities.

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