Spring 学习笔记 02：Spring Data JPA 基础——实体映射与 Repository

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>runtime</scope>
</dependency>
<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <scope>runtime</scope>
</dependency>

# application.properties
spring.datasource.url=jdbc:h2:mem:librarydb
spring.datasource.driver-class-name=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=

spring.jpa.show-sql=true
spring.jpa.hibernate.ddl-auto=create-drop
spring.h2.console.enabled=true

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-h2console</artifactId>
</dependency>

# application-mysql.properties
spring.datasource.url=jdbc:mysql://localhost:3306/library?useSSL=false&serverTimezone=Asia/Singapore
spring.datasource.username=library_user
spring.datasource.password=${DB_PASSWORD:}
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver

spring.jpa.hibernate.ddl-auto=validate

package com.example.library.domain;

import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.EnumType;
import jakarta.persistence.Enumerated;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.Table;
import jakarta.persistence.UniqueConstraint;

@Entity
@Table(name = "books",
        uniqueConstraints = @UniqueConstraint(name = "uk_books_isbn", columnNames = "isbn"))
public class Book {

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

    @Column(nullable = false, length = 200)
    private String title;

    private String author;

    @Column(length = 13)
    private String isbn;

    @Column(name = "published_year")
    private Integer publishedYear;

    @Enumerated(EnumType.STRING)
    @Column(nullable = false, length = 20)
    private BookStatus status = BookStatus.AVAILABLE;

    /** JPA 需要无参构造器，设为 protected 既满足要求又不暴露给业务代码 */
    protected Book() {
    }

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

    public Long getId() {
        return id;
    }

    public String getTitle() {
        return title;
    }

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

    public String getAuthor() {
        return author;
    }

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

    public String getIsbn() {
        return isbn;
    }

    public void setIsbn(String isbn) {
        this.isbn = isbn;
    }

    public Integer getPublishedYear() {
        return publishedYear;
    }

    public void setPublishedYear(Integer publishedYear) {
        this.publishedYear = publishedYear;
    }

    public BookStatus getStatus() {
        return status;
    }

    public void setStatus(BookStatus status) {
        this.status = status;
    }
}

package com.example.library.domain;

public enum BookStatus {

    AVAILABLE("可借"),
    BORROWED("已借出"),
    RESERVED("已预约"),
    LOST("遗失");

    private final String label;

    BookStatus(String label) {
        this.label = label;
    }

    public String getLabel() {
        return label;
    }
}

package com.example.library.repository;

import com.example.library.domain.Book;
import org.springframework.data.jpa.repository.JpaRepository;

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

package com.example.library.service;

import com.example.library.domain.Book;
import com.example.library.repository.BookRepository;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

import java.util.List;
import java.util.NoSuchElementException;

@Service
@Transactional(readOnly = true)
public class BookServiceImpl implements BookService {

    private final BookRepository bookRepository;

    public BookServiceImpl(BookRepository bookRepository) {
        this.bookRepository = bookRepository;
    }

    @Override
    @Transactional
    public Book add(String title, String author) {
        return bookRepository.save(new Book(title, author));
    }

    @Override
    public Book findById(Long id) {
        return bookRepository.findById(id)
                .orElseThrow(() -> new NoSuchElementException("找不到图书：" + id));
    }

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

package com.example.library;

import com.example.library.domain.Book;
import com.example.library.repository.BookRepository;
import org.springframework.boot.CommandLineRunner;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import java.util.List;

@Configuration
public class DataInitializer {

    @Bean
    CommandLineRunner loadSampleData(BookRepository repository) {
        return args -> {
            if (repository.count() > 0) {
                return;
            }
            repository.saveAll(List.of(
                    new Book("重构", "Martin Fowler"),
                    new Book("代码大全", "Steve McConnell"),
                    new Book("设计模式", "Erich Gamma")
            ));
        };
    }
}