JPA 与数据访问

JPA 核心概念

Entity - 实体类

@Entity
@Table(name = "users")
public class User {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @Column(nullable = false, unique = true)
    private String email;
    
    @Column(nullable = false)
    private String username;
    
    @Column(name = "created_at")
    private LocalDateTime createdAt;
    
    @PrePersist
    protected void onCreate() {
        createdAt = LocalDateTime.now();
    }
    
    // getters and setters
}

Repository - 数据访问接口

public interface UserRepository extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);
    List<User> findByUsernameContaining(String username);
    boolean existsByEmail(String email);
}

关系映射

一对多 (One-to-Many)

@Entity
public class Author {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    private String name;
    
    @OneToMany(mappedBy = "author", cascade = CascadeType.ALL)
    private List<Book> books = new ArrayList<>();
}

@Entity
public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    private String title;
    
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author_id")
    private Author author;
}

多对多 (Many-to-Many)

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @ManyToMany
    @JoinTable(
        name = "student_course",
        joinColumns = @JoinColumn(name = "student_id"),
        inverseJoinColumns = @JoinColumn(name = "course_id")
    )
    private Set<Course> courses = new HashSet<>();
}

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @ManyToMany(mappedBy = "courses")
    private Set<Student> students = new HashSet<>();
}

查询方法

方法名查询

public interface ProductRepository extends JpaRepository<Product, Long> {
    
    List<Product> findByName(String name);
    List<Product> findByPriceGreaterThan(BigDecimal price);
    List<Product> findByNameContainingIgnoreCase(String name);
    Optional<Product> findFirstByOrderByPriceDesc();
    
    @Query("SELECT p FROM Product p WHERE p.price BETWEEN :minPrice AND :maxPrice")
    List<Product> findByPriceRange(
        @Param("minPrice") BigDecimal minPrice,
        @Param("maxPrice") BigDecimal maxPrice
    );
}

事务管理

@Service
public class OrderService {
    
    @Transactional
    public Order createOrder(OrderRequest request) {
        Order order = new Order();
        order.setUserId(request.getUserId());
        
        for (OrderItem item : request.getItems()) {
            Product product = productRepository.findById(item.getProductId())
                .orElseThrow(() -> new ResourceNotFoundException("Product not found"));
            
            if (product.getStock() < item.getQuantity()) {
                throw new InsufficientStockException("Not enough stock");
            }
            
            product.setStock(product.getStock() - item.getQuantity());
            productRepository.save(product);
            
            order.addItem(item);
        }
        
        return orderRepository.save(order);
    }
}

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