LazyInitializationExceptionSpring引导

Question

为了简单起见，没有提供getter、setter、ToString和构造函数。我能够保存对象到数据库和数据库已经填充。

@Entity
@Table(catalog = "spring_project")
public class Car {

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

private String model;

@ManyToMany(cascade=CascadeType.ALL, fetch = FetchType.LAZY)
@JoinTable( name = "car_color", catalog = "spring_project",
            joinColumns         = { @JoinColumn(name = "car_id") },
            inverseJoinColumns  = { @JoinColumn(name = "colors_id") }
)
private List<Color> colors = new ArrayList<>();

@ManyToOne(cascade=CascadeType.ALL, fetch = FetchType.LAZY)
@JoinColumn(name="brand_id", referencedColumnName="id")
private Brand brand;

@Entity
@Table(catalog = "spring_project")
public class Brand {

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

private String name;

@OneToMany(mappedBy = "brand", fetch = FetchType.LAZY)
private List<Car> cars = new ArrayList<>();

@Entity
@Table(catalog = "spring_project")
public class Color {

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

如果我像Eager一样获取，一切都运行得很好，但我知道这是一种不好的做法，应该使用懒惰加载。但我一直得到LazyInitializationException。

我从错误中了解到需要一个会话，但我不知道如何提供一个，因为我使用的是Spring Data JPA，也不应该在哪里声明一个...

@SpringBootApplication
public class SrpingJpaApplication {

private static final Logger log = 
LoggerFactory.getLogger(SrpingJpaApplication.class);

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

@Bean
public CommandLineRunner demo(CarRepository carRepository,
                              ColorRepository colorRepository,
                              BrandRepository brandRepository) {
    return (args) -> {

        log.info("Reads all cars....");
        for (Car c : carRepository.findAll()) {
            System.out.println(c.toString());
        }

    };
}

在c.toString（）上抛出错误；

错误：由：org.hibernate.lazyInitializationException引起：无法初始化代理[com.readiness.moita.srpingjpa.models.brand#1]-没有会话

Answer 1

因为brand的fetchtype是惰性的，所以它不会通过调用fetchall()自动加载到会话中。要将其自动加载到会话中，您需要：

改变

@ManyToOne(cascade=CascadeType.ALL, fetch = FetchType.LAZY)
@JoinColumn(name="brand_id", referencedColumnName="id")
private Brand brand;

到

@ManyToOne(cascade=CascadeType.ALL, fetch = FetchType.EAGER)

@ManyToOne(cascade=CascadeType.ALL, fetch = FetchType.EAGER)
@JoinColumn(name="brand_id", referencedColumnName="id")
private Brand brand;

@Component
public CarService implements ICarService {

    @Autowired
    CarRepository carRepository;

    @Transactional
    public void printAllCars() {
        for (Car c : carRepository.findAll()) {
            System.out.println(c.toString());
        }
    }   
}