MS Lesson12: LazyInitializationException, n + 1 problem


1. LazyInitializationException


Hibernate session/persistence context artiq baglidirsa Hibernate child-i yukleye bilmir ve LazyInitializationException atir. 


package guru.springframework.cruddemo.entity;

import jakarta.persistence.CascadeType;
import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.FetchType;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.OneToMany;
import jakarta.persistence.OneToOne;
import jakarta.persistence.Table;
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;
import lombok.ToString;

import java.util.ArrayList;
import java.util.List;

@Getter
@Setter
@Entity
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
@Table(name = "persons")
public class Person {

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

    @Column(nullable = false)
    private String name;

    @Column(nullable = false, unique = true)
    private String email;

//    @OneToOne(mappedBy = "person", cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.LAZY)
//    @OneToOne(mappedBy = "person", orphanRemoval = true)
//    private Address address;

    @Builder.Default
    @OneToMany(mappedBy = "person", cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.EAGER)
    private List<Car> cars = new ArrayList<>();

}



package guru.springframework.cruddemo.entity;

import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.FetchType;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.JoinColumn;
import jakarta.persistence.OneToOne;
import jakarta.persistence.Table;
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;
import lombok.ToString;

@Getter
@Setter
@Entity
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
@Table(name = "addresses")
public class Address {

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

    @Column(nullable = false)
    private String country;

    @Column(nullable = false)
    private String city;

//    @ToString.Exclude
//    @OneToOne(optional = false)
//    @JoinColumn(name = "person_id", nullable = false, unique = true)
//    private Person person;

}


package guru.springframework.cruddemo.entity;

import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.FetchType;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.JoinColumn;
import jakarta.persistence.ManyToOne;
import jakarta.persistence.Table;
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;
import lombok.ToString;

@Getter
@Setter
@Entity
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
@Table(name = "cars")
public class Car {

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

    @Column(nullable = false)
    private String brand;

    @Column(nullable = false)
    private String model;

    @ToString.Exclude
    @ManyToOne(optional = false, fetch = FetchType.LAZY)
    @JoinColumn(name = "person_id", nullable = false)
    private Person person;

}


package guru.springframework.cruddemo;

import guru.springframework.cruddemo.entity.Person;
import guru.springframework.cruddemo.repository.PersonRepository;
import lombok.RequiredArgsConstructor;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import java.util.List;

@SpringBootApplication
@RequiredArgsConstructor
public class CrudDemoApplication implements CommandLineRunner {

    private final PersonRepository repository;

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

    @Override
    public void run(String... args) {
        List<Person> all = repository.findAll();

    }

}



LazyInitializationException-u aradan qaldirmaq ucun ilk variant fetch type EAGER etmekdir. Lakin bu yaxshi hell sayilmir. Cunki bize child entity(ler) lazim olmaya biler ve boshuna cagirmish olacayiq.


Ikinci yox ise @Transactional ishletmekdir, lakin bu da oz novbesinde yaxshi hell sayilmir. Lazy problemini gizletmek ucun bu hellin istifadesi yolverilmezdir, hemcinin tranzaksiya boyunca connection obyektini ishlek saxlamaq duzgun sayilmir.


Ucuncu yox - native query yazmaqdir, lakin bu dolayi hell demekdir. Bu zaman bize projection veya dto lazim olur:

package guru.springframework.cruddemo.projection;

import java.math.BigDecimal;
import java.time.Instant;

public interface PersonPassportRow {

    Long getPersonId();

    String getPersonName();

    String getPersonEmail();

    BigDecimal getScholarship();

    Instant getCreatedAt();

    Long getPassportId();

    String getPassportPin();

}


package guru.springframework.cruddemo.dto;

import java.math.BigDecimal;
import java.time.Instant;

public record PersonPassportDto(
        Long personId,
        String personName,
        String personEmail,
        BigDecimal scholarship,
        Instant createdAt,
        Long passportId,
        String passportPin
) {
}


package guru.springframework.cruddemo.repository;

import guru.springframework.cruddemo.entity.Person;
import guru.springframework.cruddemo.projection.PersonPassportRow;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;

import java.util.List;
import java.util.Optional;

public interface PersonRepository extends JpaRepository<Person, Long> {

    @Query(value = """
            select p.id as personId,
                   p.name as personName,
                   p.email as personEmail,
                   p.scholarship as scholarship,
                   p.created_at as createdAt,
                   pp.id as passportId,
                   pp.pin as passportPin
            from person p
            left join passport pp on pp.person_id = p.id
            where p.id = :personId
            """, nativeQuery = true)
    Optional<PersonPassportRow> findPersonWithPassportNative(Long personId);

}


package guru.springframework.cruddemo.service.impl;

import guru.springframework.cruddemo.dto.PersonPassportDto;
import guru.springframework.cruddemo.entity.Parent;
import guru.springframework.cruddemo.projection.PersonPassportRow;
import guru.springframework.cruddemo.repository.ParentRepository;
import guru.springframework.cruddemo.repository.PersonRepository;
import guru.springframework.cruddemo.service.PersonService;
import lombok.RequiredArgsConstructor;
import org.springframework.stereotype.Service;

@Service
@RequiredArgsConstructor
public class PersonServiceImpl implements PersonService {

    private final PersonRepository repository;
    private final ParentRepository parentRepository;

    @Override
    public Parent findById(Long id) {
        return parentRepository.findById(id).orElseThrow();
    }

    @Override
    public PersonPassportDto findPersonWithPassportNative(Long personId) {
        PersonPassportRow row = repository.findPersonWithPassportNative(personId).orElseThrow();
        return new PersonPassportDto(
                row.getPersonId(),
                row.getPersonName(),
                row.getPersonEmail(),
                row.getScholarship(),
                row.getCreatedAt(),
                row.getPassportId(),
                row.getPassportPin()
        );
    }

}



Dorduncu yol - join fetch.

@Query("""
        select p
        from Person p
        join fetch p.passport
        where p.id = :personId
        """)
Optional<Person> findByIdWithPassportJoinFetch(Long personId);


Beshinci yol EntityGraph.

package guru.springframework.cruddemo.repository;

import guru.springframework.cruddemo.entity.Parent;
import org.springframework.data.jpa.repository.EntityGraph;
import org.springframework.data.jpa.repository.JpaRepository;

import java.util.Optional;

public interface ParentRepository extends JpaRepository<Parent, Long> {

    @EntityGraph(attributePaths = {"child"})
    Optional<Parent> findById(Long id);

}


veya 

package guru.springframework.cruddemo.repository;

import guru.springframework.cruddemo.entity.Parent;
import org.springframework.data.jpa.repository.EntityGraph;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;

import java.util.Optional;

public interface ParentRepository extends JpaRepository<Parent, Long> {

    @EntityGraph(attributePaths = {"child"})
    @Query("select p from Parent p where p.id = :id")
    Optional<Parent> foo(Long id);

}


Altinci yol NamedEntityGraph.

package guru.springframework.cruddemo.entity;

import jakarta.persistence.Entity;
import jakarta.persistence.FetchType;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.JoinColumn;
import jakarta.persistence.NamedAttributeNode;
import jakarta.persistence.NamedEntityGraph;
import jakarta.persistence.OneToOne;
import lombok.Getter;
import lombok.RequiredArgsConstructor;
import lombok.Setter;
import lombok.ToString;
import org.hibernate.proxy.HibernateProxy;

import java.util.Objects;

@Getter
@Setter
@ToString
@RequiredArgsConstructor
@Entity
@NamedEntityGraph(
        name = "parent-with-child",
        attributeNodes = @NamedAttributeNode("child")
)
public class Parent {

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

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "child_id")
    private Child child;

    @Override
    public final boolean equals(Object o) {
        if (this == o) return true;
        if (o == null) return false;
        Class<?> oEffectiveClass = o instanceof HibernateProxy ? ((HibernateProxy) o).getHibernateLazyInitializer().getPersistentClass() : o.getClass();
        Class<?> thisEffectiveClass = this instanceof HibernateProxy ? ((HibernateProxy) this).getHibernateLazyInitializer().getPersistentClass() : this.getClass();
        if (thisEffectiveClass != oEffectiveClass) return false;
        Parent parent = (Parent) o;
        return getId() != null && Objects.equals(getId(), parent.getId());
    }

    @Override
    public final int hashCode() {
        return this instanceof HibernateProxy ? ((HibernateProxy) this).getHibernateLazyInitializer().getPersistentClass().hashCode() : getClass().hashCode();
    }
}


package guru.springframework.cruddemo.repository;

import guru.springframework.cruddemo.entity.Parent;
import org.springframework.data.jpa.repository.EntityGraph;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;

import java.util.Optional;

public interface ParentRepository extends JpaRepository<Parent, Long> {

    @EntityGraph(value = "parent-with-child")
    @Query("select p from Parent p where p.id = :id")
    Optional<Parent> foo(Long id);

}




N + 1 problemi nedir?

1 sorgu ile esas obyekti getiririk bu zaman n eded netice gelir, sonra ise child-lari getirmek ucun her bir entity ucun 1 sorgu atilir, netice etibari ile 1 + n sorgu alinir. 

Mes: 1 query ile 100 Person getiririk, ve her Person ucun ayrica Passport cekirik. toplam 1 + 100 = 101 query alinir.


package guru.springframework.cruddemo;

import guru.springframework.cruddemo.entity.Person;
import guru.springframework.cruddemo.repository.PersonRepository;
import lombok.RequiredArgsConstructor;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.transaction.annotation.Transactional;

import java.util.List;

@SpringBootApplication
@RequiredArgsConstructor
public class CrudDemoApplication implements CommandLineRunner {

    private final PersonRepository repository;

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

    @Override
    @Transactional(readOnly = true)
    public void run(String... args) throws Exception {
        List<Person> all = repository.findAll(); // 1 query
        for (Person person : all) {
            int courseCount = person.getCourses().size(); // N extra queries (one per person)
            System.out.println("personId=" + person.getId() + ", courseCount=" + courseCount);
        }
    }

}




N + 1 problemini hell etmek ucun bir nece usul var.

1. join fetch

2. entity graph

3. named entity graph

4. dto/projection

5. batch fetch

spring:
  datasource:
    url: jdbc:postgresql://${DB_HOST:localhost}:${DB_PORT:7777}/${DB_NAME:crud}
    username: ${DB_USERNAME:crud}
    password: ${DB_PASSWORD:crud}

    hikari:
      pool-name: MyPostgresHikariCP
      maximum-pool-size: 100
      minimum-idle: 10          # Lowered this so Hikari has "extra" connections to kill
      idle-timeout: 5000         # 10 seconds (time an idle connection survives)
      max-lifetime: 5000         # 30 seconds (minimum allowed by Hikari)
      connection-timeout: 20000
      auto-commit: true      # Default is true; Spring @Transactional overrides this



  jpa:
    open-in-view: false
    hibernate:
      ddl-auto: update
    show-sql: true
    properties:
      hibernate:
        dialect: org.hibernate.dialect.PostgreSQLDialect
        default_batch_fetch_size: 50


package guru.springframework.cruddemo;

import guru.springframework.cruddemo.entity.Person;
import guru.springframework.cruddemo.repository.PersonRepository;
import lombok.RequiredArgsConstructor;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.transaction.annotation.Transactional;

import java.util.List;

@SpringBootApplication
@RequiredArgsConstructor
public class CrudDemoApplication implements CommandLineRunner {

    private final PersonRepository repository;

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

    @Override
    @Transactional(readOnly = true)
    public void run(String... args) throws Exception {
        System.out.println("=== findAll + lazy courses with batch fetch ===");
        List<Person> all = repository.findAll(); // 1 query
        for (Person person : all) {
            int courseCount = person.getCourses().size();
            System.out.println("personId=" + person.getId() + ", courseCount=" + courseCount);
        }
    }

}





























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