Java Notes

Spring Data JPA

Complete guide to Spring Data JPA - entity mapping, repository interfaces, query methods, JPQL, native queries, relationships, and database operations.

What is Spring Data JPA?

Spring Data JPA is a layer on top of JPA (Java Persistence API) that eliminates boilerplate database code. You define repository interfaces, and Spring automatically generates the implementation.

Entity Mapping

java example

@Entity
@Table(name = "students")
@Data
@NoArgsConstructor
@AllArgsConstructor
@Builder
public class Student {

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

    @Column(nullable = false, length = 100)
    private String name;

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

    @Column(name = "phone_number")
    private String phone;

    @Enumerated(EnumType.STRING)
    private Gender gender;

    @Column(name = "date_of_birth")
    private LocalDate dateOfBirth;

    @Column(name = "created_at", updatable = false)
    private LocalDateTime createdAt;

    @Column(name = "updated_at")
    private LocalDateTime updatedAt;

    @PrePersist
    protected void onCreate() {
        createdAt = LocalDateTime.now();
        updatedAt = LocalDateTime.now();
    }

    @PreUpdate
    protected void onUpdate() {
        updatedAt = LocalDateTime.now();
    }

    public enum Gender {
        MALE, FEMALE, OTHER
    }
}

Repository Interface

java example

@Repository
public interface StudentRepository extends JpaRepository<Student, Long> {

    // Derived query methods (Spring generates SQL automatically)
    Optional<Student> findByEmail(String email);
    List<Student> findByName(String name);
    List<Student> findByGender(Student.Gender gender);
    List<Student> findByNameContainingIgnoreCase(String keyword);
    List<Student> findByAgeGreaterThan(int age);
    List<Student> findByCreatedAtAfter(LocalDateTime date);
    List<Student> findByNameAndEmail(String name, String email);
    List<Student> findByGenderOrderByNameAsc(Student.Gender gender);
    boolean existsByEmail(String email);
    long countByGender(Student.Gender gender);
    void deleteByEmail(String email);

    // Pagination and Sorting
    Page<Student> findByGender(Student.Gender gender, Pageable pageable);

    // JPQL Queries
    @Query("SELECT s FROM Student s WHERE s.email LIKE %:domain")
    List<Student> findByEmailDomain(@Param("domain") String domain);

    @Query("SELECT s FROM Student s WHERE YEAR(s.dateOfBirth) = :year")
    List<Student> findByBirthYear(@Param("year") int year);

    @Query("SELECT s.gender, COUNT(s) FROM Student s GROUP BY s.gender")
    List<Object[]> countByGenderGrouped();

    // Native SQL Queries
    @Query(value = "SELECT * FROM students WHERE TIMESTAMPDIFF(YEAR, date_of_birth, CURDATE()) > :age",
           nativeQuery = true)
    List<Student> findOlderThan(@Param("age") int age);

    // Update query
    @Modifying
    @Transactional
    @Query("UPDATE Student s SET s.email = :email WHERE s.id = :id")
    int updateEmail(@Param("id") Long id, @Param("email") String email);
}

JPA Relationships

One-to-Many / Many-to-One

java example

@Entity
@Table(name = "departments")
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @OneToMany(mappedBy = "department", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<Employee> employees = new ArrayList<>();
}

@Entity
@Table(name = "employees")
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "department_id")
    private Department department;
}

Many-to-Many

java example

@Entity
public class Student {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @ManyToMany
    @JoinTable(
        name = "student_courses",
        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;
    private String title;

    @ManyToMany(mappedBy = "courses")
    private Set<Student> students = new HashSet<>();
}

One-to-One

java example

@Entity
public class User {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String username;

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "profile_id")
    private UserProfile profile;
}

Service Layer with JPA

java example

@Service
@RequiredArgsConstructor
@Transactional(readOnly = true)
public class StudentService {

    private final StudentRepository studentRepository;

    public List<Student> getAllStudents() {
        return studentRepository.findAll();
    }

    public Page<Student> getStudentsPaginated(int page, int size, String sortBy) {
        Pageable pageable = PageRequest.of(page, size, Sort.by(sortBy));
        return studentRepository.findAll(pageable);
    }

    public Student getStudentById(Long id) {
        return studentRepository.findById(id)
                .orElseThrow(() -> new ResourceNotFoundException("Student not found: " + id));
    }

    @Transactional
    public Student createStudent(Student student) {
        if (studentRepository.existsByEmail(student.getEmail())) {
            throw new DuplicateResourceException("Email already exists");
        }
        return studentRepository.save(student);
    }

    @Transactional
    public Student updateStudent(Long id, Student details) {
        Student student = getStudentById(id);
        student.setName(details.getName());
        student.setEmail(details.getEmail());
        student.setPhone(details.getPhone());
        return studentRepository.save(student);
    }

    @Transactional
    public void deleteStudent(Long id) {
        Student student = getStudentById(id);
        studentRepository.delete(student);
    }
}

Derived Query Method Keywords

Keyword	Example	SQL Equivalent
findBy	findByName(name)	WHERE name = ?
findByNot	findByNameNot(name)	WHERE name != ?
And	findByNameAndAge(n, a)	WHERE name=? AND age=?
Or	findByNameOrEmail(n, e)	WHERE name=? OR email=?
Between	findByAgeBetween(a, b)	WHERE age BETWEEN ? AND ?
LessThan	findByAgeLessThan(age)	WHERE age < ?
GreaterThan	findByAgeGreaterThan(age)	WHERE age > ?
Like	findByNameLike(pattern)	WHERE name LIKE ?
Containing	findByNameContaining(str)	WHERE name LIKE %?%
StartingWith	findByNameStartingWith(s)	WHERE name LIKE ?%
OrderBy	findByGenderOrderByName()	ORDER BY name
IsNull	findByPhoneIsNull()	WHERE phone IS NULL
In	findByAgeIn(List ages)	WHERE age IN (?,?,?)
Top/First	findTop5ByOrderByAge()	LIMIT 5

Interview Key Points

Spring Data JPA generates repository implementations automatically
JpaRepository extends PagingAndSortingRepository extends CrudRepository
Method names are parsed into queries (findByNameAndEmail → WHERE name=? AND email=?)
@Query for complex JPQL/native queries
@Modifying + @Transactional required for UPDATE/DELETE queries
FetchType.LAZY loads related entities on access; EAGER loads immediately
cascade = CascadeType.ALL propagates operations to related entities
@PrePersist / @PreUpdate for automatic timestamps

Summary

In this chapter, we learned about Spring Data JPA in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Spring Data JPA.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, spring, framework, boot

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Java Notes

Spring Data JPA

Complete guide to Spring Data JPA - entity mapping, repository interfaces, query methods, JPQL, native queries, relationships, and database operations.

What is Spring Data JPA?

Spring Data JPA is a layer on top of JPA (Java Persistence API) that eliminates boilerplate database code. You define repository interfaces, and Spring automatically generates the implementation.

Entity Mapping

java example

@Entity
@Table(name = "students")
@Data
@NoArgsConstructor
@AllArgsConstructor
@Builder
public class Student {

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

    @Column(nullable = false, length = 100)
    private String name;

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

    @Column(name = "phone_number")
    private String phone;

    @Enumerated(EnumType.STRING)
    private Gender gender;

    @Column(name = "date_of_birth")
    private LocalDate dateOfBirth;

    @Column(name = "created_at", updatable = false)
    private LocalDateTime createdAt;

    @Column(name = "updated_at")
    private LocalDateTime updatedAt;

    @PrePersist
    protected void onCreate() {
        createdAt = LocalDateTime.now();
        updatedAt = LocalDateTime.now();
    }

    @PreUpdate
    protected void onUpdate() {
        updatedAt = LocalDateTime.now();
    }

    public enum Gender {
        MALE, FEMALE, OTHER
    }
}

Repository Interface

java example

@Repository
public interface StudentRepository extends JpaRepository<Student, Long> {

    // Derived query methods (Spring generates SQL automatically)
    Optional<Student> findByEmail(String email);
    List<Student> findByName(String name);
    List<Student> findByGender(Student.Gender gender);
    List<Student> findByNameContainingIgnoreCase(String keyword);
    List<Student> findByAgeGreaterThan(int age);
    List<Student> findByCreatedAtAfter(LocalDateTime date);
    List<Student> findByNameAndEmail(String name, String email);
    List<Student> findByGenderOrderByNameAsc(Student.Gender gender);
    boolean existsByEmail(String email);
    long countByGender(Student.Gender gender);
    void deleteByEmail(String email);

    // Pagination and Sorting
    Page<Student> findByGender(Student.Gender gender, Pageable pageable);

    // JPQL Queries
    @Query("SELECT s FROM Student s WHERE s.email LIKE %:domain")
    List<Student> findByEmailDomain(@Param("domain") String domain);

    @Query("SELECT s FROM Student s WHERE YEAR(s.dateOfBirth) = :year")
    List<Student> findByBirthYear(@Param("year") int year);

    @Query("SELECT s.gender, COUNT(s) FROM Student s GROUP BY s.gender")
    List<Object[]> countByGenderGrouped();

    // Native SQL Queries
    @Query(value = "SELECT * FROM students WHERE TIMESTAMPDIFF(YEAR, date_of_birth, CURDATE()) > :age",
           nativeQuery = true)
    List<Student> findOlderThan(@Param("age") int age);

    // Update query
    @Modifying
    @Transactional
    @Query("UPDATE Student s SET s.email = :email WHERE s.id = :id")
    int updateEmail(@Param("id") Long id, @Param("email") String email);
}

JPA Relationships

One-to-Many / Many-to-One

java example

@Entity
@Table(name = "departments")
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @OneToMany(mappedBy = "department", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<Employee> employees = new ArrayList<>();
}

@Entity
@Table(name = "employees")
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "department_id")
    private Department department;
}

Many-to-Many

java example

@Entity
public class Student {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @ManyToMany
    @JoinTable(
        name = "student_courses",
        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;
    private String title;

    @ManyToMany(mappedBy = "courses")
    private Set<Student> students = new HashSet<>();
}

One-to-One

java example

@Entity
public class User {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String username;

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "profile_id")
    private UserProfile profile;
}

Service Layer with JPA

java example

@Service
@RequiredArgsConstructor
@Transactional(readOnly = true)
public class StudentService {

    private final StudentRepository studentRepository;

    public List<Student> getAllStudents() {
        return studentRepository.findAll();
    }

    public Page<Student> getStudentsPaginated(int page, int size, String sortBy) {
        Pageable pageable = PageRequest.of(page, size, Sort.by(sortBy));
        return studentRepository.findAll(pageable);
    }

    public Student getStudentById(Long id) {
        return studentRepository.findById(id)
                .orElseThrow(() -> new ResourceNotFoundException("Student not found: " + id));
    }

    @Transactional
    public Student createStudent(Student student) {
        if (studentRepository.existsByEmail(student.getEmail())) {
            throw new DuplicateResourceException("Email already exists");
        }
        return studentRepository.save(student);
    }

    @Transactional
    public Student updateStudent(Long id, Student details) {
        Student student = getStudentById(id);
        student.setName(details.getName());
        student.setEmail(details.getEmail());
        student.setPhone(details.getPhone());
        return studentRepository.save(student);
    }

    @Transactional
    public void deleteStudent(Long id) {
        Student student = getStudentById(id);
        studentRepository.delete(student);
    }
}

Derived Query Method Keywords

Keyword	Example	SQL Equivalent
findBy	findByName(name)	WHERE name = ?
findByNot	findByNameNot(name)	WHERE name != ?
And	findByNameAndAge(n, a)	WHERE name=? AND age=?
Or	findByNameOrEmail(n, e)	WHERE name=? OR email=?
Between	findByAgeBetween(a, b)	WHERE age BETWEEN ? AND ?
LessThan	findByAgeLessThan(age)	WHERE age < ?
GreaterThan	findByAgeGreaterThan(age)	WHERE age > ?
Like	findByNameLike(pattern)	WHERE name LIKE ?
Containing	findByNameContaining(str)	WHERE name LIKE %?%
StartingWith	findByNameStartingWith(s)	WHERE name LIKE ?%
OrderBy	findByGenderOrderByName()	ORDER BY name
IsNull	findByPhoneIsNull()	WHERE phone IS NULL
In	findByAgeIn(List ages)	WHERE age IN (?,?,?)
Top/First	findTop5ByOrderByAge()	LIMIT 5

Interview Key Points

Spring Data JPA generates repository implementations automatically
JpaRepository extends PagingAndSortingRepository extends CrudRepository
Method names are parsed into queries (findByNameAndEmail → WHERE name=? AND email=?)
@Query for complex JPQL/native queries
@Modifying + @Transactional required for UPDATE/DELETE queries
FetchType.LAZY loads related entities on access; EAGER loads immediately
cascade = CascadeType.ALL propagates operations to related entities
@PrePersist / @PreUpdate for automatic timestamps

Summary

In this chapter, we learned about Spring Data JPA in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Spring Data JPA.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, spring, framework, boot

Continue learning this concept

PreviousSpring Boot Introduction NextValidation in Spring Boot