Complete and Categorized JPA Notes with Detailed Explanation and Code Examples

🧩 1. Overview of JPA

• JPA (Java Persistence API) is a specification to manage relational data in Java applications.
• It provides a bridge between the object-oriented domain model and the relational database.
• Common implementations: Hibernate, EclipseLink, OpenJPA.

🏷️ 2. Entity Annotations

Annotation	Purpose
@Entity	Marks a class as a JPA entity
@Table(name = "table_name")	Optional table name
@Id	Primary key
@GeneratedValue(strategy = GenerationType.AUTO)	Auto generation of primary key
@Column(name = "col_name")	Customize column mapping
@Transient	Field not persisted
@Lob	For large binary/text data (e.g., BLOB)
@Embedded	For embeddable classes
@JoinColumn(name = "col")	Foreign key column for relationships

🔗 3. Entity Relationships

One-to-One

Example: A user has exactly one profile. Both share a one-to-one relationship.

@Entity
public class User {
    @Id
    @GeneratedValue
    private Long id;

    private String username;

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "profile_id") // foreign key in User table
    private Profile profile;
}

@Entity
public class Profile {
    @Id
    @GeneratedValue
    private Long id;

    private String address;
    private String phone;
}

One-to-Many

Example: A user can place many orders. This is a one-to-many relationship.

@Entity
public class User {
    @Id
    @GeneratedValue
    private Long id;

    private String username;

    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
    private List<Order> orders = new ArrayList<>();
}

@Entity
public class Order {
    @Id
    @GeneratedValue
    private Long id;

    private String item;

    @ManyToOne
    @JoinColumn(name = "user_id") // foreign key in Order table
    private User user;
}

Many-to-One

Example: Each order is placed by one user — a many-to-one mapping (inverse of one-to-many).

@ManyToOne
@JoinColumn(name = "user_id")
private User user;

Many-to-Many

Example: A user can have multiple roles and a role can belong to multiple users.

@Entity
public class User {
    @Id
    @GeneratedValue
    private Long id;

    private String username;

    @ManyToMany
    @JoinTable(
        name = "user_role",
        joinColumns = @JoinColumn(name = "user_id"),
        inverseJoinColumns = @JoinColumn(name = "role_id"))
    private Set<Role> roles = new HashSet<>();
}

@Entity
public class Role {
    @Id
    @GeneratedValue
    private Long id;

    private String name;

    @ManyToMany(mappedBy = "roles")
    private Set<User> users = new HashSet<>();
}

🔍 4. Queries

JPQL

@Query("SELECT u FROM User u WHERE u.status = 1")
List<User> findAllActiveUsers();

Native Query

@Query(value = "SELECT * FROM users WHERE status = 1", nativeQuery = true)
List<User> findAllActiveUsersNative();

Named Query

@Entity
@NamedQuery(name = "User.findByEmail", query = "SELECT u FROM User u WHERE u.email = ?1")
public class User {}

📚 5. Pagination & Sorting

Page<User> findAll(Pageable pageable);
List<User> findAll(Sort sort);

@Query("SELECT u FROM User u ORDER BY id")
Page<User> findAllWithPagination(Pageable pageable);

Example usage:

@Entity
public class User {
    @Id
    @GeneratedValue
    private Long id;

    private String name;
    private String email;
    private int age;
}

//in repository 
public interface UserRepository extends JpaRepository<User, Long> {
    
    // Custom with pagination and sorting
    Page<User> findByAgeGreaterThan(int age, Pageable pageable);

    // Custom sorting using derived query
    List<User> findByNameContaining(String keyword, Sort sort);
}

//in service / controller

@Autowired
private UserRepository userRepository;
public Page<User> getUsersPaged(int page, int size) {
    Pageable pageable = PageRequest.of(page, size);
    return userRepository.findAll(pageable);
}

//pagination with sorting
public Page<User> getSortedUsersPaged(int page, int size, String sortBy, boolean desc) {
    Sort sort = desc ? Sort.by(sortBy).descending() : Sort.by(sortBy).ascending();
    Pageable pageable = PageRequest.of(page, size, sort);
    return userRepository.findAll(pageable);
}

//example controller
@GetMapping("/users")
public Page<User> getUsers(
    @RequestParam(defaultValue = "0") int page,
    @RequestParam(defaultValue = "5") int size,
    @RequestParam(defaultValue = "name") String sortBy,
    @RequestParam(defaultValue = "false") boolean desc) {

    Sort sort = desc ? Sort.by(sortBy).descending() : Sort.by(sortBy).ascending();
    Pageable pageable = PageRequest.of(page, size, sort);
    return userRepository.findAll(pageable);
}

// sample api call
GET /users?page=0&size=5 → First 5 users
GET /users?page=1&size=5&sortBy=age&desc=true → Second page sorted by age in descending order

🧠 6. Common Query Keywords

Keyword	Example
And	findByFirstNameAndLastName
Or	findByAgeOrName
Between	findByAgeBetween(20, 30)
LessThan	findByAgeLessThan(18)
Like	findByEmailLike("%gmail.com")
StartingWith	findByNameStartingWith("A")
In	findByIdIn(List<Long> ids)
IsNull	findByEmailIsNull()
True/False	findByActiveTrue()

🛠️ 7. Modifying Queries (Update, Delete)

@Modifying
@Query("UPDATE User u SET u.status = :status WHERE u.name = :name")
int updateStatus(@Param("status") Integer status, @Param("name") String name);

🔁 8. Entity Graphs (For Performance Tuning)

@Entity
@NamedEntityGraph(name = "Group.detail", attributeNodes = @NamedAttributeNode("members"))
public class Group {
  @ManyToMany
  List<Member> members;
}

Repository usage:

@EntityGraph(value = "Group.detail", type = EntityGraph.EntityGraphType.LOAD)
Group findByName(String name);

🚀 9. Cascade Types

CascadeType	Description
PERSIST	Child saved automatically
MERGE	Merge changes to children
REMOVE	Delete children too
REFRESH	Refresh child state
DETACH	Detach child from session
ALL	Applies all of the above

@OneToMany(cascade = CascadeType.ALL)
private List<Order> orders;

🗃️ 10. Fetch Types

Type	Description
LAZY	Data loaded on-demand (recommended)
EAGER	Data loaded immediately

@OneToMany(fetch = FetchType.LAZY)
private List<Order> orders;

🔢 11. Indexed and Named Parameters

Indexed

@Query("SELECT u FROM User u WHERE u.status = ?1 AND u.name = ?2")
User findByStatusAndName(Integer status, String name);

Named

@Query("SELECT u FROM User u WHERE u.status = :status AND u.name = :name")
User findByStatusAndNameNamed(@Param("status") Integer status, @Param("name") String name);

🆔 12. Natural IDs and UUIDs

Natural ID (e.g., Email, SSN)

@NaturalId
@Column(unique = true)
private String email;

UUID as Primary Key

@Id
@GeneratedValue(generator = "uuid")
@GenericGenerator(name = "uuid", strategy = "uuid2")
private String id;

📌 13. Additional Best Practices

• Use @Repository to enable Spring Data exception translation.
• Prefer DTOs over exposing entities in REST responses.
• Use @EntityGraph for optimizing performance instead of EAGER fetch.

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