Entity Class and JPA Repositroy

1. Entity Class

• A POJO that represents a table in the database.
• Fields are mapped to columns in the table.
• Class must have:
  • A no-arg constructor
  • An @Id field (primary key)
• @Entity
  • Table name defaults to class name (can customize with @Table).
  • Part of JPA (javax.persistence.Entity).
  • Required for Hibernate to manage the class.

@Id : Primary key of the entity.

@Column : Maps field to DB column.

@Table : Maps entity to DB table.

@GeneratedValue

used with @Id to specify how the primary key should be generated. Strategies

• AUTO : JPA chooses strategy based on DB dialect.

  • MySQL : uses AUTO_INCREMENT (IDENTITY).
  • PostgreSQL : uses SEQUENCE.
  • Oracle : uses SEQUENCE

• IDENTITY :

  • Uses DB auto-increment column.
  • ID manage by DB.

• TABLE : uses an extra intermediate table for ID generation.

  Here,

  • sequence_name = 'User' identifies the sequence for the User entity.
  • next_val is the next id Hibernate will assign.

• Entity with @TableGenerator
  

  Example Code

     @Entity
     public class User {
  
       @Id
       @GeneratedValue(strategy = GenerationType.TABLE, generator = "user_table_gen")
       @TableGenerator(
         name = "user_table_gen",           // generator name for Hibernate
         table = "hibernate_sequences",     // table to store sequence values
         pkColumnName = "sequence_name",    // column that stores sequence names
         valueColumnName = "next_val",      // column that stores next value
         pkColumnValue = "User",            // sequence name for this entity
         initialValue = 1,                  // starting value
         allocationSize = 1                 // increment per call
       )
       private int id;
       private String name;
       private String email;
     }
  

• SEQUENCE

  • Uses a database sequence to generate unique IDs.
  • Supported by Oracle, PostgreSQL, DB2, etc.

• Entity with @SequenceGenerator

  Step 1: Create the sequence in DB

    -- PostgreSQL / Oracle
    CREATE SEQUENCE user_seq
    START WITH 1
    INCREMENT BY 1
  

  here ,

  • user_seq : sequence name.
  • START WITH 1 : first ID = 1
  • INCREMENT BY 1 : each call increases by 1

  Step 2: Use @SequenceGenerator in Hibernate

Code

   @Entity
   public class User {

       @Id
       @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "user_seq_gen")
       @SequenceGenerator(
           name = "user_seq_gen",      // generator name in Hibernate
           sequenceName = "user_seq", // database sequence name
           allocationSize = 1        // increment per Hibernate call
       )
       private int id;
       private String name;
       private String email;
   }

without use of database sequence name

@Entity
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "user_seq_gen")
    @SequenceGenerator(
        name = "user_seq_gen", // generator name in Hibernate
        initialValue = 10,    // starting value of the sequence
        allocationSize = 1   // increment per Hibernate call
    )
    private int id;
    private String name;
    private String email;
}

Entity class Example

Example Code

  @Table(name="user-table")
  public class User{

    @id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name="sid")
    private int id;

    @Column
    private String name;
    //Getters and Setters 
  }

2. JPA Repository

Pre-Defind interfaces provied by Spring JPA module.

There are two types intefaces avaiable :

1. CrudRepository (basic CRUD) : Parent of JpaRepository
2. JpaRepository (CRUD + extra features like pagination, sorting).

Question: How to create JPA Repository?

• Create an interfaces by extending CrudRepository or JpaRepository .

CrudRepository and JpaRepository

provides database operations .

• Part of Spring Data JPA.
• Provides CRUD operations for entities.
• After extends, Spring auto-generates implementation (no SQL/DAO needed).
• Common Method: save(), findAll(), findById(), deleteById()

Example:

JpaRepository<T, ID>

Here

• T : Entity class name
• ID :
  • Data type of identifier column of entity class.
  • use wrapper class (Integer for int ,Double for double)

Example: JPA Repository

public interface StudentRepository extends JpaRepository<User, Integer> {
    //logic here 
}
public interface StudentRepository extends CrudRepository<User, Integer> {
    //logic here 
}

Here,

• StudentRepository manages the Student entity.
• JpaRepository<User, Integer> and CrudRepository<User, Integer> here :
  • User : entity type.
  • Integer : type of the primary key.

3. UUID(Universally Unique Identifier)

• 128-bit value, usually represented as a 36-character string like: 550e8400-e29b-41d4-a716-446655440000

• Very useful for distributed systems where auto-increment IDs can clash.

• Using in Java

private UUID id = UUID.randomUUID();

1. Using in Hibernate

  @Entity
  public class User {
      @Id
      @GeneratedValue(strategy = GenerationType.UUID)
      private String id;
      private String name;
  }

1. Using in Column (MySql)

CREATE TABLE user (
    id VARCHAR(255) PRIMARY KEY,
    name VARCHAR(255)
);

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