"Welcome To Ashok IT"				
			                     "Spring Boot and MicroServices"
                                               Topic : Spring Data JPA - Generators
                                              	 Date  : 12/04/2025
                                                  (Session - 26)                  
_____________________________________________________________________________________________________________________________
Last Session
============

CrudRepoistory Interface Methods
================================
1) T save(T t) ::: This method is used for Saving given entity (or) Inserting Record.

2) Iterable<T> saveAll(Iterable<S> items) ::: This method is used for saving group of entities.

   NOTE 
   ====
   * Iterable is root interface of Collection interface and came in the version of Java 1.5

   * Iterable interface provides capability processing the collection object using "Enhanced For Loop" which came in the                    version of Java 1.5.

3) Optional<T> findById(Serializable id) ::: To retreive record based on the given primary key value.

   NOTE
   ====
   * Optional is an utility class from java8 Version which helps us to avoid the null checks in Java program.

   * Optional Class contains some utility methods to check whether object is present or not & We can get the object from                    optional object as below

       i) boolean isPresent()  

      ii) T get();

4) Iterable<T> findAllById(Iterable ids)  ::: To retreive records based on given primary key values.

5) Iterable<T> findAll() ::: To Retreive all the records from table.

6) void delete(T t) ::: Deleting record based on given entity.

7) void deleteById(Serializable id) ::: Deleting record based on given primary key

8) void deleteAllById(Iterable ids) ::: Deleting records based on given primary key values .

9) void deleteAll() ::: Deleting all records from table with out primary key

10) void deleteAll(Iterable<T> objects) ::: Deleting all records from table with out primary key

11) int count() :::: To get total records as count from table.

12) boolean existsById(Serializable id) ::: Check whether the record is existed or not for given primary key.

Today Session
==============
1) spring.jpa.show-sql
======================
* This property will displays underlying sql queries used by ORM Framework.

* By default this entry value as "false" means it will not display underlying SQL Queries on console, If we make it as true     means will displayed underlying sql queries on console

 Example
 =======
  spring.jpa.show-sql=false(default)
  spring.jpa.show-sql=true

OUTPUT
======
Hibernate: 
    select
        c1_0.customer_id,
        c1_0.contact_no,
        c1_0.email,
        c1_0.location,
        c1_0.name 
    from
        shopping_customers c1_0

2) spring.jpa.properties.hibernate.format_sql
=============================================
* This property is used to display underlying SQL Queries in structured format (or) Formatted.

* By default value for this entry is "false"

OUTPUT
======
Hibernate: select c1_0.customer_id,c1_0.contact_no,c1_0.email,c1_0.location,c1_0.name from shopping_customers c1_0

* If we change the value for above entry as "true"

OUTPUT
======
Hibernate: 
    select
        c1_0.customer_id,
        c1_0.contact_no,
        c1_0.email,
        c1_0.location,
        c1_0.name 
    from
        shopping_customers c1_0

3) spring.jpa.hibernate.ddl-auto
=================================
* In the earlier we just created the Database table manually through SQL Developer/SQL Command Line interface.

* When we are working With ORM We no need to create the Database Table manually, Instead of this we need to add                 configuration in  application.properties for creating Database table automatically during application execution.
  
  Example :
  =========
  spring.jpa.hibernate.ddl-auto=create,create-drop,update,validate,none

  1) create      ::: First will drop the existing the table and then always will create new table.

  2) create-drop ::: First will drop the existing the table and then will perform creating table and populating data will                           drop table immediately during startup only by observing log messages.

  3) validate    ::: Just Verifying the schema whether database tables present or not.

  4) update      ::: If table is not available then only will create otherwise it will reuse the existing table.

  5) none        ::: Programmer need to create the database table manually.

  Recommended value
  =================
  spring.jpa.hibernate.ddl-auto = update

* If we are providing the value for Primary key column means will have chance of getting "Constraint Violation exeception",     if user is trying to insert record with same primary key value.

* Inorder to overcome the above problem we have two solution

   1) Always primary key column value should be auto generate from Database side.

         Ex: Oracle(Sequence), MySQL(AutoIncrement)

Sequence Creation
=================
*** create sequence <sequence_name> start with <initial_value> increment by <increment_Value>

     Ex: create sequence demo_sequence start with 1000 increment by 1;

*** To select the next val from sequence need to use below command

     Ex: select demo_sequence.nextval from dual;

*** To select the current val from sequence need to use below command

     Ex: select demo_sequence.currval from dual;

*** Utilizing the Sequence while inserting the record into database table

     Ex: insert into shopping_customers values(demo_sequence.nextval,'test@gmail.com','Hyderabad','Test4');
         

   2) Always primary key column value should be auto generate through ORM Framework.
   
       Ex: Generators

Generators
==========
* Basically in application development will have multiple database tables.

* Each Database table will have mandatorly primary key column which is combination of two constraints i.e.,Unique + not null    constraint.
 
* While inserting records into database table, If end user supplied primary key value might have chance for entering            duplicate value for primary key column.

* To Overcome this problem ORM Framework provided the generators concept.

* Generators are used in application for generating the Primary key column value based on some mechanism supported by           database vendor.

* When we use generators in application before saving record primary key value will be generated with in application then  
  record will be inserted into table along with primary key.

* ORM Will support's the four types of Generators are as below 

       1) AUTO

       2) SEQUENCE

       3) IDENTITY

       4) TABLE

* We can use the following annotation @GeneratedValue helps for generating value for primary key based on strategy defined.

* @GeneratedValue annotation should be defined on top of primary key column field in entity class.

************************
1) GenerationType.AUTO
************************
>> This is the default generation type strategy for generating the primary key value of Database column.

>> We can define the auto strategy on top of the primary key field in Entity Class with below annotation
       
       @Id
       @GeneratedValue(strategy=GenerationType.AUTO)
       private Integer productId;

>> If we are using Oracle database internally Hibernate will create sequence database object with the name of                  hibernate_sequence.
       
   Ex:  create sequence hibernate_sequence starts with 1 and increment by 1;  >>>> SpringBoot 2.7.X version

        create sequence shopping_customer_seq start with 1 increment by 50 >>>> Spring Boot 3.X Table specific Sequence.

        create sequence mahesh_customers_seq start with 1 increment by 50  >>>>>> Spring Boot 3.X Table Specific Sequence.

>> Before inserting the record into database table Hibernate will select value for hibernate_sequence/table specific sequence    and insert into primary key column value.

     Ex:  select hibernate_sequence.nextval from dual;  >>>>>>> SpringBoot 2.7.x Version

          select shopping_customers_seq.nextval from dual; >>>>>>> SpringBoot 3.x Version

          insert into shopping_customers(?,?,?,?,?);

>> If we are using MYSQL database internally Hibernate Will create table with name of Hibernate_sequence because MySQL           doesn't supports the sequences concept and inorder to generate the primary key column value in MYSQL will have auto           increment facility.

       create table hibernate_sequence(
          next_val bigint;
       );

       insert into hibernate_sequence values(1);

NOTE
====
Hibernate: 
    create table shopping_customers (
        customer_id integer not null,
        contact_no varchar(255),
        email varchar(255),
        location varchar(255),
        name varchar(255),
        primary key (customer_id)
    ) engine=InnoDB

Hibernate: 
    create table shopping_customers_seq (
        next_val bigint
    ) engine=InnoDB

Hibernate: 
    insert into shopping_customers_seq values ( 1 )

Hibernate: 
    select
        next_val as id_val 
    from
        shopping_customers_seq for update
Hibernate: 
    update
        shopping_customers_seq 
    set
        next_val= ? 
    where
        next_val=?
Hibernate: 
    insert 
    into
        shopping_customers
        (contact_no, email, location, name, customer_id) 
    values
        (?, ?, ?, ?, ?)

>> Before inserting the record into database table Hibernate will select value for hibernate_sequence table and insert into      primary column value.

Example Application
===================

Customer.java
=============
package com.ashokit.enities;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="mahesh_cutomers")
public class Customer {
	
	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	private Integer customerId;
	
	private String customerName;
	
	private String customerLocation;

	public Integer getCustomerId() {
		return customerId;
	}

	public void setCustomerId(Integer customerId) {
		this.customerId = customerId;
	}

	public String getCustomerName() {
		return customerName;
	}

	public void setCustomerName(String customerName) {
		this.customerName = customerName;
	}

	public String getCustomerLocation() {
		return customerLocation;
	}

	public void setCustomerLocation(String customerLocation) {
		this.customerLocation = customerLocation;
	}

	@Override
	public String toString() {
		return "Customer [customerId=" + customerId + ", customerName=" + customerName + ", customerLocation="
				+ customerLocation + "]";
	}
	
}


CustomerDao.java
================
package com.ashokit.dao;

import org.springframework.data.repository.CrudRepository;

import com.ashokit.enities.Customer;

public interface CustomerDao extends CrudRepository<Customer, Integer> {

}


Application.java
================
package com.ashokit;

import java.util.Arrays;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import com.ashokit.dao.CustomerDao;
import com.ashokit.enities.Customer;

@SpringBootApplication
public class Application implements CommandLineRunner{
	
	@Autowired
	private CustomerDao customerDao;

	public static void main(String[] args) {
		SpringApplication.run(Application.class, args);
	}
	
	@Override
	public void run(String... args) throws Exception {
		
		//inserting the record for an customer
		Customer c = new Customer();
		c.setCustomerName("Mahesh");
		c.setCustomerLocation("Hyderabad");
		
		Customer c2 = new Customer();
		c2.setCustomerName("Ashok");
		c2.setCustomerLocation("Bangalore");
		
		Iterable<Customer> savedCustomers = customerDao.saveAll(Arrays.asList(c,c2));
		savedCustomers.forEach(cust -> System.out.println(cust));
	}
}


application.properties
======================
#configuring the Database properties
spring.datasource.driver-class-name=oracle.jdbc.OracleDriver
spring.datasource.url=jdbc:oracle:thin:@localhost:1521:xe
spring.datasource.username=system
spring.datasource.password=manager

#Hibernate Configuration
spring.jpa.hibernate.ddl-auto=update
spring.jpa.show-sql=true

********************************
Problem With hibernate_sequence 
********************************
  ShoppingCustomers  >>>>>>  shopping_customers(DBTable) >>>>>> hibernate_sequence >>>>> Oracle Database

  ShoppingEmployee   >>>>>>  shopping_employees(DBTable) >>>>>> hibernate_sequence >>>>> Oracle Database

  1) You Performed 5 insertions into shopping_customers DB Table  >>>>>> CustomerIds(1,2,3,4,5)
          
  2) You Performed 5 inserstion into shopping_employees DB Table >>>>>> employeeids(6,7,8,9,10)

  3) You Performed 5 insertions into shopping_customers DB Table >>>>>> customerIds(11,12,13,14,15)

  4) You Performed 5 insertions into shopping_employees DB Table >>>>>> employeeids(16,17,18,19,20)

  5) Viewing the data for shopping_customers :1,2,3,4,5,11,12,13,14,15

  6) Viewing the data for shopping_employees :6,7,8,9,10,15,16,17,18,19,20

  If you observe the data for above two tables primary key column values are not in sync i.e.,Data Inconsistency.

  Inorder to overcome this problem we need to create independent sequence per database table.

        shopping_customers >>>>> shopping_customers_sequence  >>>> GenerationType.SEQUENCE

        shopping_employees >>>>> shopping_employees_sequence  >>>> GenerationType.SEQUENCE