Hibernate

Identifier : strategies

• @GeneratedValue(Strategy = GenerationType."XXXXX")

AUTO

• hibernate will choose automatically, based on dialect.
• Oracle, PostgreSQL : Uses SEQUENCE because Oracle supports sequences.
• MySQL, SQL Server, H2 : Uses IDENTITY because MySQL supports auto-increment columns.

IDENTITY :

• Auto-increment, can apply on Number types.

TABLE

• @GeneratedValue () @TableGenerator()
• less common, PK is generated using a "table" that holds a set of unique keys.

  CREATE TABLE ID_GEN (
      GEN_NAME VARCHAR(255) NOT NULL,
      GEN_VALUE BIGINT NOT NULL,
      PRIMARY KEY (GEN_NAME)
  );
  
  @Id
  @GeneratedValue(strategy = GenerationType.TABLE, generator = "my_table_generator")  <<<
  @TableGenerator(
  name                = "my_table_generator", 
  table               = "ID_GEN",
  pkColumnName        = "GEN_NAME",
  valueColumnName     = "GEN_VALUE",
  pkColumnValue       = "MY_ID_GEN_1",
  allocationSize      = 1 
  )
  private Long id;
  
  - SELECT GEN_VALUE FROM ID_GEN WHERE GEN_NAME = 'MY_ID_GEN_1';
  - UPDATE ID_GEN SET GEN_VALUE = GEN_VALUE + 1 WHERE GEN_NAME = 'MY_ID_GEN_1';
  

SEQUENCE

• @GeneratedValue @SequenceGenerator :

  CREATE SEQUENCE MY_SEQUENCE START WITH 1 INCREMENT BY 1;             <<< 
  
  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "my_sequence")
  @SequenceGenerator(name = "my_sequence", sequenceName = "MY_SEQUENCE", allocationSize = 1)
  private Long id;
  
  allocationSize -  number of sequence values to allocate at a time
  

GENERIC - UUID/GUID

• @GeneratedValue() @GenericGenerator() - from hibernate
• Random Sequence, avoid predictable sequences for security.
• fact: Random values can lead to fragmented indexes, which can affect performance.
• UUID/GUID
• 128 bit - globally unique
• eg: 550e8400-e29b-41d4-a716-446655440000
• 32 char,string, group of 5 by Hypen.
• incorporate TimeStamp and hardware info.
• use-case : DB:Pk, DistributedSystem, registry keys, etc
• eg:
• org.hibernate.id.UUIDGenerator

      @GeneratedValue( generator = "myUUID")
      @GenericGenerator(name = "myUUID", strategy = "org.hibernate.id.UUIDGenerator")
  

• create own: CustomIdentifier.java
  

  public class CustomIdentifier implements IdentifierGenerator {
    @Override
    public Object generate(SharedSessionContractImplementor sharedSessionContractImplementor, Object o) {
        return UUID.randomUUID();
    }
  }            
  
  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "generic-1")
  @GenericGenerator(name = "generic-1", strategy = "CustomIdentifier")
  private Long id;
  

• NanoID

• unique IDs that are shorter and more URL-friendly.

• eg: DqQrAcB9jK

• TimestampId

      @Override
      public Serializable generate(SharedSessionContractImplementor session, Object object) {
          long timestamp = System.currentTimeMillis();
          int random = new Random().nextInt(999999);
          return timestamp + "-" + random;
      }
  

• Composite Identifiers
• https://www.baeldung.com/hibernate-identifiers
• @Embeddable Class ABC : Also public no-agr const, define equal and hashcode
• then inject @EmbeddedId ABC id;

---

D. Validator

• spring-boot-starter-validation
• @Min / @Max / @DecimalMin/MAX / @Range- on numbetypes
• @Size(min = 3, max = 15) : on String
• @length(min = 3, max = 15) : on Collection
• @Null/ @NotNull / @NotEmpty / @NotBlank
• @AssertTrue / @AssertFalse
• @Pattern(regexp = "^[0-9]{10}$") String phoneNo
• Date:
• @Past
• @PastOrPresent
• @Future
• @FutureOrPresent
• More (H-specific) : @URL, @email, @CreditCardNumber, @Currency, @UUID

• Custom validator:

• create custom Annotation1 and annotate with @Constraint(validatedBy = Validator1.class)
• class Validator1 implements ConstraintValidator // string:: TypeOfvalueBeingValidate
• Apply @Annotation1