what is the use of annotations Id and GeneratedValue strategy GenerationType IDENTITY Why the generationtype is identity

Question

Id   GeneratedValue strategy   GenerationType IDENTITY    Why we are using this annotations  i need to know if this autoincrement my table id values   GenerationType IDENTITY  is there any other types whats actually happening when we use this annotation  public class Author extends Domain        Id      GeneratedValue strategy   GenerationType IDENTITY       Basic optional   false       Column name    id        private Integer id        Basic optional   false       Column name    name        private String name        Column name    address        private String address         OneToMany cascade   CascadeType ALL  mappedBy    authorId       private List lt Book gt      bookList       public Author                  setServiceClassName  wawo tutorial service admin AuthorService               Is it necessary to extend Domain abstract class What is the use

User · Answer

In a Object Relational Mapping context  every object needs to have a unique identifier  You use the  Id annotation to specify the primary key of an entity   The  GeneratedValue annotation is used to specify how the primary key should be generated  In your example you are using an Identity strategy which     Indicates that the persistence provider must assign primary keys for   the entity using a database identity column    There are other strategies  you can see more here

User · Answer

Simply   Id  This annotation specifies the primary key of the entity     GeneratedValue  This annotation is used to specify the primary key generation strategy to use  i e Instructs database to generate a value for this field automatically  If the strategy is not specified by default AUTO will be used    GenerationType enum defines four strategies   1  Generation Type   TABLE   2  Generation Type  SEQUENCE  3  Generation Type  IDENTITY    4  Generation Type  AUTO  GenerationType SEQUENCE  With this strategy  underlying persistence provider must use a database sequence to get the next unique primary key for the entities    GenerationType TABLE  With this strategy  underlying persistence provider must use a database table to generate keep the next unique primary key for the entities    GenerationType IDENTITY This GenerationType indicates that the persistence provider must assign primary keys for the entity using a database identity column  IDENTITY column is typically used in SQL Server  This special type column is populated internally by the table itself without using a separate sequence  If underlying database doesn t support IDENTITY column or some similar variant then the persistence provider can choose an alternative appropriate strategy  In this examples we are using H2 database which doesn t support IDENTITY column   GenerationType AUTO This GenerationType indicates that the persistence provider should automatically pick an appropriate strategy for the particular database  This is the default GenerationType  i e  if we just use  GeneratedValue annotation then this value of GenerationType will be used     Reference - https   www logicbig com tutorials java-ee-tutorial jpa jpa-primary-key html

User · Answer

Let me answer this question  First of all  using annotations as our configure method is just a convenient method instead of coping the endless XML configuration file     The  Idannotation is inherited from javax persistence Id  indicating the member field below is the primary key of current entity  Hence your Hibernate and spring framework as well as you can do some reflect works based on this annotation  for details please check javadoc for Id    The  GeneratedValue annotation is to configure the way of increment of the specified column field   For example when using Mysql  you may specify auto increment in the definition of table to make it self-incremental  and then use    GeneratedValue strategy   GenerationType IDENTITY    in the Java code to denote that you also acknowledged to use this database server side strategy  Also  you may change the value in this annotation to fit different requirements    1  Define Sequence in database  For instance  Oracle has to use sequence as increment method  say we create a sequence in Oracle     create sequence oracle seq    2  Refer the database sequence  Now that we have the sequence in database  but we need to establish the relation between Java and DB  by using  SequenceGenerator    SequenceGenerator name  seq  sequenceName  oracle seq     sequenceName is the real name of a sequence in Oracle  name is what you want to call it in Java   You need to specify sequenceName if it is different from name  otherwise just use name   I usually ignore sequenceName to save my time     3  Use sequence in Java  Finally  it is time to make use this sequence in Java  Just add  GeneratedValue      GeneratedValue strategy GenerationType SEQUENCE  generator  seq     The generator field refers to which sequence generator you want to use  Notice it is not the real sequence name in DB  but the name you specified in name field of SequenceGenerator     4  Complete  So the complete version should be like this   public class MyTable        Id      SequenceGenerator name  seq  sequenceName  oracle seq                GeneratedValue strategy GenerationType SEQUENCE  generator  seq                      private Integer pid      Now start using these annotations to make your JavaWeb development easier

[java] what is the use of annotations @Id and @GeneratedValue(strategy = GenerationType.IDENTITY)? Why the generationtype is identity?

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