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

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.

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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.

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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

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Why are we using this annotation?

First I would like to remind everyone that the annotations, such as @Id, are providing metadata to the persistence layer(I will assume hibernate).This metadata will most likely be stored in the .class file(but not stored in the database) and is used to tell hibernate how to recognize, interpret and manage the entity. So, Why are you using the annotation? To provide your persistence layer with the proper information about how to manage the entity.

Why use the @Id annotation?

The @Id annotation is one of the two mandatory annotations needed when creating an entity with JPA. The other one being @Entity. @Id does two things for us: 1) signifies that this field will be the unique identifier for this class when mapped to a database table 2) the presence of @Id lets the persistence layer know that all other fields within this class are to be mapped to database rows

Why use @GeneratedValue?

By marking the @Id field with @GeneratedValue we are now enabling id generation. Which means that the persistence layer will generate an Id value for us and handle the auto incrementing. Our application can choose 1 of 4 generations strategies: 1) AUTO 2) TABLE 3) SEQUENCE 4) IDENTITY

If not strategy is specified then AUTO is assumed

What is strategy = GenerationType.IDENTITY actually doing?

When we specify the generation strategy as GenerationType.IDENTITY we are telling the persistence provider(hibernate) to let the database handle the auto incrementing of the id. If you were to use postgres as an underling database and specified the strategy as IDENTITY, hibernate would execute this:

create table users (
       id  bigserial not null,
        primary key (id)
    )

Notice that they type of the id is bigserial, what is bigserial? As per the postgres documentation, bigserial is a large autoincrementing integer.

Conclusion

By specifying:

@Id 
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;

you have told the underlying persistence layer to use the id field as a unique identifier within the database. Also told the persistence layer to let the database handle the auto incrementing of the id with GenerationType.IDENTITY.

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