JPA annotation - @GeneratedValue

Test

JPA annotation “@GeneratedValue” is the way to make insert operation return the auto-incremented ID (Sequence/IDENTITY) of the inserted record.

While working with MySql, you must have used auto-increment property to maintain unique values in the “ID” column of the table. If you are Oracle user, you might have used sequences.(MySql do not have sequences yet.) Sequences are more powerful way of keeping records unique in the table or across multiple tables, and use of sequences is not limited to the tables.

Let’s now see how Object Relational frameworks deal with auto-incremented values or sequences.
Assume, I have a table named "Task" in database. “Task” table has three columns - ‘ID’, ‘Name’ and ‘DueDate’, where ‘ID’ is an auto-increment field. Let’s say “Task” table is mapped to the “Task POJO (Plain Old Java Object)” by some Object-Relation framework such as Hibernate. If I want to insert "Task POJO" into the database, I can simply set the “Name” and “DueDate” using bean methods “setName” and “setDueDate” of the “Task POJO”. It is not required to set the “ID” because “ID” is an auto-incremented field of the table.We want Insert operation to return the value of the auto-incremented ID which is not known until record is actually inserted into the database (because database assigns the auto-incremented value). So, to achieve this, OR frameworks do one simple trick. Once “Task POJO” is inserted into the database, ID property of the “Task POJO” will get set for you. If you are using JPA annotations, you need to set the @GeneratedValue annotation along with the @Id annotation on ID field of the POJO (Entity). Only one @GeneratedValue annotation per POJO or Entity is allowed.

This is how it looks like in code,

@Entity
public class MyEntityimplements Serializable {
 @Id
 @GeneratedValue
 private Long id;
} 

More annotations later, stay tuned