Declaring a class as an entity and creating a table in the database – @Entity and @Table

We need a class to be declared as an entity for hibernate to use it. Hibernate considers the class as a persistent class if it is annotated with the @Entity annotation.

How to do it…

Perform the following steps to declare a class as a hibernate entity:

  1. Enter the following code on your editor:
    @Entity
public class Employee {
  // Fields and getter/setter
}

    Here, we annotate a class, Employee, with the @Entity annotation. As a result, hibernate considers the current class eligible to be persisted.

    Note

    If you build a session factory with the preceding code and the table name is not given, hibernate will create a table with the name employee, which is equal to the class name.

  2. If we want a user-defined table name rather than a default name, we can use the @Table annotation. The following code shows us how to achieve this:
    @Entity
@Table(name="tbl_employee")
public class Employee {
  // Fields and getter/setter
}

Here, we give name="tbl_employee" as a parameter in the @Table annotation. So, hibernate will override the default table name with the name "tbl_employee".

There is another attribute available with @Table annotations; let's take a look at it.

This attribute is called uniqueConstraints. It is used when we need the UNIQUE key constraint with multiple fields.

The following code shows how to do this:

@Entity
@Table(name = "tbl_employee", uniqueConstraints = @UniqueConstraint(columnNames = { "id" , "empCode"}))
public class Employee {

    @Id
    private long id;

    @Column
    private String empCode;
  
    // Fields and getter/setter

}

When a SessionFactory is created for the first time and property hbm2ddl.auto is set to create, hibernate will execute the following queries to create a table and the unique key constraints:

Hibernate: drop table if exists tbl_employee
Hibernate: create table tbl_employee (id bigint not null, empCode varchar(255), primary key (id))
Hibernate: alter table tbl_employee add constraint UK_3r763mmnyundobvaiqjv6lnj1  unique (id, empCode)

When the table is created using the preceding code, hibernate will create the UNIQUE key using two fields: id and empCode.

Take a look at the following script belonging to the table created by hibernate to understand uniqueConstraints:

CREATE TABLE `tbl_employee` (
  `id` bigint(20) NOT NULL,
  `empCode` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id` (`id`,`empCode`)
);

Note

The uniqueConstraints is useful when we need a UNIQUE constraint for multiple fields. For only one column, you can use the @Column(unique=true) annotation directly on the field.

How it works…

Hibernate uses the declared annotation at the time of compilation to get the information applied to the Java code. You can't persist a class if it is not annotated with @Entity or defined in *.hbm.xml.

The @Table annotation is an optional annotation and is used when the custom table name is required.

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