Internally default mapping of Table and Column in Hibernate

In last article, we have discussed about component mapping in hibernate using JPA annotation. Strongly recommend to read this post.

Lets start the discussion about mapping of columns with the object attributes (member variables).

@Entity
@Getter
@Setter
public class Person {

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

  @Column(nullable = false)
  private String name;

  @Embedded
  private Address address;

  public Person() {
  }

  public Person(String name, Address address) {
    this.name = name;
    this.address = address;
  }
}

@Embeddable
public class Address {

  private String street;
  private String city;
  private String zipcode;

  public Address() {
  }

  public Address(String street, String city, String zipcode) {
    this.street = street;
    this.city = city;
    this.zipcode = zipcode;
  }
}

The table is automatic create by hibernate because of hbm2ddl.auto=create-drop properties set into hibernate.cfg.xml file. But we have nowhere defined the name of the table and column of the table. Where did this "Person" table come from and columns of this table?

We have not written the schema for it, no mapping file and not used @Table JPA annotations for mapping. Even we have not used @Column annotation or mapping file to map id attribute to id column of this table. Similarly for the rest of the attributes. Hibernate automatically figure out about it. First hibernate uses the reasonable default values for xml based mapping metadata and also for annotations based mapping metadata.

So when it finds a mapping information missing. Then it uses sensible default value for that.

When It doesn't find the @Table annotation on entity. It is going to use the name of entity as table name. So here name of the entity is "Person" so it is mapped to the name of the table i.e "Person".

Similarly, It doesn't find the @Column annotation on an attribute. It is going to use the attribute name as column name of the table.

Hibernate internally treats them like below:

@Entity
@Table(name = "Person")
@Getter
@Setter
public class Person {

  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Column(name = "id")
  private Long id;

  @Column(name = "name", nullable = false)
  private String name;

  @Embedded
  private Address address;

  public Person() {
  }

  public Person(String name, Address address) {
    this.name = name;
    this.address = address;
  }
}

@Embeddable
public class Address {

  @Column(name = "street")
  private String street;
  @Column(name = "city")
  private String city;
  @Column(name = "zipcode")
  private String zipcode;

  public Address() {
  }

  public Address(String street, String city, String zipcode) {
    this.street = street;
    this.city = city;
    this.zipcode = zipcode;
  }
}

When we run the application that time It create the DDl script using the mapping metadata of the person entity and execute the schema against the "helloworld" database.

create table Person (id bigint not null auto_increment, city varchar(255), 
street varchar(255), zipcode varchar(255), name varchar(255) not null, 
primary key (id)) engine=InnoDB

The name column of the Person table is having not null constraint that is coming because of @Column(nullable=false).