Hibernate simple entities

This note is about hibernate simple entities.

references:

• web:
  • Vlad Mihalcea’s blog
  • thorben jansen’s blog
  • hibernate orm
  • hibernate reactive
  • quarkus reactive hibernate with panache
  • postgreSQL docs - current
• paperback book:
  • Vlad Mihalcea’s high performance java persistence

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intro: Entities

entities typically represent a table on a database, and each instance of an entity a row on that table. They can have logic code as seen on the active record pattern.

an entity in BMC_Anvil is a POJO representing a table on a database, and except for accessors done via lombok it has no logic.

in this application there is no code on the entities as I chose to use a repository approach instead of an active record approach to decouple the data access code from the data modeling.

when looking at the entities in BMC_Anvil you are looking at the table structure of the application.

the entities can be very simple representations of a table of very complex representations of tables, relations, fields data types, etc.

simple Entities

I do not have plain simple entities in the application, so let’s first see a proper simple entity en then a simple entity in BMC_Anvil

simple entities in general

a simple entity en general will look something like this:

example modified from quarkus.io orm with panache guide

@Entity
public class Person {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long      id;
    private String    name;
    private LocalDate birth;

    //get / set omitted for brevity 
}

via the @Entity annotation, the above means that Person is a class that will represent a table of the same name in a database

it has 4 fields with different data types that will represent the columns on the database.

the id Long field is annotated with @Id and @GeneratedValue, meaning that it will act as primary key and its value will be autogenerated leaving to hibernate to decide which strategy to use, hence the strategy = GenerationType.AUTO.

hibernate will take care of mapping the proper data type back and forth from the database to match Long, String and LocalDate to the underlying database’s data types.

simple entities in BMC_Anvil

the actual showcase application uses a few tricks to benefit from more advanced JPA features like inheritance.

I’ll show here how a simple entity looks like in the application.

@Entity
@Table(name = "comment")
@Getter
@Setter
@EqualsAndHashCode(onlyExplicitlyIncluded = true, callSuper = true)
public class CommentEntity extends BaseEntity {

  @Column(columnDefinition = "text")
  private String comment;

  @ManyToOne
  private CardEntity card;

}

let’s ignore lombok’s annotations as they are addressed elsewhere

this CommentEntity has the @Entity annotation along with @Table. The table one tell hibernate how do we want to call the table on the DB.

this entity es very simple even if we start seen some annotations on the fields. It just has 2 fields, a String and a CardEntity. let’s check these 2 JPA annotations:

• @Column(columnDefinition = "text"): a String type maps by default to a varchar(255) on the database, but we want our comment column to have no limit. By means of the @Column annotation we can configure different aspects of the column in the database, and in this case we tell the orm to create this String field as a text column on the db.
• @ManyToOne: this is an association mapping that tells hibernate that there are many comments per card. We are going to look deeper into mappings at their own articles.

seems that there is all to it, but we are missing an id of some sorts.

this simple entity class is made even simpler by reusing code. The CommentEntity extends a BaseEntity class. Let’s quickly check it!

@MappedSuperclass
@Getter
@Setter
@EqualsAndHashCode(onlyExplicitlyIncluded = true)
public abstract class BaseEntity {
  @Id
  @EqualsAndHashCode.Include
  private UUID id;

  @ManyToOne(fetch = LAZY)
  private UserEntity createdBy;

  @CreationTimestamp
  @Column(updatable = false)
  private LocalDateTime createdAt;

  @UpdateTimestamp
  private LocalDateTime updatedAt;

}

aha! the parent class has the id we were missing. By extending from this class our CommentEntity inherits everything from the BaseEntity.

we are going to see how it all works on the hibernate inheritance note. See you there!