Imagine having a tool that can automatically detect if you are using JPA and Hibernate properly. Hypersistence Optimizer is that tool!
After I wrote the article about inheriting properties from a base class entity using @MappedSuperclass, I got an avalanche of opinions, but this one from Lukas deserves a blog post:
Composition over inheritance. It seems to me that an embedded type would be a better fit
— Lukas Eder (@lukaseder) November 8, 2017
While @MappedSuperclass has its benefit, allowing you to reuse even the @Id mapping, as well as being more lenient towards Hibernate-specific auto-generated properties like @GeneratedValue, using Embeddable types is the other JPA alternative for reusing a bunch of properties among multiple entities.
In this article, we are going to see how we can reuse several audit-related properties using @Embeddable and another awesome JPA feature, @EntityListeners.
Assuming we have the following tables in our relational database:
As you can see from the diagram above, all tables share the same four audit-based columns:
created_bycreated_onupdated_byupdated_onTherefore, we want to encapsulate these four entity properties in a reusable @Embedabble type:
@Embeddable
public class Audit {
@Column(name = "created_on")
private LocalDateTime createdOn;
@Column(name = "created_by")
private String createdBy;
@Column(name = "updated_on")
private LocalDateTime updatedOn;
@Column(name = "updated_by")
private String updatedBy;
//Getters and setters omitted for brevity
}
Now, to populate these properties automatically, we are going to use the following JPA entity event listener:
public class AuditListener {
@PrePersist
public void setCreatedOn(Auditable auditable) {
Audit audit = auditable.getAudit();
if(audit == null) {
audit = new Audit();
auditable.setAudit(audit);
}
audit.setCreatedOn(LocalDateTime.now());
audit.setCreatedBy(LoggedUser.get());
}
@PreUpdate
public void setUpdatedOn(Auditable auditable) {
Audit audit = auditable.getAudit();
audit.setUpdatedOn(LocalDateTime.now());
audit.setUpdatedBy(LoggedUser.get());
}
}
The LoggedUser utility is described in this article, so I won’t repeat its definition here.
The Auditable type is an interface that looks as follows:
public interface Auditable {
Audit getAudit();
void setAudit(Audit audit);
}
Our entities are going to implement the Auditable interface so that the JPA event listener can locate the Audit embeddable type and set the appropriate audit-based properties.
Now, to make the AuditListener available to our entities, we are going to use the @EntityListeners JPA annotation.
Therefore, our four JPA entities are going to look as follows.
@Entity(name = "Post")
@Table(name = "post")
@EntityListeners(AuditListener.class)
public class Post implements Auditable {
@Id
private Long id;
@Embedded
private Audit audit;
private String title;
@OneToMany(
mappedBy = "post",
cascade = CascadeType.ALL,
orphanRemoval = true
)
private List<PostComment> comments = new ArrayList<>();
@OneToOne(
mappedBy = "post",
cascade = CascadeType.ALL,
orphanRemoval = true,
fetch = FetchType.LAZY
)
private PostDetails details;
@ManyToMany
@JoinTable(
name = "post_tag",
joinColumns = @JoinColumn(
name = "post_id"
),
inverseJoinColumns = @JoinColumn(
name = "tag_id"
)
)
private List<Tag> tags = new ArrayList<>();
//Getters and setters omitted for brevity
}
@Entity(name = "PostDetails")
@Table(name = "post_details")
@EntityListeners(AuditListener.class)
public class PostDetails implements Auditable {
@Id
private Long id;
@Embedded
private Audit audit;
@OneToOne(fetch = FetchType.LAZY)
@MapsId
private Post post;
@Lob
private byte[] image;
//Getters and setters omitted for brevity
}
We are using @MapsId for the @OneToOne mapping because it’s the best way to map a one-to-one table relationship.
@Entity(name = "PostComment")
@Table(name = "post_comment")
@EntityListeners(AuditListener.class)
public class PostComment implements Auditable {
@Id
@GeneratedValue(generator = "native")
@GenericGenerator(
name = "native",
strategy = "native"
)
private Long id;
@Embedded
private Audit audit;
@ManyToOne
private Post post;
private String review;
//Getters and setters omitted for brevity
}
We are using the native Hibernate-specific generator because, for MySQL, the AUTO generator is to be avoided.
@Entity(name = "Tag")
@Table(name = "tag")
@EntityListeners(AuditListener.class)
public class Tag implements Auditable {
@Id
private String name;
@Embedded
private Audit audit;
//Getters and setters omitted for brevity
}
Now, when inserting three Tag entities:
Tag jdbc = new Tag();
jdbc.setName("JDBC");
entityManager.persist(jdbc);
Tag hibernate = new Tag();
hibernate.setName("Hibernate");
entityManager.persist(hibernate);
Tag jOOQ = new Tag();
jOOQ.setName("jOOQ");
entityManager.persist(jOOQ);
Hibernate is going to issue the following SQL INSERT statements:
INSERT INTO tag (
created_by,
created_on,
updated_by,
updated_on,
name
)
VALUES (
'Alice',
'2017-11-20 11:17:40.453',
'NULL(VARCHAR)',
'NULL(TIMESTAMP)',
'JDBC'
)
INSERT INTO tag (
created_by,
created_on,
updated_by,
updated_on,
name
)
VALUES (
'Alice',
'2017-11-20 11:17:40.473',
'NULL(VARCHAR)',
'NULL(TIMESTAMP)',
'Hibernate'
)
INSERT INTO tag (
created_by,
created_on,
updated_by,
updated_on,
name
)
VALUES (
'Alice',
'2017-11-20 11:17:40.473',
'NULL(VARCHAR)',
'NULL(TIMESTAMP)',
'jOOQ'
)
Notice that the created_by and created_on have been properly populated by the AuditListener.
When persisting a Post along with its associated PostDetails child entity:
Post post = new Post();
post.setId(1L);
post.setTitle(
"High-Performance Java Persistence, 1st Edition"
);
PostDetails details = new PostDetails();
details.setImage(imageBytes);
post.setDetails(details);
post.getTags().add(
entityManager.find(Tag.class, "JDBC")
);
post.getTags().add(
entityManager.find(Tag.class, "Hibernate")
);
post.getTags().add(
entityManager.find(Tag.class, "jOOQ")
);
entityManager.persist(post);
Hibernate takes care of the audit-based columns:
INSERT INTO post (
created_by,
created_on,
updated_by,
updated_on,
title,
id
)
VALUES (
'Alice',
'2017-11-20 11:17:40.552',
NULL(VARCHAR),
NULL(TIMESTAMP),
'High-Performance Java Persistence, 1st Edition',
1
)
INSERT INTO post_details (
created_by,
created_on,
updated_by,
updated_on,
image,
post_id
)
VALUES (
'Alice',
'2017-11-20 11:17:40.56',
NULL(VARCHAR),
NULL(TIMESTAMP),
[1, 2, 3, 4, 5, 6, 7, 8, 9],
1
)
INSERT INTO post_tag (post_id, tag_id) VALUES (1, 'JDBC')
INSERT INTO post_tag (post_id, tag_id) VALUES (1, 'Hibernate')
INSERT INTO post_tag (post_id, tag_id) VALUES (1, 'jOOQ')
When updating the Post entity:
Post post = entityManager.find(Post.class, 1L);
post.setTitle(
"High-Performance Java Persistence, 2nd Edition"
);
Hibernate populates the updated_by and updated_on columns as well:
UPDATE post
SET
created_by = 'Alice',
created_on = '2017-11-20 11:17:40.552',
updated_by = 'Alice',
updated_on = '2017-11-20 11:17:40.605',
title = 'High-Performance Java Persistence, 2nd Edition'
WHERE
id = 1
Great!
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As demonstrated, JPA allows you to provide entity event listeners which you can register via the @EntityListeners annotation. This way, we can encapsulate the audit-based properties in an @Embeddable type and make it available to multiple entities using the @Embedded annotation.
This way, you can reuse both the data structures (e.g. @Embeddable) and behavior as well (e.g. @EntityListeners).
I follow every step But @Embedded audit is null while saving so it’s thrown null pointer exception
All the code is on my High-Performance Java Persistence GitHub repository, and it works like a charm. Check it out and compare with your code base.
public interface Auditable {
Audit getAudit();void setAudit(Audit audit);
}
where is the implenetation of getAudit();
Just read the article carefully and you are going to find the answer to your question.