JPA瞬态注释

Question

I am learning Spring , Hibernate

I have read the following and other tutorial sources. What I understand is @Transient will prevent the object field from serializing and stop it from persisting to the database.

Transient variable in java

Transient annotation

JPA Transient annotation and JSON

My question is what if you have an object's field that you don't want to persist to database but you want the field to be retrieved when getting data from server?

For example on your MySql createDate set to CURRENT_TIMESTAMP so when a data is inserted a TIMESTAMP is then created my MySql.

MySql is set up something like this

ALTER TABLE `books` 
ADD COLUMN `dateCreated` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON 
UPDATE CURRENT_TIMESTAMP AFTER `dateCreated`;

My controller is

@RequestMapping(path="/booklist", method = RequestMethod.GET, produces="application/json")
@ResponseBody
public Map<String, Object> getListBooks(@RequestParam(name="search", required=false) String search, 
                @RequestParam(name="ordering", required=false) String ordering){


    Map<String, Object> data = new HashMap<String, Object>();

    // Get list of books from server
    List<Book> books = bookDao.getBookList(search, ordering);

    System.out.println(books.get(0));

    // do other things and return map
}

My return data is

Book [id=1, name=Murach Jquery, 2rd Edition, ispn=978-1890774912, price=4.24, dateCreated=null, datePublished=null]

My Book object is as follows

@Entity
@Table(name="books")
@Component
public class Book implements Serializable{

    private static final long serialVersionUID = -2042607611480064259L;

    @Id
    @GeneratedValue
    private int id;

    @NotBlank
    private String name;

    @NotBlank
    @Size(min=2, max=16)
    private String ispn;

    @DecimalMin(value = "0.1")
    private double price;

    @Transient
    private Timestamp dateCreated;

    private Date datePublished;

    // getter and setters

}

Thank you.

Answer 1

@Transient wont help in your case.

Use @PrePersist on a method in your entity class.

@PrePersist
public void beforePersist() {
    this.dateCreated = new Date();
}

beforePersist() method annotated with @PrePersist will help you automatically initilize entity fields before you want to persist a new entity. For your case you can initialize your date property inside this method. You dont have to manually set it everytime you persist a new entity.

Now when you fetch entities from your database you will get your date property.

Also look at @PreUpdate which helps you update entity fields when you update an entity.

Answer 2

Use @Formula annotation for the field and just pass the column. See the example

@Formula("createDate")
private Timestamp dateCreated;