Explicit constructor using Lombok?

Question

Hello there,

I'm rewriting some messy code that manages a database, and saw that the original programmer created a class mapped to the database like so:

(I've removed unnecessary code that has no purpose in this question)

@Entity
@Data
@EqualsAndHashCode(callSuper = false, of = { "accessionCode", "header", "date" })
@SuppressWarnings("PMD.UnusedPrivateField")
public class PDBEntry implements Serializable {
    @Id
    @NaturalId
    @NotEmpty
    @Length(max = 4)
    private String accessionCode;

    @NaturalId
    @NotEmpty
    private Date date;

    @NaturalId
    // We allow for the header to be 'null'
    private String header;

    private Boolean isValidDssp;

    @Temporal(TemporalType.TIMESTAMP)
    private Date lastUpdated = new Date(System.currentTimeMillis());

    protected PDBEntry(){}

    public PDBEntry(String accessionCode, String header, Date date){
        this.accessionCode = accessionCode;
        this.header = header;
        this.date = date;
    }
}

I am still a beginner at Hibernate and using Lombok, but wouldn't this do the same thing and wouldn't Lombok automatically create the needed constructor for you?

@Entity
@Data
@SuppressWarnings("PMD.UnusedPrivateField")
public class PDBEntry implements Serializable {
    @Id
    @NaturalId
    @NotEmpty
    @NonNull
    @Length(max = 4)
    private String accessionCode;

    @NaturalId
    @NotEmpty
    @NonNull
    private Date date;

    @NaturalId
    // We allow for the header to be 'null'
    private String header;

    private Boolean isValidDssp;

    @Temporal(TemporalType.TIMESTAMP)
    private Date lastUpdated = new Date(System.currentTimeMillis());
}

Also, the original programmer of this code says he allows for the header to be 'null', yet he explicitly created a constructor that needs a value for header. Am I missing something or is this a bit contradictory?

Answer 1

That bit is contradictory you're right. I've not used Lombok before but with hibernate if you want to be able to create a bean and persist you need the default constructor as given above as far I was aware. It uses Constructor.newInstance() to instantiate new objects.

Here is some hibernate documentation which goes into more detail.

Hibernate Documentation

Answer 2

Have a look at @NoArgsConstructor, @RequiredArgsConstructor, @AllArgsConstructor.

The constructor behavior of @Data is like @RequiredArgsConstructor:

@RequiredArgsConstructor generates a constructor with 1 parameter for each field that requires special handling. All final fields get a parameter, as well as any fields that are marked as @NonNull that aren't initialized where they are declared.

Given that none of your fields are either final or @NonNull, this will result in a no-argument constructor. However, this is not the most expressive way to achieve this behavior.

What you'll probably want in this case is a @NoArgsConstructor (optionally combined with a @AllArgsConstructor), to clearly communicate the intended behavior, as is also indicated in the documentation:

Certain java constructs, such as hibernate and the Service Provider Interface require a no-args constructor. This annotation is useful primarily in combination with either @Data or one of the other constructor generating annotations.