使用Lombok的显式构造函数？

Question

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

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.

Answer 2

If you are using @Data with a @NonNull field and still want a noargs-constructor, you might wanna try to add all 3 annotation together

@NoArgsConstructor
@RequiredArgsConstructor
@AllArgsConstructor

Apparently an old intelliJ bug which I did replicate in Eclipse Kepler and lombok v0.11.4

Answer 3

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 4

@NoArgsConstructor, 
@RequiredArgsConstructor, 
@AllArgsConstructor

Generate constructors that take no arguments, one argument per final / non-null field, or one argument for every field. Read this lombok-project

@Data
@RequiredArgsConstructor /*Duplicate method Someclass() in type Someclass*/
@NoArgsConstructor(access=AccessLevel.PRIVATE, force=true)  /*Duplicate method Someclass() in type Someclass*/
@Entity
public class Someclass {      
    @Id
    private  String id;
    private  String name;
    private  Type type; 

    public static enum Type { X , Y, Z}
}

Fixed it by making member variables final

@Data
@RequiredArgsConstructor 
@NoArgsConstructor(access=AccessLevel.PRIVATE, force=true)
@Entity
public class Someclass {

    @Id
    private final String id;
    private final String name;
    private final Type type; 
    public static enum Type { X , Y, Z}
}