Is there a way to map an empty String to null using MapStruct?

Question

I am using MapStruct for mapping a DTO to an entity and vice versa in my Spring Boot application.

Is there a way to map empty strings to null using @Mapping?

Drakensberg

Answer 1

You can simply use the expression within the @Mapping

@Mapping(target = "name", expression = "java(source.name.isEmpty() ? null : source.name)")
Dog convert(Cat source);

Sherr

If the field has a private access, we may use the getter method in place of the field name. Eg: @Mapping(target = "name", expression = "java(source.getName.isEmpty() ? null : source.getName)") Dog convert(Cat source); – Vestavestal

What a boilerplate! – Presidium

Answer 2

Using an expression, as suggested in another answer, is possible. I personally prefer to do this a bit safer, and perhaps a bit more declarative.

@Mapper
public interface MyMapper {

    @Mapping( target = "s2",  qualifiedBy = EmptyStringToNull.class )
    Target map( Source source);

    @EmptyStringToNull
    default String emptyStringToNull(String s) {
        return s.isEmpty() ? null : s;
    }

    @Qualifier
    @java.lang.annotation.Target(ElementType.METHOD)
    @Retention(RetentionPolicy.CLASS)
    public @interface EmptyStringToNull {
    }

    class Source {

        private String s1;
        private String s2;

        public String getS1() {
            return s1;
        }

        public void setS1(String s1) {
            this.s1 = s1;
        }

        public String getS2() {
            return s2;
        }

        public void setS2(String s2) {
            this.s2 = s2;
        }
    }

    class Target {

        private String s1;
        private String s2;

        public String getS1() {
            return s1;
        }

        public void setS1(String s1) {
            this.s1 = s1;
        }

        public String getS2() {
            return s2;
        }

        public void setS2(String s2) {
            this.s2 = s2;
        }
    }
}

You can re-use the EmptyStringToNull qualifier as many times as you'd like and your not dependent on the parameter name.

Lanti

Is there any way to do that for all strings in all mappers? – Editorial

Answer 3

If you would like it to work globally in the mapper class and intercept any String-Mappings you can simply implement the following:

default String mapEmptyString(String string) {
 string != null && !string.isEmpty() ? string : null;
}

This will map any null or empty String source to null.

Jesusitajet

Are you sure this works? – Presidium

Answer 4

You can also use a Condition like the documentation suggests.

@Mapper
public interface MyMapper {

     Target map( Source source);

     @Condition
     default boolean isNotEmpty(String value) {
        return value != null && !value.isEmpty();
     }
}

Bestraddle