lotabout / transformer-playground

Annotation processor @Transform for creating transformers for classes.

An annotation processor @Transformer for creating transformers for classes.

(Note that it is only for learning purpose and I'll not maintain this repo for product usage)

Scenario

In order to achieve encapsulation, sometimes we need to duplicate the code a little bit. For example, you have a Business Object named ApplicantBo:

public class ApplicantBo {
    private int id;
    private String name;
    private List<EducationVo> educationList;

    // don't want to go public
    private ZonedDateTime lastUpdate;

    // business logic here
}

public class EducationVo {
    private ZonedDateTime startTime;
    private ZonedDateTime endTime;
    private String school;
    private List<String> courses;
}

Somehow we need to send the data to other person/services. For example if we want to send tha applicant information to browser, we might want to serialize it to JSON. And for other cases, we might need to create a data class that duplicate most of the fields:

public class ApplicantPojo {
    private int id;
    private String name;
    private List<EducationPojo> educationList;
}

public class EducationPojo {
    private ZonedDateTime startTime;
    private ZonedDateTime endTime;
    private String school;
    private List<String> courses;
}

And things become tedious that we need to convert these two objects. So that we need to write boilerplates for transforming:

public class ApplicantBoTransformer {
    public static ApplicantPojo toPojo(ApplicantBo bo) {
        // ...
    }
}

public class EducationVoTransformer {
    public static EducationPojo toPojo(EducationVo vo) {
        // ...
    }
}

This is where @Transformer helps, it will generate the boilerplates automatically.

What @Transformer does

With the ApplicantBo, ApplicantPojo defined in last section, we can add annotation to it:

@Transformer(to={ApplicantPojo.class})
public class ApplicantBo {
    // ...
}

@Transformer(to = EducationPojo.class)
public class EducationVo {
    // ...
}

And @Transformer will grab the fields in two classes with the same name, and generate the transformer automatically:

public final class ApplicantBoTransformer {
  public static ApplicantPojo toApplicantPojo(ApplicantBo from) {
    ApplicantPojo to = new ApplicantPojo();;
    to.setId(from.getId());
    to.setName(from.getName());
    to.setEducationList(from.getEducationList().stream().map(l1 -> EducationVoTransformer.toEducationPojo(l1)).collect(Collectors.toList()));
    // Fields only in ApplicantBo
    // ZonedDateTime: lastUpdate
    // Fields only in ApplicantPojo
    return to;
  }
}

public final class EducationVoTransformer {
  public static EducationPojo toEducationPojo(EducationVo from) {
    EducationPojo to = new EducationPojo();;
    to.setStartTime(from.getStartTime());
    to.setEndTime(from.getEndTime());
    to.setSchool(from.getSchool());
    to.setCourses(from.getCourses().stream().map(l1 -> l1).collect(Collectors.toList()));
    // Fields only in EducationVo
    // Fields only in EducationPojo
    return to;
  }
}

Note that the EducationVoTransformer is detected automatically.

Details

• syntax: @Transformer(to = {a.class, b.class}, from = {c.class, d.class})
• @Transformer will first find the fields with the same name from two classes (the one annotated and the one specified as "to" or "from").
• Then it will check if the types of the fields can be transformed or not
  • Two types can be transformed if they have the same type (primitive, declared class but not List or Map).
  • Two types can be transformed if @Transformer is used to connect the two types.
  • List can be transformed only if their bounded types are transformable.
  • Map can be transformed only if the type of the key is directly transformable and the value can be transformed by transformer.
• Array and other generic types are not supported.